Responses of common bean (Phaseolus vulgaris L.) to
photosynthetic irradiance levels during three
phenological phases
Walelign Worku, Arne Skjelvåg, Hans Gislerød
To cite this version:
Walelign Worku, Arne Skjelvåg, Hans Gislerød. Responses of common bean (Phaseolus vulgaris
L.) to photosynthetic irradiance levels during three phenological phases. Agronomie, EDP
Sciences, 2004, 24 (5), pp.267-274. .
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267
Agronomie 24 (2004) 267–274
© INRA, EDP Sciences, 2004
DOI: 10.1051/agro:2004024
Original article
Responses of common bean (Phaseolus vulgaris L.) to photosynthetic
irradiance levels during three phenological phases
Walelign WORKUa*, Arne O. SKJELVÅGb, Hans R. GISLERØDb
a
b
Debub University, Awassa College of Agriculture, PO Box 5, Awassa, Ethiopia
Agricultural University of Norway, Department of Plant and Environmental Sciences, PO Box 5003, N-1432 Ås, Norway
(Received 17 January 2003; accepted 7 April 2004)
Abstract – As a dominant component crop in intercropping systems, common bean is exposed to radiation deficit during various phases. An
indeterminate cultivar was examined from twenty-seven treatments consisting of all possible combinations of three levels of photosynthetic
irradiance, 100, 250 and 400 µmol m–2 s–1, applied during three phenological phases. Acclimation characteristics to reduced irradiance included
lower chlorophyll a/b ratio, reduced stomatal density, increased specific leaf area and leaf area ratio and increased shoot-root ratio.
Susceptibility of the phases varied when comparisons were made based on entire phases and a magnitude that considered timing and light
interception. Number of pods per plant, the predominant yield component, responded to irradiance level during all phases but most during
flowering. For number of seeds per pod the only relevant phase was seed filling while seed weight responded during flowering and seed filling.
A significant interaction between the irradiance levels of phases was observed for pod number.
Phaseolus vulgaris / irradiance / phenological phases / acclimation characteristics / seed yield / yield components
1. INTRODUCTION
Intercropping is practiced by the majority of farmers in the
tropics and subtropics as a yield maximization and risk minimization strategy. Nearly 80% of all beans produced are grown
with maize at some time during the annual crop cycle [8]. It is
mainly intercropped with late-maturing cultivars of maize and
sorghum. The amount of light under the taller cereal canopy
changes during the various phenological phases though the variation during the latter phases is smaller. For instance, in late
cultivars of maize, the maize crop starts to have a closed canopy
starting from six weeks from emergence and continues up to
the beginning of physiological maturity. The poorer performance of legumes when intercropped may be partly because the
quantity of light reaching their canopy is reduced by the taller
companion crop [28]. Dry matter production by a canopy of a
given size varies in proportion to the amount of intercepted
energy, at a rate governed by its radiation use efficiency [27].
The review by Laing et al. (1984) indicates that competition for
light among components is the most important factor determining productivity of the intercropping system when other factors
are non-limiting.
As an understorey component, the amount of light that
reaches the legume crop is largely reduced. In a maize-bean
intercropping, Gardiner and Craker (1981) indicated that at low
* Corresponding author: [email protected]
(18 000 plants/ha) and high (55 000 plants/ha) maize densities,
the amount of solar radiation available for the bean was 50%
and 20% of the incident light for the low and high densities,
respectively. The highest density was accompanied by 70%
yield reduction as compared with the sole crop yield. Chickoye
et al. (1996) in their simulation study of competition for photosynthetically active radiation showed that shading of the bean
canopy by common ragweed, Ambrosia artemisiifolia, accounted
for 50–70% of the yield loss when both emerged together.
Grain yield responses of crops to artificially induced shade
has been studied in maize [4, 13, 24], in wheat [11, 16], in peanut [12] and in Vicia faba [29]. The physiological stage of plant
development when shading is imposed [4], as well as shading
duration [11] and the intensity of shading have varying impacts
on seed yield and other agronomic parameters. Early et al.
