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Growth, and nitrogen uptake and flow in maize plants affected by root
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growth restriction
3
Liangzheng Xu1,2, Junfang Niu3 Chunjian Li1* Fusuo Zhang1
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(1 The Key Laboratory of Plant Nutrition, MOA, Department of Plant Nutrition, China Agricultural
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University, Beijing 100193, China
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Department of Biology, Jiaying University, Meizhou 514015, China
Centre of Agricultural Resources, Institute of Genetics and Developmental Biology, the Chinese Academy
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of Sciences, Shijiazhuang 050021, China)
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Running title:
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Root Growth Restriction, and Nitrogen Uptake and Flow in Maize
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*Corresponding author: [email protected]
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Address: Department of Plant Nutrition, China Agricultural University,
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Yuanmingyuan West Road 2, Beijing 100193, PR China
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Phone: +86 10 6273 3886
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Fax:
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E-mail: [email protected]
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Number of tables: 6
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Number of figure: 2
+86 10 6273 1016
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1
1
Abstract
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3
Preventing the development of shoot-borne roots produces maize plants with an elongated
4
mesocotyl between the shoot and the absorption roots (those roots contributing to water and
5
nutrient absorption), and confines xylem and phloem transport to a single pathway in the
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mesocotyl. The aim of the present experiments was to investigate the influence of a reduced
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root-system size on root growth and nitrogen (N) uptake and flow within plants. Maize plants
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cultured in quartz sand with three types of root systems were grown: 1) intact (control); 2)
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embryonic (growth of shoot-borne roots restricted); and 3) primary (growth of shoot-borne
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roots restricted and seminal roots excised). Xylem sap from different leaves was collected by
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application of pressure to the root system. Restriction of shoot-borne root growth caused a
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strong decrease in the absorption root:shoot dry weight ratio and a reduction in shoot growth.
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On the other hand, compensatory growth and an increased N uptake rate in the remaining
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roots were observed. The N uptake rate of plants with embryonic or primary root systems was
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4.8 and 5.5 times higher, respectively, than that of plants with an intact root system. Despite
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the limited long-distance transport pathway in the mesocotyl with restriction of shoot-borne
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root growth, N cycling within these plants was higher than those in control plants, implying
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that xylem and phloem flow velocities via the mesocotyl were considerably higher than in
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plants with an intact root system. As a result, leaf N concentration in the constituent organs of
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plants with restricted root systems was not reduced. The removal of the seminal roots in
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additional to restricting shoot-borne root development did not affect whole plant growth and
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N uptake, except for the stronger compensatory growth of the primary roots. Our results
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suggest that, in spite of a strong decrease in the absorption root:shoot dry weight ratio under
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root growth restrictions, an adequate N supply to the plant is maintained by compensatory
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growth of the remaining roots, increased N uptake rate and flow velocities within the xylem
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and phloem via the mesocotyl, and reduction in the shoot growth rate.
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Key words: Root system components; nitrogen uptake; nitrogen flow; Zea mays.
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2
1
The root systems of terrestrial plants perform two primary functions: acquisition of soil-based
2
resources and anchorage (Fitter 2002). Shoot growth and development in plants is strongly
3
dependent on concomitant and unrestricted development of the root system. Reducing root
4
size by root pruning (Jesko 1972; Carmi and Koller 1978) or restriction of the root volume
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(Carmi et al. 1983; Robbins and Pharr 1988) leads to a commensurate decrease in shoot
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growth. On the other hand, plants with restricted root development can continue to grow
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(Jesko 1972; Jeschke et al. 1997; Shane and McCully 1999) and, at least in grass species such
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as wheat (Passioura 1972) or maize (Hetz et al. 1996), even attain maturity supported only by
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the primary root system. This suggests that plants exhibit a strong compensatory growth and
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uptake mechanism to acquire soil-based resources with the remaining roots after root pruning.
