Variability of key biochemical parameters in senescent leaves of sugar beet at nitrate deficiency and surplus accumulation of glucose

Parameters of sugar beet (Beta vulgaris L.) leaf senescence were investigated in the stage of vegetative growth in plants grown at normal nitrate level (N) or under its deficiency (DefN). Accelerated senescence was initiated by the 41-h-long exposure of leaf discs on the surface of water with alternating darkness and light. In plants grown at DefN, the number of leaves and their average area decreased; after the incubation of the discs from such leaves on water, the content of soluble carbohydrates (sCarb) and especially of glucose sharply increased as compared with normal level of nitrate (N), whereas the content of soluble protein (sProt) and Rubisco activity considerably decreased, which is characteristic of the negative hexokinase (HXK1) effect of glucose. The rate of a decrease in the content of sProt in the course of leaf senescence calculated for the leaf of each strorey was lower than the rate of a decrease in RuBisCO activity. A decrease in the content of sProt and RuBisCO activity in all the storeys of leaves grown at joint action of nitrate deficit (DefN) and incubation on water was on the average greater than in each of these treatments separately but less than the sum of these effects. The imperfection of the putative mechanism of signal transduction at DefN and excess glucose and their interaction in senescent sugar beet leaves is discussed as well as the opportunity to use the ratio between sCarb and sProt for the evaluation of the manifestation of the negative hexokinase effect of glucose.