(1967) reported shading during flowering was detrimental in
maize while Reed et al. (1988) observed that both flowering and
seed filling were equally susceptible. On the other hand, Hang
et al. (1984) found the podding and maturing phases, which
comprised largely the seed-filling phase, to be more sensitive
to 75% shade than the flowering and pegging phases in peanut.
They suggested that as an indeterminate plant it has more flexibility to recover from shading during the flowering stage. The
discrepancies may be partly attributed to species and cultivar
differences, a variable shading period following different growth
268
W. Worku et al.
durations, interactive effects between phases and lack of uniformity in designating phenological phases. The latter is relevant especially in crops with indeterminate growth habits
where there is considerable overlapping of reproductive events.
In addition to comparing entire phases to determine relative
sensitivity, there is a need for the comparison to be made based
on a quantity that will take both the duration and light interception of the various phases into consideration, because there are
differences between the various phases in growth rate, duration,
leaf area index and light interception.
There is a need to understand responses of common bean to
irradiance levels during various phases in order to devise and
manipulate more efficient crop combinations. The purpose of
this study was: (1) to investigate acclimation characteristics and
growth of an indeterminate cultivar; (2) to compare the sensitivity of phases to low irradiance with respect to seed yield and
its components, and (3) to observe the contribution of interactions between developmental phases in modifying the responses
to irradiance.
2. MATERIALS AND METHODS
2.1. Setup
The experiment was conducted in three growth rooms (each
3.25 × 2.30 m) with photosynthetic photon flux densities
(PPFD) of 100, 250 and 400 µmol m–2 s–1 ± 10%, at the Agricultural University of Norway. The three irradiance levels,
which are for convenience designated as ‘low’ (L), ‘medium’
(M) and ‘high’ (H), were provided by 400 W high-pressure
sodium lamps and had a R/FR ratio of 1.93 ± 0.01. The ‘high’
light intensity is low compared with the natural growing environment with an average growing season irradiance level of
650–750 µmol m–2 s–1. In this context, ‘low’ and ‘medium’
intensities were compared with the ‘high’ level, approximating
‘severe’ and ‘moderate’ shade, respectively. The temperature
of all the rooms was kept at 21/18 °C ± 1 day/night, photoperiod
at 12 h and relative humidity was 60–70% during the light
period. Seeds of the cultivar Red Wolaita, which is an indeterminate Ethiopian cultivar, were sown in plastic trays at a spacing of 6 × 7 cm with one seed per hill. Germination occurred
after a week in a greenhouse with 20/18 °C day/night temperature. Then uniform seedlings were transplanted into plastic
pots of 15 cm diameter and 2.6-liter capacity, two in each pot.
The growth medium was a greenhouse organic soil consisting
of peat, clay and sand and one g kg–1 of a fully water-soluble
complete fertilizer, Superba (Hydro Agri, Rotterdam, Holland), containing 14:4:21:2:2.4% of NPKMgS and micronutrients (Fe, Mn, Cu, Zn, B, Mo). Watering was done as often
as required to keep the pots near field capacity. The complete
fertilizer, Superba, was added at the rate of 300 mg pot–1 four
times: the first a week after emergence and then at ten-day intervals thereafter. Manual watering was done, generally, every 2–
3 days until flowering, while during flowering and seed filling
it was daily. The frequency was reduced towards physiological
maturity.
The 243 pots were randomly assigned to the three growth
rooms in April 1999, four days after emergence. The experiment was arranged as a 3 × 3 × 3 factorial comprising all possible 27 treatment combinations. The factors were irradiance
levels during the vegetative (IV), the flowering (IF) and the
seed-filling (ISF) phases, each phase receiving irradiance levels of 100, 250 or 400 µmol m–2 s–1. At the end of the vegetative
and flowering phases, the pots were moved and redistributed
among the different growth rooms such that at the end of the
third phase all 27 combinations of three phases and irradiance
levels (L, M, H) were represented: from LLL through LLM,
LLH, LML, and so forth to HHH. This means that the group
of plants represented at any light level during a specific phase
involved an orthogonal combination of light treatments and
phases during the other two phases. Thus, at maturity, determination and comparison of an effect during a particular phase
could be made specifically. Nine pots were assigned for each
treatment. The pots were spaced 23 cm apart though they were
less crowded in the later growth phases due to harvesting of
some pots.
