EFFICIENCY OF BIOMASS ACCUMULATION BY SUNFLOWER AS AFFECTED BY GLUCOSE REQUIREMENT OF BIOSYNTHESIS AND LEAF NITROGEN-CONTENT

The efficiency of biomass accumulation of sunflower (Helianthus annuus L.) may decrease after anthesis as a result of decreased leaf nitroge n content and increased glucose requirement for biosynthesis. The gene rality of this phenomenon was examined in a field experiment conducted in 1994 near Temple, Texas with three levels of nitrogen supply (0, 1 14, and 228 kg N ha(-1)). Intercepted photosynthetically active radiat ion (IPAR) and plant biomass measurements were made on nine sampling d ates from the head-visible stage to physiological maturity. Radiation- use efficiency (RUE) was evaluated as the slope of the relationship of dry matter to cumulative IPAR. RUE decreased after 17 days after the end of anthesis, but was not significantly affected by leaf nitrogen c ontent. Thus, in the condition of this experiment, a decrease in speci fic leaf nitrogen is unlikely to explain the decline in RUE late in th e season. TDM expressed in g of glucose increased linearly with cumula tive IPAR until physiological maturity. This suggests that the decreas e in the efficiency of biomass accumulation resulted from an increase in glucose requirement of biomass.