Physiology of yield determination of mung bean (Vigna radiata (L.) Wilczek) under various irrigation regimes in the dry and intermediate zones of Sri Lanka

Field Crops Research 61 (1999) 1±12

Physiology of yield determination of mung bean (Vigna radiata (L.) Wilczek) under various irrigation regimes in the dry and intermediate zones of Sri Lanka W.A.J.M. De Costaa,*, K.N. Shanmugathasana, K.D.S.M. Josephb a

Department of Crop Science, Faculty of Agriculture, University of Peradeniya, Peradeniya, Sri Lanka b Field Crops Research and Development Institute, Maha-Illuppallama, Sri Lanka Received 10 October 1997; received in revised form 22 February 1998; accepted 20 August 1998

Abstract Drought is a major factor limiting yield improvement of mung bean (Vigna radiata (L.) Wilczek) in the sub-humid, dry and intermediate zones of Sri Lanka. Therefore, the objective of this study was to analyze the yield response of mung bean to irrigation at various phenological stages in terms of radiation interception, radiation-use ef®ciency and harvest index. Four ®eld experiments were carried out at two sites (Maha-Illuppallama and Kundasale) during the short, dry yala season over two years (1995 and 1996). The life cycle of mung bean was divided into three stages: vegetative (from germination to appearance of ®rst ¯ower); ¯owering (from appearance of ®rst ¯ower to 75% pod initiation); and pod-®lling (from 75% pod initiation to maturity). Eight irrigation treatments were de®ned as all possible combinations of irrigation during the three stages. Maximum potential soil water de®cits (PSWD) ranging from 127 to 376 mm developed as a result of keeping different combinations of stages unirrigated. Maximum LAI (Lm) and the fraction of incoming radiation intercepted (F) increased signi®cantly with the number of stages irrigated. Speci®cally, treatments which included irrigation during the vegetative stage achieved large Lm and F. Radiation-use ef®ciency (RUE), maximum total biomass (Wm), harvest index (HI) and seed yield (Y) also showed a signi®cant positive response to the number of stages irrigated. However, all the above parameters were signi®cantly greater in treatments which included irrigation during the pod-®lling and ¯owering stages. The treatment which received irrigation only during the vegetative stage had signi®cantly lower RUE, Wm, HI and Y despite having higher Lm and F. Therefore, irrigation is critical during pod-®lling and ¯owering stages mainly because of the higher LAI during these periods and, consequently, the greater demand for water. Lack of irrigation during these critical stages resulted in the development of signi®cant PSWD with adverse effects on photosynthesis and consequently decreased RUE. Moreover, water stress during ¯owering and pod-®lling stages signi®cantly reduced pod initiation and pod growth rates and thereby reduced HI. It is concluded that to maximize mung bean yields in the dry season of the sub-humid zones of Sri Lanka, irrigation should extend across all phenological stages, specially the pod-®lling stage. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Mung bean; Irrigation; Water stress; Radiation use; Harvest index; Yield

*Corresponding author. Tel.: 94-8-88354; fax: 94-8-88041; e-mail: [email protected] 0378-4290/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0378-4290(98)00141-5

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1. Introduction Mung bean (Vigna radiata (L.) Wilczek), which is grown mainly in the central and southeastern regions of Asia (Shanmugasundaram, 1988) is one of the least researched and under-exploited of the major grain legume crops (Lawn and Ahn, 1985). In Sri Lanka, mung bean is an important component of the maize± legume crop rotation in the uplands and the rice± legume crop rotation in the lowlands of the sub-humid, dry and intermediate zones (Jayasekara and Ariyaratne, 1988). Mung bean crops are grown in the short rainy season (locally known as yala), in Sri Lanka and are subjected to signi®cant soil water de®cits at various stages of growth. Consequently, the current yields (i.e. around 800 kg haÿ1) are signi®cantly below the potential (i.e. around 3000 kg haÿ1) (Lawn and Ahn, 1985). Therefore, yield improvement of mung bean, either through breeding of higher yielding cultivars or through improved crop management, should be based on countering the effects of water stress. The physiological basis of yield determination of mung bean can be considered by expressing the seed yield (Y) in terms of the following components: Y ˆ SF…RUE:HI†

