Physiological, Biochemical, and Fluorescence Parameters of Senescing Sugar Beet Leaves in the Vegetative Phase of Growth

The phase of vegetative growth of sugar beet (Beta vulgaris L., single-sprout form) was conditionally subdivided into four periods according to leaf number and size (including already withered leaves): (A) 8 +/- 1 weeks after seedling emergence (wase) (5-7 leaves); (B) 11 +/- 1 wase (10-12 leaves); (C) 14 wase (13-15 leaves); (D) 15 wase (15-18 leaves). It took each next leaf about 1 week to come into view. In the course of leaf senescence, palisade parenchyma became less ordered; cells, vacuoles, and intercellular spaces expanded; leaf area and thickness increased. Chloroplasts became swollen, starch grains (and later osmiophilic globules) accumulated and degraded. In every growth period, the highest levels of soluble carbohydrates (sCH), chlorophyll (Chl (a + b)), soluble protein, and the highest activities of rubisco and soluble carboanhydrase usually preceded the full leaf expansion. In unfolded leaves at the growth period B, the maximum values of biochemical characteristics were as a rule higher than at the growth periods A and C and especially D. The only exception was Chl (a + b) with its peak level somewhat increased with plant age. Occurrence of peak values of individual characteristics depended on plant growth period. These characteristics started diminishing asynchronously, with a minimum in old operational leaves. Only the sCH content in the leaves at the periods C and D was stable. Changes in quantum yield at PSII and nonphotochemical fluorescence quenching reflected the age-associated differences in biochemical characteristics. The results are discussed in the light of the idea that leaf senescence is a normal stage of development directly related to the changes in source-sink relations. Biochemically, this stage comprises the degradation of temporarily stored products and partial utilization of the breakdown products for maintenance of the growth of newly formed leaves and root.