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Nitrogen (N) shows high mobility within plants. Nitrogen cycling, i.e., retranslocation in
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the phloem from the shoots to root system, and translocation of cycled nutrients back to the
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shoot in the xylem, is important for plant growth and development, especially under stressful
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conditions (Marschner et al. 1997). Nitrogen transport and partitioning within plants varies
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among species and environmental conditions. Enhanced N retranslocation from shoots to
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roots under lower N supply has been reported in some species such as wheat (Lambers et al.
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1982), tobacco (Rufty et al. 1990), pea (Duarte and Larsson 1993), castor bean (Peuke et al.
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1994) and maize (Niu et al. 2007). It is important to know the impact of growth restriction of
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the root system on nutrient flow within plants.
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The maize plant has a complex root system composed of different root types formed at
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different stages of plant development. The embryonic root system consists of a single primary
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root and a variable number of seminal roots. The post-embryonic root system consists of
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shoot-borne adventitious roots (Hochholdinger et al. 2004). If seedlings are kept in the dark
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soon after germination, they develop a strongly elongated mesocotyl. By preventing the
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development of shoot-borne roots in such plants in a preliminary experiment, we established
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that maize plants only with the embryonic root system could be obtained. By also excising the
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seminal roots, in addition to restricting development of shoot-borne roots, a maize plant
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possessing only a primary root can be obtained. Unlike the control plants, which developed a
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large number of shoot-borne roots, all of the xylem and phloem transport in the plants with
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only the embryonic and primary root system was confined to the vascular tissues in the stele
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of the mesocotyl.
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The aim of the present experiments was to investigate the influence of differentially
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restricted root systems of maize, consisting of either embryonic roots (primary root + seminal
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roots) or only a primary root, on plant growth and N uptake and supply to the shoots. In
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1
addition, the morphological and physiological compensatory responses of the remaining roots
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after root pruning, N flow resulting from reduced size of the root system, and the effect of
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confining xylem and phloem transport to the mesocotyl were investigated.
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Results
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Plant growth and development
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At both harvests, restriction of shoot-borne root growth and additional removal of seminal
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roots reduced growth of the whole plant, especially of the upper leaves, although the total leaf
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number among the treatments was the same. The absorption roots (those roots contributing to
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water and nutrient absorption) to shoot dry weight (DW) ratio was also dramatically
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decreased compared to the control plants (Table 1). However, restriction of shoot-borne root
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growth stimulated embryonic root growth. Compensatory growth of the remaining roots,
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represented by the increased DW, length and surface area of the primary and seminal roots,
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was observed (Figure 1C, Table 2). The additional excision of the seminal roots in
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conjunction with growth restriction of shoot-borne roots further stimulated growth of the
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remaining primary root (Table 2), and even stimulated growth of the upper leaves, as
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compared to plants with an embryonic root system (Table 1).
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Changes in net N gain, N use efficiency and N uptake rate
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Changes in net N increment in response to growth restriction of the shoot-borne roots and
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additional removal of seminal roots were similar to those for net dry matter gain. At the first
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harvest, N content was reduced only in the upper leaves of the plants with restricted root
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systems. Net N gain in the upper leaves and whole plants was further reduced by the restricted
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development of the shoot-borne roots after an additional 10 d growth under the same
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conditions. Net N export from the lower leaves of all plants was evident (Table 3). Although
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restriction of shoot-borne root growth reduced the gain in net root system DW at the second
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harvest, especially with additional removal of the seminal roots (Table 1), the net N gain in
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the roots (Table 3) and the N concentration in constituent organs of the root-restricted plants
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(Table 4) did not decrease compared with those in the control plants.
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Nitrogen use efficiency (NUE; defined as biomass production per plant N content) of the
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plants with different root systems over the 10-d study period did not differ significantly.
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However, the root N uptake rate of the plants with restricted shoot-borne root growth was
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markedly higher than that of the control plants (Table 5).
