Response of an Indeterminate Soybean {Glycine Max (L.) Merr} to Chronic Water Deficit During Reproductive Development Under Greenhouse Conditions

Abstract

Although crops are commonly exposed to chronic moderate water deficit stress, most research has been focused on catastrophic and extreme moisture deficit stress. A greenhouse experiment was conducted to: (i) assess the impact of chronic water stress, during reproductive development, on soybean growth and productivity; (ii) determine the effects of water stress on soybean canopy architecture; and (iii) evaluate changes in canopy reflectance due to water deficit. Soybean plants were grown under three moisture regimes: daily watering at 100% (control), 50% (medium stress W2) and 25% (severe stress W1) of evapotranspiration. Water stress during reproductive development decreased plant physiological activity, vegetative growth, and productivity and had a visible impact on plant canopy architecture at both stress levels. Water deficit reduced leaf chlorophyll content at the severe stress level. At the physiological level, the major effect of chronic water stress was a decrease in stomatal conductance. Water stress decreased the rate of plant vegetative growth for all variables measured, suggesting that water deficit during reproductive growth might have induced an early switch of plant development from vegetative to reproductive. Water deficit, at W1 and W2, did not affect the pod dry weight to total canopy dry weight ratio. This suggested that the proportional allocation of dry matter to reproductive structures was maintained and, although the stress was imposed during reproductive development, vegetative and reproductive structures were equally affected. This may be a function of the indeterminate nature of the soybean cultivar. Paraheliotropism behaviour was observable at W2, but not at W1. Thus, there might be a threshold stress level beyond which leaf movement cannot be activated, preventing control of transpiration through canopy movement. Water stress affected the spectral reflectance signature of soybean leaves, increasing leaf reflectance in the visible and decreasing it in the infrared spectral ranges at both stress levels.

Bien que les cultures soient communment exposes la scheresse chronique modre, la recherche se concentre sur les dficits hydriques catastrophiques et extrmes. Dans cet article, nous dcrivons une exprimentation conduite en serre pour : (i) valuer l'impact du dficit hydrique sur la croissance et productivit du soja (Glycine max (L.) Merr.); (ii) dterminer les effets du stress sur l'architecture de la canope du soya; et (iii) valuer l'effet du stress sur la rflectivit de la canope. Les plantes de soja furent cultives sous trois rgimes d'irrigation : arrosage journalier 100% (contrle W3), 50% (stress moyen W2) et 25% (stress svre W1) de l'vapotranspiration. Le stress hydrique diminua l'activit physiologique, la croissance vgtative, et productivit du soja, et affecta l'architecture de la canope aux deux niveaux de stress. Le stress hydrique a diminu le contenu foliaire en chlorophylle pour le stress svre. Au niveau physiologique, l'effet majeur du stress fut la diminution de la conductance stomatique. Le stress a rduit le taux de croissance vgtative, suggrant qu'un dficit hydrique durant la croissance reproductive du soja pourrait induire un changement du dveloppement vgtatif au dveloppement reproductif. Le dficit hydrique n'a pas affect le ratio du poids sec de cosse sur le poids sec total de la canope. Ceci suggra que l'attribution proportionnelle de matires sche aux structures reproductives fut maintenu, et que, bien que le stress fut impos durant la priode reproductive, les structures vgtatives et reproductives furent affectes dans la mme ampleur. Ceci peut tre du la nature indtermine du soja. Le comportement parahliotropique fut observ W2, mais pas W1. Ainsi, il existe peut tre un niveau seuil au-del duquel le mouvement foliaire n'est plus activable, empchant le contrle de la transpiration par le mouvement de la canope. Le stress hydrique a affect la signature spectrale des feuilles de soya, augmentant la rflectivit foliaire dans le visible et la diminuant dans l'infrarouge aux deux niveaux de stress.