(1)

where, S is the seasonal total of incoming radiation, F the fraction of incoming radiation intercepted over the growing season, RUE the mean radiation-use ef®ciency and HI the harvest index. RUE is de®ned as the amount of biomass produced per unit of radiation intercepted (Monteith, 1977) and is a measurement of the ef®ciency of biomass production by photosynthesis. The total biomass of the crop is given by the product of the components S, F and RUE. Harvest index is the ratio of total seed dry weight to ®nal total biomass, thus indicating the fraction of total biomass partitioned to seeds. One management option for preventing the adverse effects of drought on mung bean yield in the subhumid zones of Sri Lanka is irrigation. Therefore, the objective of the present paper is to examine the response of yield and its components shown in Eq. (1) under different irrigation regimes and thereby determine its physiological basis of yield variation. The number of pods initiated and their rate of growth are important determinants of harvest index in grain legumes (Lawn and Ahn, 1985). Moreover,

the ability to translocate pre-anthesis assimilates during the pod-®lling period, especially during periods of water stress, could be crucial in stabilizing legume yields (Goldsworthy, 1984; Evans, 1993). Therefore, a secondary objective was to investigate the effects of irrigation at different growth stages on the above aspects of pod growth. 2. Material and methods 2.1. Site description Four experiments were carried out during the yala seasons of 1995 and 1996 at two sites, representing the dry and intermediate zones of Sri Lanka (7±88N and 80±818E). The dry zone was represented by MahaIlluppallama (MI) and the intermediate zone by Kundasale (KS). The annual average rainfalls are 1000 mm at MI and 1400 mm at KS, with both sites having a distinctly bi-modal distribution (Panabokke, 1996). The soil at MI belongs to the group Rhodustalfs with a sandy clay loam texture. It is moderately welldrained with a pH of 6.5, a cation exchange capacity (CEC) of 15 me 100gÿ1 and a C : N ratio of 10. Quartz and kaolinite are the dominant minerals present and Ca is the dominant basic cation. The soil at KS is a moderately well-drained sandy clay loam belonging to Rhodudults. The pH, CEC and C : N ratio are, respectively, 6.5, 16 me 100gÿ1 and 10. Dominant minerals are quartz and kaolinite with Ca and Mg being the dominant cations. The sub-soil base saturation is 75% at MI and 25% at KS. The respective soil water contents at ®eld capacity and permanent wilting point were 28.9% and 15.6% at MI and 26% and 12% at KS (Mapa and Pathmarajah, 1995). 2.2. Crop establishment and management Crops (cultivar MI-5) were established by direct seeding in rows 30 cm apart, following yala rains on 04 May 1995 and 11 July 1996 at MI and on 06 June 1995 and 07 June 1996 at KS. The delayed planting at MI in 1996 was a result of the delayed onset of rains. The emerged seedlings were thinned two weeks after sowing to obtain a plant population of 42 plants mÿ2. A basal application of fertilizer (35 kg haÿ1 urea; 140 kg haÿ1 triple super phosphate; 75 kg haÿ1 muri-

W.A.J.M. De Costa et al. / Field Crops Research 61 (1999) 1±12

ate of potash) was given at sowing and a top dressing (30 kg haÿ1 urea) was applied 30 days after sowing (DAS) in all experiments. Weed control was done manually. Pests and diseases were controlled by periodic spraying of appropriate chemicals. Final harvests of the respective crops were taken on 24 July (at MI in 1995), 28 September (MI in 1996), 4 September (KS in 1995) and 22 August (KS in 1996). 2.3. Experimental treatments and design In order to establish irrigation treatments at different growth stages, the total crop duration was divided into the following three stages: vegetative; ¯owering; and pod-®lling. The vegetative stage was the period from sowing to the appearance of ®rst ¯ower. The period from the appearance of ®rst ¯ower until 75% pod initiation was de®ned as the ¯owering stage and the subsequent period until the ®nal harvest was de®ned as the pod-®lling stage. Table 1 shows the

Table 1 Durations of the three growth stages (in days) of mung bean in two sites and seasons Stage Vegetative Flowering Pod-filling Total duration

Maha-Illuppallama

Kundasale

1995

1996

1995

1996

28 14 39 81

29 16 34 79

32 18 40 90

30 16 32 78

3

durations of the three stages during the two seasons at the two sites. Eight experimental treatments were de®ned as irrigations at all possible combinations of the three growth stages (Table 2). The treatments were laid out in a randomized complete block design with three replicates. The plot size was 3 m3 m with a distance of 1 m in-between. Plots were prepared as sunken beds with bunds around them for holding irrigation water. Surface irrigation was applied, in appropriate treatments, at the frequency recommended for the farmers by the Department of Agriculture, Sri Lanka, i.e. every four days during the ®rst four weeks and every seven days thereafter, until eight weeks after sowing (Anonymous, 1990). Irrigation is not recommended during late-pod-®lling stage because it promotes more vegetative growth at the expense of reproductive growth in mung bean. Each irrigation and/or suf®cient rainfall brought the soil water content in the respective plots to ®eld capacity. 3. Measurements 3.1. Potential soil water deficit (PSWD) A simple index of soil water de®cit was calculated as the difference between the daily rainfall and pan evaporation. The daily values were cumulated over the cropping period. It was assumed that at the beginning of the season, the soil was at ®eld capacity and hence PSWD was zero. Whenever a rainfall during mid-

Table 2 Definition of experimental treatments Treatment

VNFNPN VIFIPI VIFNPN VIFIPN VIFNPI VNFIPI VNFNPI VNFIPN a b

Not-irrigated. Irrigated.