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Nitrogenous compounds in xylem sap
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Total N concentration in the xylem sap of upper or lower leaves of the plants with different
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root systems was not significantly different, although N concentration was lower in the upper
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leaves and higher in the lower leaves of the control plants than those in the respective leaves
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of plants in both root treatments. The major nitrogenous compound in the xylem sap of all
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plants was NO3–, ranging between 78% and 93% of the total nitrogenous compounds. The
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amino-N fraction in the xylem sap of both leaf strata of the root-treated plants was higher than
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those of control plants (Table 6).
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Estimation of net N flow within plants
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In all plants, the largest sink for N accumulation was the upper leaves, which accounted for
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93%, 84%, and 95% of the total N taken up in plants with intact, embryonic and primary root
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systems, respectively (Figure 2). In plants with restricted root systems, the N taken up by
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embryonic roots or primary roots and translocated via xylem in the mesocotyl to the shoot
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was not only distributed to the different leaves, but also to the growth-restricted shoot-borne
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roots. Since the sum of the xylem-transported nitrogenous compounds exceeded the sum of N
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uptake for the whole plant, phloem retranslocation of N from the shoot to roots must have
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taken place. The amount of N retranslocated in the phloem contributed to 26%, 24% and 19%
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of the N transported in the xylem of the plants with intact, embryonic and primary root
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systems, respectively. The phloem-retranslocated N came from different leaves (Figure 2).
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There was even net N export from the lower leaves, since the N exported via the phloem
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exceeded the N imported via the xylem in these leaves.
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Discussion
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Effects on plant growth and N uptake
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Restriction of shoot-borne root growth reduced the growth of new leaves and total N uptake,
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which supports the conclusion that growth and development of the shoot of a whole plant is
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strongly dependent on the concomitant and unrestricted development of the adventitious root
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system (Jesko 1972; Robbins and Pharr 1988; Jeschke et al. 1997). On the other hand, plants
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with a greatly reduced absorption root/shoot DW ratio were able to maintain sustained growth
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(Table 1) and showed no symptom of nutrient deficiency (Figure 1). Additional removal of the
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seminal roots, which are part of the embryonic root system, did not further impact on plant
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growth or N uptake except for inducing stronger compensatory growth of the primary root
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system (Figure 1, Tables 1–3). At the second harvest, the primary-root DW, length and
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surface area of plants with only the primary root system were significantly higher than that of
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plants with embryonic root systems and especially those of the control plants (Table 2). Root
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growth is regulated by shoot growth and intrinsic developmental programs (Malamy 2005;
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Wang et al. 2006). Plant hormones play important roles in the regulation of root growth.
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Auxin, for instance, is the key signal that controls lateral root formation (Bhalerao et al. 2002;
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Marchant et al. 2002; Friml et al. 2006; Osmont et al. 2007), and abscisic acid also stimulates
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lateral root growth (Biddington and Dearman 1982; Hartung and Heilmeier 1992; Chen et al.
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2006).
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Restriction of shoot-borne root growth resulted in a dramatic decrease in the DW ratio of
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absorption roots to shoots (Table 1). It was expected that the nutrient concentration, for
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example N, in leaves of the root-restricted plants would be lower than that in controls. In fact,
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leaf N concentrations in the treated plants were not reduced significantly (Table 4), in spite of
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the reduced total net N increment (Table 3). In order to meet the nutrient demands of the shoot,
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the N uptake rate of the absorption roots was markedly increased. The N uptake rate of plants
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with embryonic and primary root systems was 4.8 and 5.5 times higher, respectively, than that
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of plants with an intact root system (Table 5). As a result, the net N gain of plants with
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embryonic and primary root systems during the 10-d study period was 65% and 74%,
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respectively, while the net dry matter gain was 57% and 62%, respectively, compared to those
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of the control plants. Besides the compensatory growth and increased surface area of the
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remaining root system, the increased N uptake rate could be explained by a shoot
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demand-driven regulatory mechanism of nutrient uptake (Drew and Saker 1984; Cooper and
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Clarkson 1989; Drew et al. 1990; Imsande and Touraine 1994; Marschner et al. 1996).
