Agronomie 11 (1991) 737-746
DOI: 10.1051/agro:19910904

Water deficits during reproductive growth of soybeans. I. Their effects on dry matter accumulation, seed yield and its components

JM Andriani, FH Andrade, EE Suero and JL Dardanelli

EEA Balcarce, Instituto Nacional de Tecnologia Agropecuaria (INTA), Facultad de Ciencias Agrarias, Univ Nac de Mar Del Plata, CC 276, (7620) Balcarce, BS AS, Argentina

Abstract - The objective of this study was to determine the effect of water deficits during the reproductive period of an indeterminate soybean (Glycine max (L) Merr) crop on growth, seed yield and its components, and to establish whether the R₁-R₄ or the R₄-R_6.3 period was the most sensitive to drought. Group maturity III cultivar Asgrow 3127 was planted in the 1986-1987 and 1987-1988 growing seasons on a loamy soil (typic argiudol). The trial consisted of 3 deficiency treatments starting with 50% of soil-available water at the soil depth explored by the roots at the time of initiation, and with a control (II) constantly maintained at > 50% of soil available water. The treatment drought periods were: 0I) from R₁ to R₄; 10) from R₄ to R _6.3; and 00) from R₁ to R₄ and from R₄ to R_6.3. Plots were individually irrigated and during the drought periods protected from rainfall with plastic covers. The control treatment (II) had greater dry matter production than 0I and 10, and these 2 treatments produced more dry matter than 00. Drought from R₁ to R₄ decreased leaf area production and drought from R₄ to R_6.3 accelerated leaf senescence. Leaf area index was related to percent solar radiation interception showing a critical LAI of = 5.5. Consequently, water deficits affected solar radiation interception. The reduction in PAR intercepted by the crop due to water deficits was no greater than 12%. The utilization efficiency of the intercepted PAR that ranged from 1.5-1.9 g per MJ was clearly affected by the drought. Drought during the first period affected the vegetative growth with little effect on seed yield. This effect was reflected on harvest index and on dry matter remobilization efficiency. On the other hand, the reduction in crop growth rate induced by drought during the R₄-R_6.3 period was associated with important decreases in the number of reproductive structures per unit area as well as in total vegetative dry matter. The number of pods/ha was the yield component most affected by the treatments. For 0I, the number of seeds/pod and the weight of the seeds compensated the reduction in number of pods, relative to the control.

Résumé - Déficit hydrique durant la phase reproductive du soja. I. Ses effets sur l'accumulation de la matière sèche, le rendement en grain et ses composants. L'effet d'un déficit hydrique durant la phase reproductive du soja a été étudié pendant 2 ans dans une expérience en conditions contrôlées en plein champ. Les traitements ont consisté en : II) un témoin irrigué, et 3 périodes d'imposition de sécheresse, à savoir : 01) sécheresse de R₁ à R₄; 10) sécheresse de R₄ à R_6.3; et 00) sécheresse de R₁ à R₄ et de R₄ à R_6.3. Le témoin irrigué produisit une moyenne de 12 t de matière sèche/ha et 5 t de grains/ha. Une sécheresse imposée de R₁ à R₄ provoque une réduction de la surface foliaire et de la croissance végétative, avec seulement un léger effet sur le rendement en grain. Au contraire, une sécheresse imposée entre R₄ et R_6.3 diminue la production de matière sèche végétative et de graines. Le nombre de gousses/m² est la composante du rendement qui est la plus affectée par les traitements. En général, une augmentation du poids des graines compense dans le rendement la réduction du nombre de gousses.

Key words: water deficit / Glycine max (L) Merr / dry matter production / seed yield / yield component / harvest index / leaf water potential / leaf area index / interception of radiation

Mots clés : déficit hydrique / Glycine max (L) Merr / production de matière sèche / rendement en graines / composantes du rendement / indice de récolte / potentiel hydrique de la feuille / indice de surface foliaire / radiation interceptée