Growth stages and irrigations

No. of stages irrigated

vegetative

flowering

pod-filling

Na Ib Ib Ib Ib Na Na Na

Na Ib Na Ib Na Ia Na Ib

Na Ib Na Na Na Ib Ib Na

0 3 1 2 2 2 1 1

Total no. of irrigations MI 95

MI 96

KS 95

KS96

0 9 4 6 7 5 3 2

0 7 3 6 4 4 1 3

0 9 5 7 7 4 2 2

0 9 5 7 7 4 2 2

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season exceeded the cumulative PSWD up to that point, PSWD was brought back to zero. This index gives a measurement of the maximum water stress experienced by the crops and was termed `potential soil water de®cit' by Penman (1962). This is an adequate substitute when detailed soil water measurements, using for example the neutron probe, are not feasible due to lack of resources. All meteorological data were obtained from weather stations located at the experimental sites. 3.2. Crop growth Leaf area index (LAI) and total dry matter were measured by destructive sampling of 10 randomly selected plants at two-week intervals. Total leaf area of all harvested plants were measured by an automatic leaf area meter. LAI was computed as the ratio between leaf area and land area occupied. Total dry weight was obtained by oven-drying the harvested plant sample at 808C to a constant weight. Seasonal time courses of LAI and total dry weight were used to identify the maximum levels of LAI and total biomass that were achieved under various the irrigation regimes. 3.3. Radiation interception Radiation interception was measured in all experimental plots by a pair of tube solarimeters and microvolt integrators (Delta-T Devices, UK) at weekly intervals. All measurements were made on clear days between 1000 and 1400 h. Simultaneous measurements were made over periods of 15 min with one solarimeter outside the plot (measuring incident radiation ± I0) and the other inside the plot at ground level oriented at 908 to the crop rows (measuring transmitted radiation ± I). Applying the Beer's Law to light interception by crop canopies (Monsi and Saeki, 1953), the canopy light extinction coef®cient (k) was estimated as the slope of the linear regression between loge (I/I0) and LAI. Daily of radiation interception (Ri) was estimated as: Ri ˆ I d f i

(2)

where Id is the daily incident radiation (MJ mÿ2 dayÿ1) and fi the fraction of incident radiation inter-

cepted. Id was estimated from daily values of sunshine duration obtained from weather stations located at experimental sites using the following empirical equation based on Angstrom's formula as adopted for Sri Lanka by Samuel (1991): Id ˆ I0;d ‰a ‡ b…S=Z†Š

(3)

where I0,d is the daily global radiation (MJ mÿ2 dayÿ1) at the top of the atmosphere, S the measured sunshine duration (h dayÿ1) and Z the day length (h). The parameters a and b depend on the location and the respective values are 0.29 and 0.39 for MI and 0.27 and 0.42 for KS (Samuel, 1991). Daily values of I0,d and Z for the experimental duration were computed by a set of meteorological equations (Rosenberg et al., 1983; Monteith and Unsworth, 1990). Daily fi was estimated as: fi ˆ 1 ÿ eÿkL

(4)

where k is the canopy light extinction coef®cient and L the leaf area index. Daily L values were estimated by ®tting a polynomial curve (Hunt, 1982) to the fortnightly-measured values of L. A similar procedure was followed by Jamieson et al. (1995) to estimate daily radiation interception from a single measurement of radiation transmission. Radiation-use ef®ciency (RUE) was computed as the ratio between the total biomass at ®nal harvest and the total amount of radiation intercepted over the entire season. Mean seasonal fractional radiation interception (F) was calculated as the ratio between the total intercepted radiation and the total incident radiation. 3.4. Podding behaviour In the experiments at Maha-Illuppallama, ten plants from the central area of the plots were selected randomly and tagged. The number of pods and podding nodes per plant were recorded at 3±4-day intervals from the time of pod initiation until about 1±2 weeks before ®nal harvest. The maximum number of pods per plant was identi®ed by examining its time course in each irrigation treatment. Possible translocation of pre-anthesis assimilates to pods was examined by comparing pod growth rates during the pod®lling period with the corresponding overall crop growth rates.