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Amounts of nutrients taken up are not determined by root size, but by the demands of the
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shoot (Jiang et al. 2001; Wang et al. 2006; Yang et al. 2007). The shoot apex is a strong sink
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and thus imports assimilate from source leaves and mineral nutrients either taken up by the
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root system or remobilized from other tissues.
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In the control plants, transport of water and nutrients is mainly through the xylem of a
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large number of shoot-borne roots (Hoppe et al. 1986), whereas in plants of the present study
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with restricted root systems it was solely through the xylem in the mesocotyl of one (primary
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root system) or a few (embryonic root system) roots. Since the number and diameter of the
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xylem conduits in the basal portion of these roots are determined in very young maize
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seedlings and no new conduits can be added during the life of the root, plants with both of the
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primary and embryonic root systems must accommodate the full supply of water and nutrients
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needed by the shoot through the few major vessels available in the mesocotyl (Jeschke et al.
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1997; Shane and McCully 1999).
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Flows and partitioning of N within plants
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In addition to the compensatory root growth and higher N uptake rate in the plants with
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restricted shoot-borne root growth (Table 5), N cycling within these plants was also higher
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than that in control plants. The amount of the xylem-transported N in plants with embryonic
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and primary root systems was 1.3 and 1.2 times more than that of total uptake during the 10-d
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study period, and was higher than that of control plants (1.1 times more) (Figure 2). As a
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result, leaf N concentration in the treated plants was not significantly reduced (Table 4).
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Furthermore, the total N concentration in the xylem sap of the upper leaves—fast growing
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organs of the plants—was higher than that in control plants, although the differences were not
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statistically significant (Table 6).
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In both plants with intact and restricted root systems, the amount of N transported in the
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xylem was far more than that taken up by the roots in a given period, which suggests that the
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excess must have been compensated by export via the phloem. The phloem-cycled N
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amounted to 26%, 24% and 19% of N transported in the xylem of the plants with intact,
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embryonic and primary root systems, respectively, while phloem-retranslocated N from the
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shoot to roots was derived from the six lower leaves. During the 10-d study period, net N
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exported from the lower leaves took place in both the treated and control plants (Table 3,
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Figure 2), thus it is not a consequence of restriction of shoot-borne root growth. However, the
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total phloem area in the mesocotyl of both restricted root system types is subject to the same
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developmental limitation as the xylem. Nevertheless, the exported N from the lower leaves in
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these plants moved downwards to the roots rather than directly feeding younger leaves higher
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up the shoot, as in the control plants (Figure 2).
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The results in the present study indicate that, in spite of a decreased overall root growth and
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concomitant reduction in shoot growth in plants subject to restricted root growth,
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morphological and physiological (uptake and translocation) compensation by the remaining
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roots can supply sufficient N to meet the demands for aboveground plant growth. As a result,
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N concentrations in the different organs of the root-restricted plants were not significantly
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reduced in comparison with the control plants. The results highlight the high degree of N
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cycling via xylem and recycling via phloem in plants with restricted shoot-borne root growth,
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even when the transport pathway in the mesocotyl is very limited. The restricted nutrient flow
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in the xylem and phloem might have been overcome or compensated by a strongly increased
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flow velocity.
