COMPETITION BETWEEN NITROGEN ACCUMULATION AND GRAIN-GROWTH FOR CARBOHYDRATES DURING GRAIN FILLING OF WHEAT

It has been hypothesized that N accumulation and carbohydrate synthesi s compete for energy and carbon skeletons during the reproductive grow th phase of small grain cereals. This study was conducted to determine whether an increased N accumulation during grain filling of wheat (Tr iticum aestivum L.) may reduce carbohydrate availability for grain for mation. In a 2-yr field experiment, four spring wheat genotypes, grown under three early N regimes, were supplied with 0, 50, and 100 kg N h a-1 at heading. The effect of late N application on photosynthesis rat e (only in 1991), water soluble carbohydrates (WSC), grain growth and yield was recorded. N application at heading reduced WSC content of th e shoot at anthesis by 8.5 g WSC per gram of accumulated N. During gra in filling, a steady regeneration of WSC reserves in the stem occurred , though N accumulation was still enhanced by late application of N. B y 320 degree days (d-degrees) after anthesis, differences in stem WSC reserves between late N levels had disappeared. Measurements of photos ynthesis rate and leaf area duration indicated that photosynthetic cap acity of the canopy was increased by application of late N. Apart from the detrimental effect of late tillers which were induced by late N a pplication, late N application generally resulted in higher grain yiel ds. Since N compounds can be invested in light interception (photosynt hesis rate, leaf area duration) before they are translocated to the gr ains, the consumption of energy and carbon skeletons for assimilating N cannot be considered alone as reducing carbohydrate availability for grain formation.