The three phenological phases were defined as following.
Vegetative: from emergence to first flowering. Flowering:
from commencement of flowering to first pod at full length.
Seed filling: from full-length pod to physiological maturity of
the first pod. Accordingly, the durations of the vegetative, flowering and seed-filling phases were 34, 15 and 32 days, respectively. Harvesting for seed yield was carried out when the first
pod was dry to touch. As Red Wolaita is an indeterminate cultivar, it is important to note that flowering and pod set occurred
concurrently.
2.2. Chlorophyll determination
Samples for chlorophyll (chl) determination were taken
from the treatments LLL, MMM and HHH three times: 22, 40
and 66 days after germination, representing the vegetative,
flowering and seed-filling phases. Sample leaf disks were taken
on the second fully-expanded unshaded leaf from the top using
a cork borer 9 mm in diameter. Five replicate samples were
taken from each irradiance level. For each replication, three leaf
disks of 64 mm2, one from the center of each leaflet, were taken.
The absorbance readings of extracted chlorophyll were taken
at 647, 664 and 750 nm wavelengths with a Shimadzu UV-2101
PC scanning spectrophotometer. The chl content was determined using formulae developed by Porra et al. (1989).
2.3. Stomatal density determination
Stomatal density (number of stomata per unit area) was
determined from samples taken on both sides of the second
fully-expanded leaf from the top, 30 days after emergence. A
thin coat of office glue was applied to the lamina of sample
leaves and left to dry for about five minutes. A double-sided
adhesive tape was fixed on slides. The slides with the fixed
adhesive tape were gently pressed on the coat of glue and the
impression was carefully removed without damaging the leaf.
Seven samples were taken from each irradiance level. Within
each sample, four to five counts were carried out at different
spots by a light microscope.
2.4. Growth parameters
For determination of growth parameters, samples from the
treatments LLL, MMM and HHH were harvested 40 and 55 days
Common bean responses to irradiance
269
after emergence. Specific leaf area (SLA) is the ratio of leaf area
per unit of leaf dry weight. The leaf area ratio (LAR) is the ratio
of the assimilatory area per unit of plant dry weight. The leaf
weight ratio (LWR) is the ratio of the assimilatory material per
unit of leaf dry weight. The net assimilation rate (NAR) is the
increase in plant material per unit assimilatory material per unit
of time. Leaf area was measured by a 3100 area meter (Li-Cor,
Lincoln, USA), taking all non-senesced leaves from all the
sample plants. The relative growth rate (RGR) is the rate of
increase in biomass per unit of biomass present.
RGR = LAR*NAR
(1)
LAR = SLA*LWR
(2)
NAR = (W2–W1)(lnA2–lnA1)/(A2–A1)(T2–T1),
(3)
where W is biomass as dry weight and A is the assimilatory surface area as leaf area and T time.
The
equations used for growth analysis were taken from
v
Kv e t et al. (1971).
Figure 1. Effects of photosynthetic irradiance level on (A) chl a, (B)
chl b, (C) chla a + b and (D) the chl a/b ratio during the vegetative
(V), flowering (F) and seed-filling (SF) phases. n = 5; mean ± standard error of the mean.
2.5. Dry matter production
Samples for dry matter production were taken two times:
with three pots per treatment at the end of the vegetative phase
and with four to six pots after ripening. At the first harvest plant
material was separated into stem and leaf. During the final harvest it was partitioned into stem, leaf, pod and seed. Senescent
and dropped leaves were included in shoot dry matter. Root
samples were taken for three treatments, LLL, MMM and
HHH, at the final harvest. Roots were soaked in water to loosen
soil and washed manually afterwards under running tap water.
Dry weight of all the components including seed yield was
determined after oven drying at 70 °C for 48 hours.
Intercepted photosynthetically active radiation (IPAR) was
estimated from incident PAR (PARIo) based on the leaf area
index using the equation:
IPAR = PARIo (1–exp(–K*L))
(4)
where K is the extinction coefficient and L is the leaf area index.
An average extinction coefficient (K) value of 0.8 was used
[27].