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Materials and methods
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Plant culture and growth conditions
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Hybrid maize seeds (Zea mays L. cv NE1), provided by the maize breeding group of the
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Department of Plant Nutrition, China Agricultural University, were surface-sterilized in 10%
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H2O2 solution for 30 min and washed in running tap water. Thereafter, seeds were germinated
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between filter papers moistened with a saturated CaSO4 solution. When the primary root was
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about 5 mm long, selected uniform seedlings were transferred to 2.1 L PVC pots without lids
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(110 mm diameter, 230 mm height) containing quartz sand (granules 0.25–0.50 mm in
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diameter). The seeds were placed 5 mm deep into the substrate with one seedling per pot. In
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order to produce plants with only an embryonic root system, all pots including those of the
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control plants were kept in the dark until the seedlings developed a strongly etiolated
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mesocotyl. The coleoptile node was about 30–50 mm above the sand surface. The plants were
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watered initially with a half-strength nutrient solution. After one week a full-strength solution
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of the following composition was applied (all concentrations mM): K2SO4 0.75, KCl 0.1,
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KH2PO4 0.25, MgSO4 0.65, Ca(NO3)2 2, H3BO3 1×10-3, ZnSO4 1.0×10-3, CuSO4 1.0×10-3,
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MnSO4 1.0×10-3, Fe-EDTA 0.15, and (NH4)6Mo7O24 5.0×10-6. The initial pH of the nutrient
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solution was adjusted to 6.0. The plants were watered every second day in the morning with
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an excess of the nutrient solution. Small holes were provided in the bottom of the pots to
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allow for drainage. The plants were grown in greenhouse under the following conditions: a 14
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h/10 h light/dark photoperiod, 32/25°C day/night temperature, and 45–55% relative humidity.
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The photosynthetically active radiation at the top of the pot, 220–270 µmol m-2 s-1, was
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provided by reflector sunlight metal halide lamps (Philips HPI Plus, 250 W, Belgium). The
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placement of pots was randomized and adequate space between pots was provided. The
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position of pots in the greenhouse was changed every second day, in order to minimize edge
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effects.
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Treatments and harvest procedures
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The plants were divided into three groups of 14 plants, each of similar size and development.
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On the 4th day after transfer into pots, the three groups of plants were treated as follows: 1)
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additional sand was applied to the sand surface in the pot to cover the elongated mesocotyl, so
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that the shoot-borne roots could grow into the quartz sand substrate (control, Figure 1A); 2)
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no additional sand was applied, so the initiated shoot-borne roots were unable to reach the
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quartz sand substrate and the plants were dependent on their embryonic roots only (embryonic
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root plants, Figure 1B); 3) in addition to restricting growth of the shoot-borne roots as in 2,
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the seminal roots were carefully removed so that the plants only possessed the primary root
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(primary root plants). During cultivation, shoots of the plants of treatments 2 and 3 were
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supported to avoid breakage of the elongated mesocotyl. The first harvest was performed 38 d
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after transfer to the pots when plants were at the ‘ninth leaf expanding’ stage. The second
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harvest was performed 10 d later. There were seven replicates per treatment in both harvests.
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Leaves were numbered in ascending order, starting with the first leaf to develop after
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germination, which was designated as leaf no. 1. At the first harvest, the youngest visible
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unfolded leaf was designated no. 9. At harvest, plants were separated into lower leaves (nos
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1–6), upper leaves (nos 7–9 at the first harvest, and nos 7–11 at the second harvest), primary
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root, seminal roots, and shoot-borne roots. The sheath was included with each leaf. Roots
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were washed free of sand with tap water. All plant parts were killed at 105°C for 30 min, dried
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at 70°C to a constant weight, weighed (dry weight) and ground into a powder. Appropriate
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amounts of the ground plant material were used to determine the total N content using a
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modified Kjeldahl digestion method that included reduction of nitrate (Nelson and Somers
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1973). Calcium in the tissue was analyzed using a flame spectrophotometer (Cole-Parmer
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2655-00, USA).
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Root length and root surface area analyses
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The roots from the second harvest were digitally scanned (Epson 1680, Indonesia) prior to
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drying for further analyses. For scanning the root samples were placed in a rectangular glass
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dish (200 mm × 150 mm) containing a layer of water about 4–5 mm deep to untangle the
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roots and minimize root overlap. When necessary, a root was separated into subsamples until
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they fitted into the glass dish. The images were analyzed with the software WinRHIZO
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version 5.0 (Regent Instruments, Canada), by which means the total root length and the
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surface area of each root part was calculated.
