Effect of surplus glucose on physiological and biochemical characteristics of sugar beet leaves in relation to the age of the leaf and the whole plant

Effect of surplus glucose on physiological and biochemical parameters of leaves of different age was investigated in sugar beet (Beta vulgaris L., subsp. saccharifera) plants in the stages of vegetative growth (SVG). Early and late SVG were differentiated by the ratio between the weights of roots and aboveground organs (0.10 and 0.35, respectively). The excess of Glu was produced by incubation of the disks excised from detached leaves in water or 0.1 M Glu at radiant flux density of 250 μmol/(m2 s) with the light regime pattern described as night/day/night/light (8/16/8/3 h). In all the leaf disks incubated in water and glucose solution, the content of Glu and other soluble carbohydrates considerably increased as compared with their content in the leaves they were taken from. After disk incubation in water and glucose solution, the content of chlorophyll (a + b) rose as compared with its level in respective leaves in early SVG; in late SVG, it declined. In early SVG, the rate of the O2 photosynthetic evolution (Ph) in the ageing leaves under saturating concentration of NaHCO3 after incubation in water and Glu solution declined more considerably than in young leaves. In late SVG, incubation of leaf disks in water and Glu solution weakly affected Pn. The rate of O2 dark consumption in the leaf disks of all the types of treatment increased after incubation in water and especially in Glu solution. Activity of soluble carbonic anhydrase (sCA) in the extracts from young leaves in early SVG after their incubation in water and Glu solution was essentially the same, but after the incubation of ageing leaves in Glu solution, it reliably decreased. In late SVG, sCA activity sharply decreased after incubation in water and Glu solution irrespective of the leaf age. In late SVG, activity of Rubisco in the young leaves did not change after their incubation in water but decreased after incubation of the leaves of the three ages in Glu solution. In early SVG, nonphotochemical fluorescence quenching (NPQ) in the young intact leaf was lower than in the ageing leaf, and after leaf incubation in water and Glu solution, it rose. In late SVG, the value of NPQ was greater than in early SVG and, in contrast to the leaves of early SVG, it declined after leaf incubation; in water, this decline was more pronounced than in the Glu solution. In early SVG, efficient quantum yield of photosystem II (PSII) was much greater than in late SVG and it declined in the leaves incubated with Glu. It was concluded that surplus Glu can maintain biosynthetic processes in the young leaves of young sugar beet plants (trophic function). A decline in the level of chlorophyll and the activities of sCA and Rubisco in the course of leaf development and senescence is considered as a symptom of the suppression of biosynthesis of proteins of chlorophyll-protein complexes and the enzymes (Rubisco and sCA).