W.A.J.M. De Costa et al. / Field Crops Research 61 (1999) 1±12

3.5. Yield and harvest index The central 1 m2 area of the plot was harvested in two-to-three picks, depending on sites and seasons. The ®rst pick was done around 57 to 63 days after sowing (DAS), the second around 71 to 79 DAS and the ®nal pick around 77 to 90 DAS. Harvest index was computed as the ratio between sum of seed dry weights of all three picks and the total plant dry weight at ®nal harvest which included pod weights from all picks. 3.6. Data analysis Analysis of variance (ANOVA) was used to detect the signi®cance of treatment effects on measured variables. Least signi®cant difference (LSD) was used to identify differences between treatment means. 4. Results 4.1. Meteorological conditions Variations of relevant meteorological factors at the two sites during the experimental periods (Table 3) clearly showed that MI which had lower rainfall and

5

higher pan evaporation was signi®cantly drier than KS. KS had a greater number of rain days (>1 mm) than MI. However, at both sites, except during May at MI in 1995 and June at KS in 1996, monthly pan evaporation exceeded rainfall during the cropping periods. The shortwave radiation and mean ambient temperature at MI were signi®cantly greater than at KS. 4.2. Potential soil water deficit (PSWD) The maximum PSWD experienced by the treatments during the three growth stages are presented in Table 4. Within a given treatment, the respective magnitudes of PSWD varied between sites and seasons because of variation in rainfall and pan evaporation. In general, crops at MI experienced greater PSWD than their corresponding crops at KS. Within a given experiment, any corresponding pair of irrigated and non-irrigated stages showed a clear difference in the maximum PSWD. As expected, the highest and lowest PSWD were shown in VNFNPN (rainfed throughout) and VIFIPI (irrigated throughout) treatments, respectively. The treatments VNFNPN, VNFIPI, VNFNPI and VNFIPN, which were not irrigated during the vegetative stage, experienced the greatest PSWD during the vegetative stage. The treatment VNFNPI,

Table 3 Climetological records at the two sites during the experimental periods Month

Incident shortwave radiation (MJ mÿ2 monthÿ1)

Rainfall (mm/month) (No. of rain days) MI 1995

MI 1996

KS 1995

KS 1996

MI 1995

MI 1996

KS 1995

KS 1996

May June July August September Total

164.1 4.1 0.4 1.5 11.9 182.0

1.8 78.3 24.4 155.0 109.2 368.7

229.4 68.2 39.1 56.6 102.3 495.6

0.8 135.1 50.5 84.1 114.3 384.8

651 586 626 662 647 3174

698 557 593 627 574 3051

562 445 499 533 563 2605

671 483 456 539 438 2590

Month

Pan evaporation (mm/month)

May June July August September Total Mean

(9) (2) (0) (1) (3) (15)

(1) (6) (2) (6) (3) (18)

(13) (12) (9) (9) (11) (54)

(0) (15) (11) (13) (19) (58)

Mean air temperature (8C)

MI 1995

MI 1996

KS 1995

KS 1996

MI 1995

MI 1996

134 145 177 187 184 829 Ð

202 129 172 179 116 800 Ð

121 72 97 96 109 497 Ð

136 90 101 95 90 514 Ð

28.4 28.4 28.6 28.8 29.1 Ð 28.7

29.7 28.2 28.5 28.6 27.4 Ð 28.5

N.B. See text for the dates of sowing and final harvests of the different crops.

KS 1995 26.0 26.1 25.5 25.6 25.4 Ð 25.7

KS 1996 27.2 25.1 25.1 25.3 24.9 Ð 25.5

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Table 4 Maximum potential soil water deficits experienced by the irrigation treatments Site and season

Growth stage

Maximum potential soil water deficit (mm) VNFNPN

VIFIPI

VIFNPN

VIFIPN

VIFNPI

VNFIPI

VNFNPI

VNFIPN

MI 1995

vegetative flowering pod-filling

104 169 376

35 28 147

35 73 280

35 28 234

35 73 147

104 28 147

104 169 147

104 28 234

MI 1996

vegetative flowering pod-filling

116 187 342

48 28 90

48 83 239

48 28 184

48 83 90

116 28 90

116 187 90

116 28 184

KS 1995

vegetative flowering pod-filling

22 56 138

8 25 56

8 40 122

8 25 108

8 40 56

22 25 56

22 56 56

22 25 108

KS 1996

vegetative flowering pod-filling

70 107 127

13 14 33

13 48 68

13 14 41

13 48 33

70 14 33

70 107 33

70 14 41

which did not receive irrigation until the pod-®lling stage, had the greatest PSWD during the ¯owering stage. Next to VNFNPN, the treatment VIFNPN, which received irrigation only during the vegetative stage, experienced the greatest PSWD during the pod-®lling stage. 4.3. Vegetative growth In the majority of treatments, leaf area index (LAI) reached a maximum during the middle of ¯owering

(i.e. around 40 DAS) and retained it until late pod®lling (around 70 DAS). On the other hand, total dry weight (TDW) peaked during the middle of pod®lling. Table 5 reveals the variation of maximum LAI (Lm) and TDW (Wm) under the irrigation regimes. Irrigation signi®cantly (p<0.05) increased both, Lm and Wm over their corresponding levels in the rainfed VNFNPN crops. While irrigation during any growth stage had a signi®cant positive effect (relative to the rainfed treatment) on Lm, the response was greatest when at least two stages were irrigated and also when

