Effect of pattern of water supply on Vicia faba L. 2. Pod retention and filling, and dry matter partitioning, production and water use.

Pod retention and pod filling of faba beans were studied under different patterns of water supply. Mild water shortage during flowering, followed by plenty of water after flowering (d-i), resulted in high seed yields at lower stem nodes (defined as the first podding node to node number 11) in cv. Alfred. The inverse treatment (i-d: plenty of water during flowering, followed by increasing water shortage after flowering), but also i-i (plenty of water during and after flowering), showed 20-60 % lower seed yields at those nodes. This effect was main­ ly due to a lower number of pods per node. In i-i, but not in i-d, the low pod retention at lower nodes was compensated at higher nodes (defined as node 12 to the last podding node). These results help to explain the mechanism of the interaction between water supply pattern and the development of reproductive sinks. The quantitative consequences of these effects on the relation between total water use and seed yield are discussed. Without taking into ac­ count different water supply patterns, a linear relation between total water use (represented by total dry matter production) and seed yield explains already 75-85 % of the variation in seed yield. If different water supply patterns are included in the regression analysis, more than 90 °!o of the variation in seed yield can be explained. The i-i patterns, compared to d-i, result in sub-optimum dry matter partitioning to reproductive organs, but show a smaller seed yield variability. This indicates that defining and maintaining the optimum level of (mild) water shortage under varying climatological conditions needs further attention.