INTERNATIONAL SYMPOSIUM
TOWARDS SUSTAINABLE LIVELIHOODS AND
ECOSYSTEMS IN MOUNTAINOUS REGIONS
7-9 March 2006, Chiang Mai, Thailand
Role of boron and zinc in flower induction of Lychee
(Litchi chinensis Sonn.)
S. Roygrong1, W. Spreer2 and V. Römheld3
1
2
The Uplands Program (SFB-564), Chiang Mai, Thailand
Institute of Agricultural Engineering, University of Hohenheim, Stuttgart, Germany
3
Institute of Plant Nutrition, University of Hohenheim, Stuttgart, Germany
Abstract
Lychee (Litchi chinensis Sonn.) is one of the most important cash crops in Northern Thailand. As
the trees require low temperature, lychee production is restricted to upland areas, where it is an
important source of income to small farm holders. As a perennial crop, it helps to protect the
steep sloping fields from erosion. Weather conditions relevant for flower induction vary over the
years. Thus, flower induction is not homogeneous from year to year and alternate bearing
imposes an important limitation for a steady production. Analysis on the nutritional status of
agricultural soils of this region revealed that boron (B) and zinc (Zn) deficiency is widespread. It
has been described that deficiency in both micronutrients B and Zn results in growth depression,
reduced flower induction and fruit set as well as decreased fruit quality. Chlorosis in sunlight
exposed parts of field grown lychee trees were observed together with an absence of flowering
and fruit set. Therefore, it was assumed that nutrient supply is involved in the process of flower
induction. Thus, objective of the presented work was to study the role of the micronutrients B
and Zn in growth as a prerequisite for flower induction of lychee in nutrient solution culture.
Lychee seedlings were precultured in a modified Hoagland nutrient solution, containing beside
other nutrients ZnSO4 and H3BO3. 30 days after planting (DAP), plants were separated into four
groups. The plants of two groups were transferred into nutrient solution without H3BO3 and the
other group without ZnSO4. As control, two groups was left with sufficient Zn and B supply.
Growth was monitored every 2 weeks. Leaf B and Zn concentration were analyzed at 60 DAP.
Then both treatments, -B and -Zn, showed signs of micronutrient deficiencies. As compared to
the control, growth was hampered and the young leaves got necrotic. The concentrations of B
and Zn were significantly decreased in -B and -Zn treatments. Unexpectedly, it was observed,
after several days with low night temperature, that the plants of the -B and -Zn treatments started
to flower, while the control plants did not.
As a conclusion B and Zn nutritional status plays a modulating role in flower induction of
lychee. An interaction with weather factors can be assumed. Thus, similar as drought stress,
nutrient deficiency might improve flower induction and thus fruit set during periods of days with
cold night temperature. The exact knowledge about this interaction is important for fertilization
recommendations. Its potential for the field use in homogenizing flowering and off-season fruit
production, respectively, has to be further investigated.
Keywords: Boron, zinc, lychee, flower induction
1
Introduction
Lychee (Litchi chinensis Sonn.) is one of the most important cash crops in Northern Thailand. As
the plants require low temperatures and a dry periods to induce flowering (Only low temperature
is needed for flowering induction; drought can improve flowering at inadequate low
temperature), lychee production is restricted to upland areas, where it is an important source of
income to small farm holders. The total planting area of bearing lychee was 11,500 ha in 1997
and increased to 11,848 ha in 2001 (Anupunt and Sukhvibul, 2005). Weather conditions relevant
for flower induction vary over the years. Thus, flower induction is not homogeneous from year
to year and alternate bearing imposes an important limitation for a steady production. Analysis
on the nutritional status of agricultural soils of this region showed that boron (B) and zinc (Zn)
deficiency is widespread (Roygrong, 2003). The relationship of nutritional status on flower
induction of lychee is not well investigated. Zn deficiency caused bronzing of leaflets and
smaller fruits with reduced flesh recovery and sugar content. It has been described that both B
and Zn deficiency result in growth inhibition, reduced flower induction and fruit set as well as
decreased fruit quality (Menzel and Simpson, 1987). Chlorosis in sunlight exposed parts of field
grown lychee plants were observed together with an absence of flowering and fruit set.
Therefore, it was assumed that nutrient supply is involved in the process of flower induction. The
objective of the work was to study the role of the micronutrients B and Zn in growth as a
prerequisite for flower induction of lychee in nutrient solution culture.