Table 5 Maximum leaf area index (LAI) and total dry weight (g mÿ2) of mung bean under various irrigation regimes at Maha-Illuppallama (MI) and Kundasale (KS) in Sri Lanka. Fractions relative to the well-watered VIFIPI treatment are given in italics Treatment

Maximum total dry weight (g mÿ2)

Maximum LAI MI

KS

1995 VNFNPN VIFIPI VIFNPN VIFIPN VIFNPI VNFIPI VNFNPI VNFIPN

0.98 2.26 1.68 2.05 2.24 1.78 1.26 1.07

LSD0.05 CV (%)

0.40 13.69

1996 (0.43) (1.00) (0.74) (0.91) (0.99) (0.79) (0.56) (0.47)

0.73 1.25 0.96 1.22 1.19 1.15 0.97 1.09 0.11 5.85

MI

1995 (0.58) (1.00) (0.77) (0.98) (0.95) (0.92) (0.78) (0.87)

1.09 2.43 2.25 2.26 1.96 1.72 1.81 1.83 0.38 8.4

1996 (0.45) (1.00) (0.93) (0.93) (0.81) (0.71) (0.74) (0.75)

0.59 2.34 1.76 1.69 1.86 1.67 1.54 1.39 0.41 14.46

(0.25) (1.00) (0.75) (0.72) (0.79) (0.71) (0.66) (0.59)

KS

1995

1996

1995

1996

212 329 237 263 266 302 279 219

377 466 387 427 445 448 401 409

163 317 252 258 254 254 268 289

158 330 261 234 285 239 284 271

(0.64) (1.00) (0.72) (0.80) (0.81) (0.92) (0.85) (0.67)

15.91 3.45

(0.81) (1.00) (0.83) (0.92) (0.95) (0.96) (0.86) (0.88)

20.16 2.74

(0.52) (1.00) (0.79) (0.81) (0.80) (0.80) (0.85) (0.91)

26.96 4.44

(0.48) (1.00) (0.79) (0.71) (0.86) (0.72) (0.86) (0.82)

38.97 8.63

W.A.J.M. De Costa et al. / Field Crops Research 61 (1999) 1±12

one of the two stages irrigated included the vegetative stage. Lm of the `fully-irrigated' VIFIPI treatment varied between 1.5 and 2.4 with a slightly greater range being shown at MI than at KS. Except at MI in 1996, Lm achieved by VIFIPI was more than twice that of Lm of the rainfed treatment. In all treatments which included an unirrigated stage, the proportional decrease of Wm relative to the fully-irrigated VIFIPI treatment were smaller than the corresponding decrease of Lm (Table 5). Similar to Lm, Wm also responded positively to irrigation at any growth stage and also to the number of stages irrigated. The greatest response was observed, however, when irrigation was provided during the ¯owering and pod-®lling stages when LAI was maximum. Except at MI in 1996, there was a nearly 100% increase in Wm from the rainfed to the `fully-irrigated' treatment. The overall differences between various combinations of irrigation regimes were clearer at MI than at KS. This was because of greater PSWD experienced at MI (Table 4). 4.4. Radiation interception and conversion Mean seasonal fraction of incident radiation intercepted (F) showed signi®cant variation between irrigation regimes in both, sites and seasons (Table 6). Irrigation at any growth stage signi®cantly increased F

7

compared to the rainfed treatment. The positive response of F to irrigation was greatest when the vegetative stage was irrigated. Except at MI in 1996, the F value of VIFNPN, which received irrigation during the vegetative stage only, was similar to those of VIFIPN and VIFNPI, both of which received irrigation during two stages. On the other hand, VNFIPI, which also received irrigation during two stages but not during the vegetative stage, had signi®cantly lower F values than VIFIPN and V I FN PI . There was signi®cant response in mean seasonal RUE in all experiments (Table 6). Except at KS in 1996, VNFIPI had signi®cantly greater RUE than even VIFIPI. RUE was highest in treatments which received irrigation during at least two stages and when one of the two stages irrigated was the pod-®lling stage (i.e. VIFIPI, VNFIPI and VIFNPI). In contrast, VIFIPN which also received irrigation during two stages but not during the pod-®lling stage, had signi®cantly lower RUE than the above treatment group. Within the group which received irrigation during one stage only, RUE was signi®cantly greater when irrigations were given during either pod-®lling (VNFNPI) or ¯owering (VNFIPN). On the other hand, when irrigation was provided only during the vegetative stage (VIFNPN), RUE was even lower than in the rainfed treatment except in 1995 at KS.