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Collection of xylem sap
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For collection of xylem sap, the plants were grown as described above. Before xylem sap
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collection, the pots were drained using a vacuum in order to remove any surplus nutrient
3
solution. The pots with the plants were then placed in a pressure chamber to apply pressure to
4
the root system. The lid of the pressure vessel was sealed with a two-component silicon
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rubber dental impression material Blend-a-gum (Blend-a-gum Forschung, Schwalbach,
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Germany) (Seel and Jeschke 1999). A major advantage of this technique is that the
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composition of the sap collected reflects that in an intact plant as many of the phloem tubes
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are uncut, and hence the normal cycling of solutes from shoot to root can still proceed (Seel
9
and Jeschke 1999). Xylem sap collection was repeated three times during the study period, i.e.
10
on the second, fifth and eighth day after the first harvest. The xylem sap collection procedure
11
essentially followed the method of Jeschke and Pate (1991). In brief, an incision was made in
12
the midrib on the adaxial surface along the full length of the leaf. Samples were taken from
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leaf nos 5 and 8. The cut surface was carefully rinsed with distilled water and a Teflon tube
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attached. Pressure was applied to both the sand substrate and roots of the treated plants. After
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slowly applying pressure, xylem sap started to exude from the midrib after reaching a
16
balancing pressure (Seel and Jeschke 1999), and the sap was collected at 100 kPa above this
17
pressure. The first exudate was discarded to avoid contamination from cut cells. Xylem sap
18
was kept on ice during collection and stored at –20°C until analysis. Calcium in the xylem sap
19
was analyzed directly after appropriate dilution using ICP (Perkin Elmer 3300 DV, USA).
20
Nitrate and ammonium in the xylem sap were analyzed with a TRAACS-2000 auto-analyser
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(Bran+Luebbe, Germany). Amino acids in the xylem sap were determined using an amino
22
acid auto-analyser (Hitachi, 8800, Japan). Total N in the xylem sap was assumed to be the
23
sum of NO3–-N, NH4+-N and amino acid-N.
24
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Estimation of the net flows of nitrogen through xylem and phloem in the whole plant
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Net flows of N in plants were estimated using the method described by Armstrong and Kirkby
27
(1979). Their model assumed that nutrients were transported solely through the xylem and
28
phloem and that Ca2+ could only be transported acropetally through the xylem without
29
mobility in the phloem.
30
The net xylem flow of Ca2+, JCa,x, transported to various plant organs during a given period
31
was the same as the net increment of Ca2+, ∆Ca, in the same organ during the same phase,
32
where:
33
34
JCa,x = ∆Cax
Under relatively stable growth conditions, the N/Ca ratio in xylem sap, [N/Ca]x, delivered to
10
1
individual organs was constant, and these values were derived from the concentration of N
2
and Ca in the collected xylem sap.
3
4
For a given period, the net xylem flow of N, JN,x, towards a organ was calculated from the
net increment of Ca2+, ∆Ca, and the N/Ca ratio, [N/Ca]x, in the xylem sap by:
5
JN,x = ∆Ca × [N/Ca]x
6
The amount of N exported through the phloem, JN,p, was equal to the difference between the
7
measured N increment, ∆N, in each organ and the net xylem import according to:
8
JN,p = ∆N − JN,x
9
A positive difference indicates net phloem import, while a negative difference implies net
10
phloem export from an organ. Working progressively along the plant from roots to leaves, the
11
net flows of N within a whole plant were obtained as shown in Fig. 2. The differences
12
between the quantities of N translocated in the phloem and in the xylem then also allowed
13
estimation of transfer between these translocation streams.
14
15
Statistical treatment
16
Analysis of variance was performed using the ProcGLM procedure of SAS version 8.02 (SAS
17
1987). Means of different treatments were compared using the least significant difference
18
(LSD) test at 5%.
19
20
Acknowledgments
21
We thank Prof. Dr F. Bangerth for valuable comments and careful correction of the
22
manuscript; Dr F. J. Chen and Dr G. H. Mi of the maize breeding group of the Department of
23
Plant Nutrition, China Agricultural University, for providing maize seeds; the State Key Basic
24
Research and Development Plan of China (No. 2007CB109302) and the State Key
25
Technologies R&D Program (No. 2006BAD25B02) for financial support.