The relationship between yield components and radiation
deficit (MJ m–2 PAR) during the three phases was analyzed by
a stepwise multiple regression. The equations obtained were:
Pod No. plant–1 = 12.56 – 1.37RDV – 3.87RDF – 1.02RDSF
(5)
– 0.027(RDV*RDF)
2
R = 0.09 + 0.39 + 0.17 + 0.03
Seed No. pod
= 6.40 – 0.01RDSF
(6)
2
R = 0.27
Seed wt (mg) = 23.18 + 0.92RDF – 0.39RDSF
R2
2.7. Data analysis
The data were orthogonal with respect to light intensity
effects and phenological phases, and they were analyzed by the
following model:
Y = µ + b1IV + b2IF + b3IV*IF + b4ISF + b5IV*ISF + b6IF*ISF
+ b7IV*IF*ISF + Є
2.6. Light interception
–1
where α level for entry in the model ≤ 0.05; n = 81; RD, radiation deficit; the subscripts V, F and SF designate the vegetative, flowering and seed-filling phases, respectively.
= 0.30 + 0.21
(7)
where IV, IF and ISF designate irradiance levels during the vegetative, flowering and seed-filling phases, respectively. Main
effects and two-factor interactions (as specified by the model)
for all the parameters from the final harvest were tested against
the three-factor interaction, IV*IF*ISF, used as an error term.
The data were analyzed using the General Linear Model of the
Statistical Analysis System (SAS, release 6.12).
3. RESULTS AND DISCUSSION
3.1. Effects on acclimation characteristics
3.1.1. Chlorophyll attributes
The low light intensity resulted in consistently significant
(P < 0.05) reductions in chlorophyll a (chl a) and chl a+b content across all the developmental phases (Fig. 1A, C). The high
light intensity consistently produced higher contents of both chl
a and chl a+b than the medium level, though this was not significant. Chlorophyll b content for the low irradiance was
reduced, but significant (P < 0.05) only for the vegetative and
270
W. Worku et al.
Table I. Effects of photosythetic irradiance on the leaf area per plant
(LA), specific leaf area (SLA), leaf weight ratio (LWR) and leaf area
ratio (LAR) at the end of the vegetative phase.
Treatment
(µmol m
100
250
400
P
Figure 2. Effects of photosynthetic irradiance level on stomatal density of the lower and upper leaf surfaces of the second fully-expanded leaf from the top, late in the vegetative phase. n = 7; mean ± standard error of the mean.
seed-filling phases (Fig. 1B). During the vegetative and flowering phases the highest chl a/b ratio was obtained under the
high light intensity (P < 0.001; Fig. 1D). The light level during
the seed-filling phase did not affect the chl a/b ratio but
increased contents of chl a and chl b similarly. Thus, the plant
did not seem to make use of the strategy to reduce the contents
of chl a and chl b differently with lowering light level towards
the end of the growth cycle. For all chlorophyll attributes under
the three irradiance levels, the values were lower during the
flowering phase. It might have been the result of more interplant shading and differences in leaf size, the leaves in the middle of the stem being wider.
Plants raised under low irradiance produced less chl a and
chl b with a smaller chl a/b ratio and these were largely consistent throughout the different phases. Similar results indicating reduction in chl content/unit leaf area were reported by
Regnier et al. (1988) in soybean and three broad leaf weeds and
Hashemi-Dezfouli and Herbert (1992) in maize. However, in
rice, Makino et al. (1997) reported a higher chl content/unit leaf
area for 350 against 1000 µmol m–2 s–1. It might be attributed
to differences in plant characteristics. For instance, they did not
observe appreciable differences in leaf area between the two
light regimes. The increased chl content of plants grown at
higher irradiance could be attributed to leaf thickness due to the
larger size and greater number of mesophyll cells. A decline in
the chlorophyll a/b ratio due to low irradiance was reported in
Amaranthus hypochondriacus [26] and in Ipomoea tricolor
[15]. The decline in the chl a/b ratio at a low level of irradiance
is attributed to an increase in the light harvesting center at the
expense of the reaction center complex, as the plants made the
most economic use of light [1, 15, 22, 26].