Table 6 Mean seasonal fractions of incident radiation intercepted and mean seasonal radiation-use efficiencies (g MJÿ1) of mung bean under various irrigation regimes at Maha-Illuppallama (MI) and Kundasale (KS) in Sri Lanka. Fractions relative to the well-watered VIFIPI treatment are given in italics Trt

MI 1995 VNFNPN VIFIPI VIFNPN VIFIPN VIFNPI VNFIPI VNFNPI VNFIPN TSR a(MJ mÿ2) LSD0.05 CV (%) a

Radiation use efficiency (g MJÿ1)

Fraction of incident radiation intercepted

0.41 0.62 0.59 0.62 0.62 0.52 0.47 0.45

MI 1996 (0.66) (1.00) (0.95) (1.00) (1.00) (0.84) (0.76) (0.73)

1658 0.052 5.53

0.34 0.48 0.44 0.49 0.45 0.40 0.37 0.39

KS 1995 (0.71) (1.00) (0.92) (1.02) (0.94) (0.83) (0.77) (0.81)

1525 0.017 2.34

0.38 0.63 0.60 0.61 0.59 0.55 0.55 0.54

(0.60) (1.00) (0.95) (0.97) (0.94) (0.87) (0.87) (0.86)

1352 0.042 3.15

Total incident radiation during the crops' lifespan.

KS 1996 0.26 0.61 0.58 0.57 0.54 0.42 0.39 0.40

(0.43) (1.00) (0.95) (0.93) (0.89) (0.69) (0.64) (0.66)

1226 0.033 3.95

MI 1995 0.30 0.37 0.24 0.33 0.36 0.45 0.30 0.41

(0.81) (1.00) (0.65) (0.89) (0.97) (1.22) (0.81) (1.11)

Ð 0.063 10.42

MI 1996

KS 1995

KS 1996

0.43 0.58 0.37 0.46 0.54 0.64 0.50 0.41

0.20 0.34 0.22 0.30 0.32 0.33 0.26 0.31

0.31 0.37 0.26 0.34 0.37 0.48 0.37 0.34

(0.74) (1.00) (0.64) (0.79) (0.93) (1.10) (0.86) (0.71)

Ð 0.070 8.13

(0.59) (1.00) (0.65) (0.88) (0.94) (0.97) (0.76) (0.91)

Ð 0.046 6.80

(0.84) (1.00) (0.70) (0.92) (1.00) (1.30) (1.00) (0.92)

Ð 0.041 6.56

8

W.A.J.M. De Costa et al. / Field Crops Research 61 (1999) 1±12

Table 7 Seed yield (kg haÿ1) and harvest index of mung bean under different irrigation regimes at Maha-Illuppallama (MI) and Kundasale (KS) in Sri Lanka. Fractions relative to the well-watered VIFIPI treatment are given in italics Treatment

Seed yield (kg haÿ1)

Harvest index (%)

MI

KS

1995 VNFNPN VIFIPI VIFNPN VIFIPN VIFNPI VNFIPI VNFNPI VNFIPN LSD0.05 CV (%)

315 1308 417 987 1164 1185 608 807

1996 (0.24) (1.00) (0.32) (0.75) (0.89) (0.91) (0.46) (0.62)

227 15.2

477 1485 636 1032 1223 1417 815 704

MI

1995 (0.32) (1.00) (0.43) (0.69) (0.82) (0.95) (0.55) (0.47)

201 11.8

266 1081 415 778 965 980 462 595

(0.25) (1.00) (0.38) (0.72) (0.89) (0.91) (0.43) (0.55)

179 10.9

KS

1996

1995

1996

1995

1996

202 967 392 803 887 859 518 589

16 34 18 28 31 31 26 26

21 35 26 30 33 36 29 29

25 36 22 30 35 37 23 25

20 (0.57) 35 (1.00) 21 (0.60) 33 (0.94) 36 (1.03) 34 (0.97) 29 (0.83) 34 (0.97)

(0.21) (1.00) (0.41) (0.83) (0.92) (0.89) (0.54) (0.61)

141 12.4

4.5. Seed yield Seed yield of crops showed signi®cant (p<0.05) variation in both, sites and seasons (Table 7). Again, the treatment group which received irrigation during at least two stages, including pod-®lling (i.e. VIFIPI, VIFNPI and VNFIPI) yielded most. On the other hand, VIFIPN had lower seed yield than the above group despite irrigation during two stages. In the treatment group receiving irrigation during one stage only, VNFNPI (pod-®lling) and VNFIPN (¯owering) had greater yields than VIFNPN (vegetative). However, irrigation during any stage increased seed yield signi®cantly over the rainfed crop. The `fully-irrigated' VIFIPI treatment achieved three-to-four-fold yield increase over the rainfed VNFNPN treatment. All treatment differences were consistent between sites and seasons. 4.6. Harvest index There were signi®cant (p<0.05) differences in harvest index in both, sites and seasons (Table 7). The response of harvest index to irrigation regime was similar to that of seed yield. One notable exception was that except at MI in 1996, harvest index of VIFNPN was not signi®cantly different from that of rainfed VNFNPN. The maximum harvest index achieved by the treatments in which two or more

(0.47) (1.00) (0.53) (0.82) (0.91) (0.91) (0.76) (0.76)