26
27
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1
2
Figure 1. Comparison of maize seedlings with different root systems at the second harvest.
3
A: Shoot base of plants with an intact root system; B: shoot base of plants with an
4
embryonic or primary root system; C left: intact root system; middle: embryonic root
5
system; right: primary root system.
6
7
Figure 2. Flow profiles for uptake, transport and utilization of nitrogen (N) in maize plants
8
with different root systems over a 10-d study period. The values of N deposition (net N
9
increment) and the statistical significance are given in Table 3. The width of arrows and the
10
height of histograms are drawn in proportion to the net flows and deposition of N. The
11
numbers indicate the values of uptake, transport and utilization (mmol N per plant over the
12
10-d study period).
13
14
15
15
1
2
A
3
4
B
C
16
1
2
3
4
Intact rootControl
plant
3.58
Embryonic root plant
2.10
Leaves
1.99
3.4
0.19
0.11
Upper
0.52
0.13
-0.35
0.85
4.10
Shoot-born roots
and
embryonic roots
3.65
Lower
-0.49
Shoot-born
roots
0.62
0.79
3.02
0.73
Embryonic
roots
0.07
2.36
1.05
0.60
Primary root plant
2.65
2.56
0.08
0.05
-0.48
Shoot-born
roots
0.52
3.22
5
0.54
Flow in xylem
Flow in phloem
0.62
Deposition in tissues
Primary roots 0.09
2.69
Net loss from tissues
17
1
Table 1. Absorption root:shoot dry weight ratio, and initial values and net increments
2
between the first and second harvest in dry weight of the constituent organs and of whole
3
maize plants with an intact, embryonic or primary root system over a 10-d study period.
Type of root
Upper
Lower
system
leaves
leaves
Whole Absorption
roots
roots*
Whole
Absorption root/
plant
shoot ratio**
Value at the first harvest (g plant-1)
Intact
1.12 a
0.53 a
0.48 a
0.48 a
2.13 a
0.29 a
Embryonic
0.88 b
0.67 a
0.37 a
0.11 b
1.92 ab
0.06 b
Primary
0.79 b
0.59 a
0.33 a
0.12 b
1.71 b
0.08 b
Net increments at the second harvest (g plant-1)
Intact
2.35 a
0.04 a
0.67 a
0.67 a
3.06 a
0.29 a
Embryonic
1.34 c –0.06 b 0.47 ab
0.06 b
1.75 b
0.05 b
Primary
1.76 b –0.10 b 0.23 b
0.05 b
1.89 b
0.05 b
4
Each value represents the mean of seven replicates. Values in columns followed by the same
5
letter for the first and second harvest are not significantly different (LSD test, p ≤ 0.05)
6
*Only the roots contributed to water and nutrient absorption
7
** Shoot = sum of shoot and shoot-borne roots that did not reach the substrate
8
18
1
2
Table 2. Dry weight, length and surface area of different parts of the root system in maize
3
plants with an intact, embryonic or primary root system at the second harvest
Root part
Type of root
system
Primary root
Seminal roots
Nodal roots
Absorption roots
Total roots
Dry weight (mg)
Intact
36.2 c
58.8 b
1062.2 a
1157.1 a
1157.1 a
Embryonic
73.8 b
100.0 a
666.1 b
173.9 b
840.0 b
Primary
163.7 a
–
691.1 b
163.7 b
854.8 b
Root length (m)
Intact
2.1 c
2.0 b
21.0 a
25.1 a
25.1 a
Embryonic
5.0 b
5.1 a
0.6 b
10.1 b
10.7 b
Primary
8.7 a
–
0.7 b
8.7 b
9.4 b
Root surface area (mm2)
Intact
4182.0 c
5428.0 b
46175.1 a
55785.1 a
55785.1 a
Embryonic
9586.7 b
7507.1 a
2525.3 b
17093.8 b
19619.1 b
Primary
