Dependence of Net Assimilation Rate on Root Growth of Isolated Leaves

Abstract

The phenomenon of inhibition of assimilation by carbohydrate accumulation has been reinvestigated using single rooted leaves of dwarf bean. Such a system has the advantage that assimilating area remains constant and carbohydrate is translocated to a single sink-the root system. The net assimilation rate of the system did not vary with season and was small compared with intact plants in the summer, suggesting that an internal factor controls assimilation rate. Evidence is given that this factor is the rate of translocation of carbohydrate from source to sink (from lamina to root) which in turn depends on growth-rate of the root system. The evidence came from experiments in which root growth was stimulated by increasing vessel size, by preliminary treatment with IAA or by raising the root temperature, or was retarded by kinetin treatment. In experiments at glasshouse temperature, lamina dry matter increased by 0.75 mg. per cm.² per week. The maximum values attained depended on the time of the experiment. In August it was greater than 7.0 mg. per cm.¹, but there was a seasonal trend. In experiments with roots at 24°C, carbohydrate accumulated in the lamina more slowly than at lower temperatures.