4.5 9.7

(0.60) (1.00) (0.74) (0.86) (0.94) (1.03) (0.83) (0.83)

2.4 4.7

(0.69) (1.00) (0.61) (0.83) (0.97) (1.03) (0.64) (0.69)

4.8 7.0

5.3 10.0

stages were irrigated was only 37%. The completely rainfed VNFNPN treatment showed signi®cantly smaller harvest indices, around 16±21%. 4.7. Initiation and growth of pods The variation in harvest index can be analyzed in terms of the number of pods initiated and mean pod growth rate (PGR) during the pod-®lling period (Table 8). In both seasons, the highest pod initiation was shown in VIFIPI and VNFIPI treatments during which both ¯owering and pod-®lling stages were irrigated. The next highest pod initiation was in VIFIPN and VIFNPI which received irrigation during either one of the above two stages. Among the treatments in which only one stage was irrigated, VNFNPI and VNFIPN had signi®cantly greater pod initiation than the rainfed VNFNPN crop which had the lowest. On the other hand, the VIFNPN treatment, which was irrigated during the vegetative stage, did not show a consistently higher pod initiation than VNFNPN. The highest PGR was shown in the `fully-irrigated' VIFIPI treatment, followed by VNFIPI and VIFNPI. All the above treatments were irrigated at least twice, including the pod-®lling stage. In contrast, the VNFNPI treatment which was irrigated only during pod-®lling had a signi®cantly lower PGR than the above group of treatments. Finally, irrigation at any stage increased PGR above that of the rainfed VNFNPN crop.

W.A.J.M. De Costa et al. / Field Crops Research 61 (1999) 1±12

9

Table 8 Maximum number of pods initiated (at 56 days after sowing), mean pod growth rates (PGR) and crop growth rates (CGR) during the podfilling period of mung bean under various irrigation regimes at Maha-Illuppallama (MI) in Sri Lanka. Fractions relative to the well-watered VIFIPI treatment are given in italics Treatment

Maximum number of pods per plant

Mean PGR (g mÿ2 dayÿ1)

Mean CGR (g mÿ2 dayÿ1)

1995

1995

1995

VNFNPN VIFIPI VIFNPN VIFIPN VIFNPI VNFIPI VNFNPI VNFIPN

3.67 14.50 4.67 10.37 10.53 13.40 7.47 9.40

LSD0.05 CV (%)

0.99 10.77

a

1996 (0.25) (1.00) (0.32) (0.72) (0.73) (0.92) (0.52) (0.65)

5.80 14.90 9.93 11.87 12.67 14.50 11.33 9.33 2.34 11.82

(0.39) (1.00) (0.67) (0.80) (0.85) (0.97) (0.76) (0.63)

1996

0.34 (0.08) 4.48 a (1.00) 1.22 a (0.27) 3.49 a (0.78) 3.74 (0.83) 3.63 (0.81) 1.44 (0.32) 2.70 (0.60) 0.92 20.02

0.03 (0.007) 4.51 (1.00) 0.89 (0.20) 2.00 (0.44) 3.11 (0.69) 3.22 (0.71) 1.51 (0.33) 0.85 a (0.19) 0.97 17.45

0.57 4.03 1.11 2.90 4.79 4.35 1.49 2.95 1.32 17.16

1996 (0.14) (1.00) (0.28) (0.72) (1.19) (1.08) (0.37) (0.73)

1.12 5.88 1.43 2.14 3.99 4.67 2.77 0.65

(0.19) (1.00) (0.24) (0.36) (0.68) (0.79) (0.47) (0.11)

1.49 19.99

Greater PGR as compared to corresponding CGR indicating mobilization of assimilates from vegetative organs to developing pods.

5. Discussion The present work examined several aspects of irrigation response in mung bean. The main focus was on the analysis of yield response to irrigation in terms of radiation interception, its ef®ciency of conversion and harvest index. In addition, pod growth behaviour was examined with a view to explain the observed variations in harvest index. Concurrently, the analysis focused on the effects on the yield formation processes of different irrigation regimes involving all possible combinations of growth stages. It is useful to ®rst analyze yield as the product of total biomass and harvest index and subsequently to consider S, F and RUE as components causing the variation of total biomass. The analysis of yield components showed that the positive yield response to irrigation of mung bean was due to increases of both, the maximum total biomass and the harvest index. Although several researchers have found increased total biomass of mung bean in response to irrigation (Lawn, 1982a; Pandey et al., 1984a; Phogat et al., 1984; Muchow, 1985a; Muchow et al., 1993a; Pannu and Singh, 1993; Haqqani and Pandey, 1994a), there are con¯icting reports about the response of harvest index. In agreement with the present experiment, Pandey et al. (1984b), Chapman et al. (1993a) and Pannu and Singh (1993) have observed a positive response of HI to irrigation. For