3.1.2. Stomatal density
Both the adaxial (upper) and abaxial (lower) leaf surface stomatal density was reduced by decreasing irradiance level
(Fig. 2). Density reductions on both surfaces were significant
(P < 0.05) for 100 versus 250 and 400 µmol m–2 s–1, while differences between the medium and high irradiances were not
significant. High irradiance increased stomatal density, by 90%
compared with low irradiance and by 12% compared with the
medium level. The stomatal density ratio of the lower to the
LA
–2 –1
s )
2
SLA
2 –1
LWR
–1
LAR
(cm )
(dm g )
(g g )
(dm2 g–1)
1642b
1930a
1464c
0.003
7.08a
5.12b
3.26c
0.001
0.731
0.717
0.732
0.153
5.19a
3.67b
2.39c
0.001
Significance is declared when P ≤ 0.05; n = 9; error df = 4; treatment
means with the same letter are not significantly different at the 5% level.
Figure 3. Effects of photosynthetic irradiance level on the net assimilation rate (NAR) and relative growth rate (RGR) between 40 and
55 days after emergence during the flowering phase. n = 3; mean ±
standard error of the mean.
upper leaf surface increased with declining light intensity. The
abaxial surface, on average, produced about 11 times more stomata than the adaxial side. The reduced stomatal density under
low irradiance may have an impact on stomatal conductance.
It may be considered as a strategy of the plant to avoid overinvestment and balance assimilation and transpiration rates.
The decreased stomatal conductance under low irradiance
could allow a sufficient influx of CO2 for the low assimilation
rate.
3.1.3. Growth parameters
Except for the leaf weight ratio, all differences in irradiance
level produced significant changes in all observed parameters
(Tab. I). Leaf area per plant peaked at 250 µmol m–2 s–1 and
decreased with both lower and higher light levels. Specific leaf
area and the leaf area ratio increased linearly with decreasing
irradiance, both by 117% from high to low level.
The net assimilation rate increased with increasing irradiance (Fig. 3). The continued increase in the net assimilation rate
with enhanced irradiance level indicates that saturation may not
have been reached at 400 µmol m–2 s–1. The relative growth
rate at low irradiance decreased by 33% compared with the high
level but there was no significant difference between plants
grown at higher irradiances (Fig. 3). Thus, the difference in
final dry matter between the medium and high irradiance levels
was the result of a greater standing biomass produced before
the first sampling time.
Common bean responses to irradiance
271
Table II. Effects of photosynthetic irradiance during three developmental phases on the seed yield, yield components and harvest index.
Treatment
Yield
–1
plant
phase
Pod No.
plant
–1
light
Reduction
Seed No.
in pod No.
pod
–1
(%)
100
HI
seed wt
(g)
(%)
Vegetative
L
M
H
9.1b
10.8a
12.0a
6.4b
7.6a
8.4a
23
10
6.0a
6.0a
5.9a
24.1a
23.3a
23.1a
46a
48a
45a
Flowering
L
M
H
8.6c
10.7b
12.6a
5.3c
7.9b
9.3a
43
15
6.1a
6.0a
5.8a
25.8a
22.8b
21.9b
43b
48a
49a
Seed-filling
L
M
H
7.3c
11.1b
13.6a
6.1c
7.6b
8.8a
30
13
5.4b
6.2a
6.3a
21.6b
24.0a
24.8a
43b
48a
48a
n = 81; error df = 8; treatment means with the same letter are not significantly different at the 5% level; L, low; M, medium; H, high; HI, harvest index.
At lower irradiances, plants increased their leaf area ratio in
order to maintain their relative growth rates. However, under
the 100 µmol m–2 s–1 irradiance level, the increase in leaf area
ratio was insufficient to offset the loss in the net assimilation
rate and as a result the relative growth rate was reduced. Since
the leaf area ratio is the function of the specific leaf area and
leaf weight ratio, and due to the absence of significant variation
in the latter, the leaf area ratio followed the magnitude of specific leaf area. The plants acclimated to a low level of irradiance
by increasing their specific leaf area. This allowed the plant to
expose more area per unit of leaf weight to maximize light
energy harvesting. This was accomplished at the expense of
photosynthetic capacity because higher net assimilation rates
were associated with smaller specific leaf area (Tab. I vs.
