Abstract
Water deficit is a major factor limiting crop yield in rainfed areas. We hypothesized that under water deficit the decrease of photosynthetic production stimulates: carbohydrate remobilization from leaves, stems and roots to reproductive organs; and decreasing flowering intensity and pod development. The present work aims to study the effect of water deficit during bloom and grain pod-filling stages in two indeterminate soybean cultivar, Vtop and Nidera. The following physiological parameters were evaluated by means of daily CO2 assimilation rate (Ai), dynamic of carbohydrates in tissues, plant growth, grain yield and yield components. The study was conducted in a greenhouse with plants sown in tanks of 0.5 m3. Regardless of the phenological phase, water deficit reduced Ai, plant growth and number of pods and seeds per plant. The fact that grain yield was less affected by water deficit at bloom than at grain pod-filling stage was attributed to larger seeds found at bloom. In both treatments, a sharp reduction on carbohydrate content was found in leaves, stem and roots at the beginning of pod formation. The high amounts of carbohydrates remobilized for seed growth, along with the high values of Ai observed in well-watered plants, indicate that grain yield of soybeans is source rather than sink limited. On the other hand, in water deficit treatments, a new stimulus for carbohydrate storage was found in the leaves and stem at the beginning of grain maturity, suggesting that grain yield was limited by sink capacity.
Highlights
The dynamic of carbon assimilation in plants is functionally controlled by a source-sink relationship (Ainsworth et al 2004; Ribeiro et al 2012)
Effects of water deficit on soybean productivity have been attributed to impairment of yield components mainly when water stress occurs during reproductive stages (Egli et al 1976, 1983; Chaves et al 2002)
Three treatments were applied as follows: T1, control, in which plants were kept in wellwatered conditions during the entire cycle; T2, water deficit imposed during the bloom stage from R2 to R5, i.e., between 38 and 51 days after emergence (DAE); and T3, water deficit during the pod-filling stage from R5.1 to R6, i.e., between 51 and 72 DAE
Summary
The dynamic of carbon assimilation in plants is functionally controlled by a source-sink relationship (Ainsworth et al 2004; Ribeiro et al 2012). Under water deficit the remobilization of carbohydrates from storage tissues to reproductive structures during bloom stages could be crucial for pods and seeds formation in soybean (Westgate and Peterson 1993; Liu et al 2003, 2004a), once photosynthesis is usually the predominant source of photoassimilates for pod and seed growth (Christy and Porter 1982) As it has been shown for maize and soybean, carbohydrates accumulated in the leaves and stem are an important source of carbon, helping to supply the high demand for photoassimilates by the growing seeds and pods (Westgate and Thompson 1989; Machado et al.1992; Liu et al 2004b). The physiological response was evaluated via analysis of the following parameters: daily CO2 assimilation rate, plant growth and determination of carbohydrate content in leaves, stems and roots
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