RESPONSE OF WHEAT CULTIVARS TO DIFFERENT SOIL-NITROGEN AND MOISTURE REGIMES .1. DRY-MATTER PARTITIONING AND ROOT-GROWTH

The balance between root and shoot growth is one of the mechanisms use d by plants to adapt to a particular environment. This balance is affe cted by nutrient supply and water availability. The objective of this study was to understand how nitrogen (N) affects root and shoot growth of three cultivars of wheat (Triticum aestivum L.) grown under two re gimes of soil moisture ('non water stressed' and 'stressed' during ste m elongation') in the greenhouse and growth chamber. Data showed that before stem elongation, shoot growth was less sensitive to high soil N levels than root growth. In fact, root growth was inhibited by excess ive soil N concentration. The cultivar 'Nesma' produced more dry matte r, absorbed N more rapidly and avoided the negative effect of high soi l N concentrations on root growth. 'Merchouch 8' produced less dry mat ter and tolerated more water stress, and appeared to require less wate r. Severe water stress masked the effect of N on root and shoot growth , and the negative effect of water stress on growth was more important under high N. Plants which were water stressed during stem elongation could not fully recover when they were rewatered from boot stage to a nthesis. Nitrogen application improved shoot but not root growth under this soil moisture situation. This study suggested that shoot and roo t growth were affected differently by N availability and that high soi l N levels enhanced the negative effect of water stress on growth.