Fig. 3). Regnier et al. (1988) reported a 108% increase in specific leaf area for soybean and three weed species under reduced
irradiance (180 against 800 µmol m–2 s–1) and noted the
absence of variation in the leaf weight ratio. Specific leaf area
is much more plastic with respect to environmental conditions
while the leaf weight ratio is rather ‘conservative’, buffering
the impact of the environment on the leaf area ratio [18].
3.1.4. Shoot-root ratio
Both shoot and root dry weights were significantly affected
by the irradiance level. Shoot biomass increased with increasing irradiance level. The values were 11.7 ± 0.87, 22.8 ± 3.83
and 39.7 ± 5.18 g plant–1 for the low, medium and high irradiance levels, respectively. The corresponding values for root dry
weight were 0.8 ± 0.24, 1.4 ± 0.03 and 3.9 ± 0.65 g plant–1.
These figures gave a shoot-root ratio of 15.5 ± 3.63 for the low,
16.2 ± 2.97 for the medium and 10.1 ± 2.76 for the high irradiance levels.
Shoot biomass almost doubled at each light level. The initial
rate of increase in dry weight with increasing irradiance was
greater for shoot dry weight than for root. As a result, the shootroot ratio of plants at the low and medium irradiance rose by
about two-thirds compared with that of plants grown at high
irradiance. The higher shoot-root ratio indicated the preferential investment in the photosynthetic apparatus under decreasing light intensity. Comparable results were reported in rice
[21] and in soybean and three weed species [25]. Makino et al.
(1997) observed that this was mainly due to the prominent
decrease in root dry weight at low irradiance. The poor root
development associated with low light intensity may have an
impact on nodulation and nodulation activity. For instance, Xia
(1995) reported a fall in nodule mass and observed early nodule
senescence under 50 and 20% shade in Vicia faba. Reduced root
growth under low irradiance may also lower the competitive
ability of the crop under intercropping conditions, especially
when moisture is limiting.
3.2. Effects on productivity
3.2.1. Seed yield
Irradiance levels during each of the three phenological
phases significantly affected seed yield (Tab. II). During the
vegetative phase only the lowest irradiance level caused a significant reduction in seed yield. This indicates that the vegetative phase may safely tolerate shading, except when it is severe,
and the crop can perform well if provided with adequate light
during the later developmental phases. On the other hand, during the flowering and the seed-filling phases there was a significant loss for each reduction in light intensity. Reduced
irradiance had the most detrimental effect during the seed-filling
phase, resulting in a 46% loss in seed yield at low irradiance
and 18% at medium irradiance compared with the high light
level. The corresponding yield reductions during the flowering
phase were 32 and 15%. During the flowering and seed-filling
phases, the level of irradiance was significant, as the yield reduction from medium to low irradiance level was always more than
that from high to medium level.
Lack of significant yield variation between medium and
high light level during the vegetative phase may indicate the
increment of photosynthesis in response to the improved light
environment. The study of Reed (1988) revealed that when
maize plants under 50% shade during the vegetative phase were
exposed to full sunlight, photosynthesis of these plants returned
to ‘normal’ level throughout flowering and grain filling. However,
it may be important to note that differences in the shade and light
environment should not be extreme for the plants to be able to
utilize the increased light intensity. Louwerse and Zweerde
272
W. Worku et al.
Table III. Partitioning of seed yield (g m–2) variance in relation to the yield components and to the main effects of radiation deficit (MJ m–2 PAR)
during three phenological phases, by stepwise multiple regression.
Variable
Intercept
Pod No. plant–1
Seed weight
Seed No. Pod–1
Model
a
Parameter
estimate
–19.01
1.42
0.42
1.49
Partial R2
Variable
Parameter
estimatea
0.76***
0.15***
0.04***
0.95
Intercept
Vegetative
Flowering
Seed filling
Model
–2.6
–1.20
–2.87
–2.34
Partial R2
0.08***
0.15***
0.37***
0.60
g MJ–1 PAR; α level for entry in the model ≤ 0.05; n = 81.
(1977) demonstrated that the rates of gross photosynthesis at
saturation irradiation increased with increasing irradiation during the preceding period in P. vulgaris and maize. Also, if the
size of the plant is too small due to extreme shading during early
growth, its canopy may be inadequate to fully utilize the increased
irradiance.
