THE INTERACTION OF SOWING DATE AND WATER AVAILABILITY IN DETERMINING PLANT ARCHITECTURE, FRUITING PATTERN AND YIELD COMPONENTS OF SOYBEAN (GLYCINE-MAX L MERR)

A 2-year trial has been carried our in northern Italy on soybean (cv. Hodgson) grown in lysimeters, comparing three soil water regimes (well -watered conditions and water stress during vegetative and reproductiv e stages) at two sowing dares. Plant evapotranspiration and water upta ke depth were calculated from volumes of water independently supplied to eight lysimeter layers; at harvest, plant architecture, yield compo nents and fruit distribution along the main stem and lateral branches were evaluated. Although water stress intensity was not severe, crop e vapotranspiration and water uptake depth were severely restricted by w ater shortage. Both low water-availability and late sowing significant ly modified the architecture of plants, decreasing total height, numbe r and length of internodes and lateral branches. Seed allocation along the stem was shifted downwards both by delaying the sowing date and b y reducing the water supply; the component most responsible for yield decrease was the number of pods per plant, while unit seed weight was only slightly affected by water stress. Grain yield reduction was high er when water availability was inadequate during the reproductive phas e in the early-sown crop and during the vegetative stage in the late-s own crop. This suggests that the intensity of the water shortage, plan t phenological stage of stress application, as well as the date of str ess application within the growing season determine the yield response of soybean.