Effect of water stress during floral initiation, flowering and podding on the growth and yield of faba bean (Vicia faba L.)

Faba bean (Vicia faba L.) is currently being adopted as a useful rotation crop on fine-textured or duplex, neutral to alkaline soils in the low rainfall Mediterranean-type environments of south-western Australia. However, seed yields in these environments are often limited by water stress. The aims of this study were to investigate the effect of water stress on growth and yield of three contrasting faba bean genotypes (ACC286, Fiord and Icarus) under glasshouse conditions and to identify any critical stages of development when faba bean is particularly sensitive to water deficits. The stress treatments were imposed by withholding water at floral initiation, 50% flowering and 50% podding, while monitoring the levels of photosynthesis, stomatal conductance, leaf water potential and leaf extension. A well-watered unstressed control was also included. The early podding stage of development was the most sensitive to water deficit in faba bean, causing a reduction in harvest indices and seed yields of at least 50% in all three genotypes. In contrast, genotypes showed a better ability to recover from stress at floral initiation and first flower stages than at early podding. As water deficits developed, leaf water potential decreased, leaves lost turgor and leaf area was reduced dramatically due to wilting. Leaflets were unable to expand and stomatal conductance and photosynthesis declined to about 50 mmol m(-2) s(-1) and 6 micromol m(-2) s(-1), respectively. While stomatal conductance and photosynthesis recovered after rewatering, leaf size was reduced permanently, especially with the stress treatment at podding. In addition, some lower leaves failed to recover turgor after rewatering in the stress treatment at podding, and this advanced leaf senescence, especially in Icarus. The genotype ACC286 appeared to cope better with moisture stress at floral initiation, first flowering and early pod set, than the other genotypes. ACC286 produced more than twice the seed yield of Fiord and Icarus in the stress treatment at podding, partly because total dry matter was not reduced as severely and there was no reduction in the number of seeds per pod. This study suggests there is considerable variation amongst faba bean genotypes in tolerance to water stress, and that maximising the number of flowers, pods and seeds is among the most important traits for maintaining stable and high seed yields under water deficit conditions.