contrast, Lawn (1982b) and Trung et al. (1985) observed that HI decreased with irrigation. Total biomass of mung bean responded to both, the number of stages irrigated and the speci®c stages that received irrigation. The response was greatest to irrigation during the ¯owering and/or pod-®lling stages. This response occurred through interaction of the fraction of radiation intercepted (F) and radiationuse ef®ciency (RUE). Irrigating a greater number of stages, including the vegetative stage, enabled mung bean to maximize leaf area index and thereby achieve greater F. Irrigation during the vegetative stage was necessary to maximize LAI, mainly because of the need for adequate water for leaf emergence and expansion (Lawn, 1982a; Squire, 1990). The sensitivity of leaf area index to water de®cits is well documented (Turner, 1986; Passioura et al., 1995) for a wide range of crops, including mung bean (Lawn, 1982a) and other legumes (Neyshabouri and Hat®eld, 1986; Acosta Gallegos and Shibata, 1989; Chapman et al., 1993b; De Costa et al., 1997). In agreement with the ®ndings of the present study, several workers have reported reduced radiation interception under water stress, especially during the vegetative stage (Green et al., 1985; Whit®eld and Smith, 1989; Jamieson et al., 1995). On the other hand, the observation of Chapman et al. (1993b) that drought during the reproductive stage did not reduce radiation interception agreed with results of the present study.

10

W.A.J.M. De Costa et al. / Field Crops Research 61 (1999) 1±12

The signi®cant positive response of RUE to irrigation observed in this study is supported by several other studies (Green et al., 1985; Muchow, 1985b; Muchow, 1989; Whit®eld and Smith, 1989; Chapman et al., 1993b; Jamieson et al., 1995). Whereas irrigation during the vegetative stage was required to maximize F, irrigation during the ¯owering and pod-®lling stages was required to maximize RUE. RUE is a measurement of the ef®ciency of canopy photosynthesis (Norman and Arkebauer, 1991; Loomis and Connor, 1992). The photosynthesis process is highly sensitive to water de®cits (Lawlor, 1995) and the greatest probability of the present mung bean crops experiencing water stress existed during the ¯owering and pod-®lling stages when LAI and, hence, the transpirational demand was maximum. Therefore, the positive effect of irrigation by alleviating water stress effects on photosynthesis and thereby on RUE would be the greatest during ¯owering and pod-®lling. This argument is supported by the observation of signi®cantly low RUE in the VIFNPN treatment which was irrigated only during the vegetative stage. This crop developed a high LAI and was able to achieve a high F. but without irrigation during subsequent stages the crops experienced signi®cant water stress. The RUE values in the well-watered treatments of the present mung bean crops (i.e. 0.34±0.58 g MJÿ1) were much smaller than the maximum value of 0.94 g MJÿ1 reported by Muchow et al. (1993b) for well-watered mung bean. However, that value was estimated by a linear regression between biomass accumulation and cumulative intercepted radiation, whereas the present estimate was the ratio between seasonal totals of biomass and intercepted radiation. Irrigation during ¯owering and pod-®lling stages increased the harvest index through greater pod initiation and higher pod growth rates. It has been observed in a range of legume crops that the number of pods per plant has a clear correlation with ®nal yield under a range of conditions including different water regimes (Lawn, 1982b; Pandey et al., 1984b; Muchow, 1985a; Neyshabouri and Hat®eld, 1986; Pannu and Singh, 1993; Haqqani and Pandey, 1994b). The greater pod growth rates were probably due to the greater availability of reproductive sinks and the greater RUE during the pod-®lling period in the irrigated treatments. Similar results were observed by Chapman et al. (1993c) and Jamieson et al. (1995).

The overall yield response under the different irrigation regimes was the net effect of variations of F, RUE and HI. Results of the present experiments indicate that, for mung bean in the sub-humid zones of Sri Lanka, the positive effects of irrigation during the ¯owering and/or pod-®lling stage on RUE and HI are much greater than the positive effect of irrigation during the vegetative stage on F. Any reduction in F due to water stress during the vegetative stage can be compensated by irrigation during subsequent stages that increase RUE and HI. However, the positive effects on F of irrigation during the vegetative stage are not large enough to compensate for water stress effects on RUE and HI during the subsequent stages. These ®ndings can form the basis of irrigation management to maximize yields of mung bean during the dry season in the sub-humid zones of Sri Lanka. Of general crop physiological importance is the ®nding that mung bean yield in the present environment has a moderately strong, positive linear relationship with RUE independently of S and F (see Tables 6±8). On the other hand, yield is curvilinearly related to HI which must exceed 25% for yield to increase above 500 kg haÿ1. These ®ndings will be important in the general management of mung bean crops in the sub-humid zones of Sri Lanka.

Acknowledgements This work was funded by a research grant from the Sri Lanka Council for Agricultural Research Policy (Grant No. 12/249/199). Mr. G.J.K. De Zoysa is thanked for his ®eld work at Kundasale.

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