2
Material and methods
Twenty four lychee seedlings were precultured in a modified Hoagland nutrient solution
containing (in mol m-3): 0.5 M Ca(NO3)2, 0.5 M MgSO4, 0.5 M K2SO4, 0.1 M K(H2PO4)2,
0.15 M FeEDTA and as μmol m-3 0.5 μM MnSO4, 0.25 μM ZnSO4, 0.2 μM CuSO4,
0.02 μM (NH4)Mo7O24, and 10 μM H3BO3. The growth experiment was conducted in a
greenhouse during October - December 2004 with a temperature between 14 and 28 °C and a
relative humidity between 30 and 60%. At 30 DAP, plants were separated into four groups of six
plants each. The plants of two groups were transferred into nutrient solutions without H3BO3 (B) and without ZnSO4 (-Zn), respectively. As control, two groups were left with sufficient Zn
(+Zn) and B (+B) supply. Nutrient solutions were changed every two weeks. Growth and the
occurrence of new shoots and flowers were monitered every two weeks by counting new flushes
and leaves. At 60 DAP, 4-6 leaves per plant were collected and dried for 2 days at 65°C. The
concentration of B was measured by ashing samples at 500 °C, dissolving the ash in 1:3 (v/v)
HNO3 and measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) while Zn
concentration was determined by ashing the samples at 500 °C and dissolving in 1:3 HCl (v/v)
and analyzed by atomic absorption spectrophotometry (AAS). Data were analysed by one-way
ANOVA.
3
Results and Discussion
At 60 DAP of both treatments, -B and -Zn, showed visual signs of micronutrient deficiencies. As
compared to the control, leaves and shoot growth was slightly inhibited and young leaves got
necrotic (Figure 1). The concentrations of B and Zn were significantly decreased in -B and -Zn
treatments with 14.8 and 20.0 mg kg-1 Dry Weight (DW), respectively, as compared to the
controls (Table 1).
2
Figure 1: Necrotic leaves (A) and dieback of buds (B) in lychee plants grown in nutrient solution
either without Zn or B supply.
Table 1: Leaf Zn and B concentration of lychee plants at 60 DAP. Different letter in a column
represent significant differences at P<0.05
Treatments
- Zn
+ Zn
n
6
6
-B
+B
6
6
Zn concentration (mg kg-1 DW)
12.5 ± 0.2 a
20.0 ± 0.1 b
B concentration (mg kg-1 DW)
14.8 ± 0.1 a
43.8 ± 0.1 b
It was clearly shown that micronutrient uptake by the roots was not hampered (adequate in the
controls) and the deficiency symptoms were related to the concentration of micronutrients in the
leaves.
Unexpectedly, it was observed, after approximately 30 days with night temperatures below 16°C,
that the plants of the -B and -Zn treatments started to flower. No flowering was observed at the
control plants with adequate B and Zn supply, which still grew vegetatively (Figure 2).
Figure 2: Lychee plants grown in nutrient solution with and without B and Zn supply at 60 DAP.
Table 2: Percentage of flowering of lychee plants in nutrient solution culture as affected by
different Zn and B supply at 60 DAP
Treatments
- Zn
+ Zn
-B
+B
n
6
6
6
6
plants flowering
4
0
3
0
Percentage
66.6
0.0
50.0
0.0
3
Obviously, the flower induction had taken place during the period of cool weather (temperature
below 16°C). However, as flower induction was not the primary subject of this study, the groups
were not homogeneous with respect to new shoots. Thus, 50 to 60% of the micronutrient
deficient plants were flowering (-B and -Zn), while the other plants from this group did not
produce a new flush. In the sufficient supplied controls, all flushes were vegetative (Table 2).
4
Conclusion
Even though it was not the main subject of this study, our observations indicate that
micronutrient supply is involved in flower induction. These findings require confirmation by
further experiments under a controlled temperature regime as temperature might be the
overriding factor and a cool temperature is still a prerequisite for flower induction (Stern and
Gazit 2003). The question whether stress resulting from micronutrient deficiency can induce
flowering, even at a persistently high temperature (>24 °C) - which is normally not flower
inducing (Chattrakul, 2005) - will clarify the interactions of different forms of stress with respect
to flower induction, such as temperature (Menzel and Simpson, 1995), drought stress (Stern et al,
1993), physical stress; girdling (Zhang, 1997) and chemical stress such as Ethephon and
Paclobutrazol (Ramburn, 2001).
The exact knowledge about interactions is important for fertilization recommendations in
combination with above mentioned methods of flower induction. Short term regulation of
nutritional status is difficult to put into practice as foliar uptake of Zn and B after spraying is
minor (The Uplands Program, 2005) and the negative effects of micronutrients deficiencies as
documented in this study (Figure 1) might out compete for the benefits a stress improved flower
induction (Table 2). Thus, the potential of controlled micronutrient supply for the field use in
homogenizing flowering and off-season fruit production, respectively, has to be further
investigated.
Acknowledgements
The financial support of this study by Deutsche Forschungsgemeinschaft (DFG) is gratefully
acknowledged.
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