3.2.2. Yield components
The responses of yield components to irradiance levels were
variable depending on the timing of exposure to the different
light levels (Tab. II). The stepwise multiple regression relating
seed yield to its components indicated that pod number per
plant accounted for about 75% of the variation (Tab. III). Seed
weight and seed number per pod together contributed to about
20% of the variation.
Number of pods per plant: Reduced irradiance level during
each of the three growth phases significantly decreased pod
number per plant except the medium versus high light during
the vegetative phase (Tab. II). The flowering phase was the
most sensitive one, showing a reduction of 43% for the low and
15% for the medium irradiance levels compared with the high
level. Irradiance level during the seed filling was also significant, with corresponding decreases of 30 and 13%. Plants
raised under the various irradiance levels during the vegetative
phase responded differently to the three light levels during the
next phase, as indicated by a significant (P = 0.028) vegetative
irradiance level x flowering irradiance level interaction. When
the light level during the flowering phase was low, the otherwise positive effect of enhanced light intensity during the preceding vegetative phase on pod number was annulled (Fig. 4).
This was also reflected in seed yield to some extent, since the
same interaction was nearly significant (P = 0.061). Changes
in the number of pods per plant in relation to radiation deficit
showed a similar trend (Eq. (5)).
The decline in the number of pods per plant due to low irradiance during the vegetative phase could be attributed to a
source limitation as the plant later failed to supply sufficient
photosynthate for every developing pod. The number of flowers produced was not important, since there were at least two
times more buds than mature pods at all irradiance levels.
Ketering (1979) and Hang et al. (1984) found a reduction in
flower number at low light intensity in groundnut. They suggested that this might have little effect on final yield, for the
same reason. The reduction in pod number during flowering at
low irradiance resulted from the high proportion of aborted
Figure 4. Interactive effects of light level during the vegetative and
flowering phases on the number of pods per plant at maturity. IF,
flowering phase irradiance level.
flowers. This could be a mechanism to limit the number of pods
in accordance with assimilate supply so that they can reach
maturity. In soybean, most abortion occurs among flowers or
small pods [14]. Similarly, in the current experiment, the majority of pods aborted some days after pollination when the pod
size was about 20 mm. During the seed-filling phase, pod
number reduction occurred due to abscission or cessation of
growth of developing pods 6–10 cm long. The seed size in some
of these pods was up to five mm in length. A full-size pod was
8–11 cm in length. Abortion of such pods was mainly observed
in the 100 µmol m–2 s–1 room on treatments that had been at a
higher irradiance level during the previous phase, i.e. the
affected plants were incapable of sustaining the number of pods
initiated during the preceding phase.
Seed number per pod: The low light level during seed filling
was the only treatment that reduced number of seeds per pod
significantly (Tab. II). This was also reflected in the regression
equation (6). Egli (1997) has shown that shading that intercepted 63% of natural incident light during the seed-filling
phase reduced seed number per pod in soybean. However, his
previous study [5] showed that the reduction in seed number
was greater when shading also included the flowering phase.
Our data show that the specific effect of low irradiance during
flowering does not seem to have a negative effect on the number
of seeds per pod. The sequential development of the reproductive sink and overlapping of reproductive events in such indeterminate cultivars may have contributed to the differences.
Common bean responses to irradiance
Seed weight: Low irradiance during flowering resulted in
significantly heavier seeds (Tab. II). On the other hand, the
response to light level during seed filling was the opposite, low
irradiance producing significantly smaller seeds. Seed weight
is determined by cell division that occurs during a period of
about two weeks after anthesis [2], and the rate and duration of
dry matter accumulation during the seed-filling period [7]. The
observed increment in seed size with decreasing irradiance during the flowering phase could be attributed to the smaller
number of pods produced. These results agree with those of
Reed et al. (1988), who showed that yield reduction due to loss
of kernel from shading of maize during the flowering phase was
partially compensated by increased kernel weight. However,
the potential to increase grain weight is limited, because a genotype has a maximum grain size [9].
273
Figure 5. Relationship between cumulative intercepted PAR and (A)
seed dry matter and (B) shoot dry matter. n = 9; VL, low irradiance
during vegetative phase; VH, high irradiance during vegetative phase.
