SHORT-TERM EFFECT OF NITRATE ON CARBON METABOLISM OF 2 SUGAR-CANE CULTIVARS DIFFERING IN THEIR BIOMASS PRODUCTION

The effect of nitrate (NO3-) on net sucrose synthesis and phosphoenolp yruvate activity was studied using two cultivars of the C-4-plant Sacc harinum officinarum, Jaronu 60-5 (Ja 60-5) and Barbados 63-118 (B 63-1 18), high- and low-biomass producers respectively. Leaf segments were treated with a N-free solution, or 40-80 mM NO3- in the light for 90 m in. Rates of NO3- uptake and assimilation were lower in Ja 60-5 than i n B 63-118. In Ja 60-5 net sucrose synthesis was high and drastically reduced by NO3-, while in B 63-118 it was lower and stimulated by NO3- assimilation. In both cultivars, the main effect of NO3- on phosphoen olpyruvate carboxylase was to reduce its sensitivity to malate. It is suggested that the relative capacities for NO3- assimilation and sucro se synthesis may be a factor involved in the control of sucrose synthe sis by NO3- and in the rapidity in which C metabolism is adjusted to N assimilation.