12194.5 a
–
2719.6 b
12194.5 c
14914.1 c
4
Each value represents the mean of seven replicates. Values in columns followed by the same
5
letter for root dry weight, length and surface area are not significantly different (LSD test, p ≤
6
0.05)
7
19
1
2
Table 3. Initial values and net increments between the first and second harvest in N content of
3
the constituent organs and of whole maize plants with an intact, embryonic or primary root
4
system over a 10-d study period
Type of root
Upper
Lower
Whole
Absorption
Whole
system
leaves
leaves
roots
roots
plant
Value at the first harvest (mmol)
Intact
2.42 a
0.95 a
0.80 a
0.80 a
4.17 a
Embryonic
2.00 b
1.20 a
0.86 a
0.23 b
4.06 a
Primary
1.69 b
0.93 a
0.75 a
0.20 b
3.37 b
Net increments at the second harvest (mmol)
Intact
3.40 a
-0.35 a
0.60 a
0.60 a
3.65 a
Embryonic
1.99 b
-0.49 a
0.86 a
0.07 b
2.36 b
Primary
2.56 b
-0.48 a
0.61 a
0.09 b
2.69 b
5
Each value represents the mean of seven replicates. Values in columns followed by the same
6
letter for the first and second harvest are not significantly different (LSD test, p ≤ 0.05)
7
8
20
1
2
Table 4. Nitrogen concentration (g kg-1 DW) in the constituent organs of maize plants with an
3
intact, embryonic or primary root system at the second harvest
Type of root
Upper
Lower
Primary
roots
Seminal
roots
Shoot-borne
roots
system
leaves
leaves
Intact
23.48 a
14.73 ab
17.08 b
18.98 a
16.93 c
Embryonic
25.16 a
16.30 a
26.00 a
23.80 a
29.67 b
Primary
23.33 a
12.86 b
23.88 a
-
38.41 a
4
Each value represents the mean of seven replicates. Values in columns followed by the same
5
letter are not significantly different (LSD test, p ≤ 0.05)
6
7
8
9
10
11
21
1
2
3
Table 5. Nitrogen (N) use efficiency (NUE; defined as biomass production per plant N
4
content) and root N uptake rate maize plants with an intact, embryonic or primary root system
5
over a 10-d study period
6
Type of root system
NUE
Root N uptake rate
(g DW g-1 N)
(g g-1 root DW [10 d]-1]
Intact
59.88 a
0.04 b
Embryonic
53.05 a
0.19 a
Primary
50.17 a
0.22 a
7
8
9
10
11
12
Each value represents the mean of seven replicates. Values in columns followed by the same
13
letter are not significantly different (LSD test, p ≤ 0.05)
14
15
16
22
1
2
Table 6. Concentrations of different nitrogen (N) forms and total N (mM) in the xylem sap of
3
different leaves of maize plants with an intact, embryonic or primary root system. The xylem
4
saps were collected between the two harvests.
Form of N
NO3–-N
NH4+-N
Amino-N
Total N
Upper leaves
Lower leaves
Intact
Embryonic
Primary
Intact
Embryonic
Primary
2.67 a
2.59 a
2.81 a
4.00 a
3.27 a
3.00 a
(91.1)
(77.8)
(85.9)
(92.0)
(92.9)
(88.5)
0.11 a
0.10 a
0.07 a
0.16 a
0.08 a
0.08 a
(3.8)
(3.0)
(2.1)
(3.7)
(2.3)
(2.4)
0.15 b
0.64 a
0.39 a
0.19 a
0.17 a
0.31 a
(5.1)
(19.2)
(11.9)
(4.4)
(4.8)
(9.1)
2.93 a
3.33 a
3.27 a
4.35 a
3.52 a
3.39 a
5
Each value represents the mean of seven replicates. The values in parentheses indicate the
6
proportion of different N forms to total N measured in the xylem sap
7
Values in rows for the same leaf stratum followed by the same letter are not significantly
8
different (LSD test, p ≤ 0.05)
9
10
23