3.2.3. Relative sensitivity of phases to reduced irradiance
The stepwise multiple regression of seed yield variation as
affected by specific light level during the various growth phases
indicated that the seed-filling phase was the most important; it
accounted for more than half of the explained variation
(Tab. III). However, the durations of each of the growth phases
were variable, which implies that the relative time of exposure
to the various irradiance levels was not similar. For instance,
the duration of the flowering phase was about half that of the
vegetative and seed-filling phases. Thus, there is a need to have
a quantity that takes both the timing and amount of light intercepted into consideration in order to compare sensitivity of
phases under an equal magnitude. Radiation deficit during each
phase was used for this purpose. The radiation deficit of each
phase was calculated as a difference between intercepted light
by the low and medium treatments with that of the high irradiance treatments. This quantity was used to quantify the effect
of reduced irradiance level and its timing on the yield and its
components. Accordingly, the highest seed yield loss per unit
of radiation deficit was observed during flowering, 2.87 g DM
MJ–1 PAR, and this was closely followed by the seed-filling
phase (Tab. III).
The larger loss of seed dry matter per unit of radiation deficit
during the flowering phase may be associated with the predominant contribution of pod number per plant in determining yield
levels. Since it is an indeterminate cultivar, flowering and pod
set occurred together and this active growth period for podding
may have made it more susceptible to radiation deficit in limiting pod number. Pod number per plant was affected most by
reduced irradiance during this phase more than in any other
phase. In the indeterminate legume Arachis hypogaea, Hang
et al. (1984) identified the podding and maturing phases as the
most sensitive, affecting seed yield under 75% shading of incident natural light. Though their shading duration was similar,
it was not compared against a similar magnitude of radiation
deficit. Also, they had separate flowering and podding phases
while the two events occurred together in our experiment.
3.2.4. Relationship between dry matter production
and intercepted radiation
The relationship between cumulative intercepted PAR and
both seed and shoot dry matter was similar, which was gener-
ally linear (Fig. 5A, B). The similar relationship for the seed
and shoot dry weight is due to the absence of large differences
in the harvest index. The two treatments that were exceptions
were those when irradiance was low during the vegetative
phase and when it was high during the same phase. These
exceptions seem to indicate that low irradiance during the vegetative phase improved radiation use efficiency while high irradiance during this phase reduced it.
4. CONCLUSION
Our experiment showed that the indeterminate cultivar Red
Wolaita of P. vulgaris made adjustments to the ambient irradiance by employing physiological, morphological, anatomical
and differential dry matter partitioning mechanisms. The plants
under reduced irradiance had lower chlorophyll content per unit
area, a reduced chl a/b ratio, low stomatal density and a reduced
net assimilation rate, which are features enhancing light harvesting at the expense of photosynthetic capacity. In order to
maintain their relative growth rate, plants at low irradiance
increased their leaf area ratio by producing leaves of larger specific leaf area. However, reduced irradiance as a result of field
shading from crop combinations is accompanied by a decreased
red-far red light ratio. This change in light quality may modify
some of the acclimation characteristics, which might have a
bearing on dry matter production.
Because of differences in growth duration and light interception, sensitivity of phases should be compared under a quantity representing a similar magnitude of radiation deficit. With
respect to seed yield, reduced irradiance during the vegetative
phase may safely be tolerated provided that it is not severe.
However, no development phase was immune from the effects
of severe light intensity reduction. The highest seed yield loss per
unit of radiation deficit was during the flowering phase, followed
by seed filling. Statistically significant two-factor interactions
were not observed for seed yield, indicating that main effects
largely explained sensitivity. This may allow simplification of
design in experiments investigating irradiance levels. Agronomic strategies should be devised to avoid or minimize shading during the susceptible phases.
274
W. Worku et al.
Acknowledgements: We thank Dr. Asbjørn Solhaug for his help with
chlorophyll determination and Dr. Halvard Baugerød for his assistance with
stomatal density measurements. We are also thankful to Dr. P. Adams for correcting the English.
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[16] Jedel P.E., Hunt L.A., Shading and thinning effects on multi and
standard floret winter wheat, Crop Sci. 30 (1990) 128–133.
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