Changes in Photosynthesis, Ribulose Bisphosphate Carboxylase, Proteolytic Activity, and Ultrastructure of Soybean Leaves during Senescence¹

Abstract

Changes in photosynthesis,rlbulose bisphosphate carboxylase (RuBPCase), and proteolytic activity were followed in the leaves of field-grown soybeans [Giycine max (L.) Merr. cv. Kent] from flowering through senescence. These parameters were followed in relation to changes in leaf resistance, chlorophyll, protein, starch, total N levels, and seed development. In addition, changes in leaf ultrastructure were observed. The initial symptoms of senescence (evident 3 to 4 weeks after flowering) were a decline in photosynthesis, chlorophyll, and total leaf N and an increase in proteolytic activity. Preceding these changes there was a swelling of the chloroplasts and a disorientation of the chloroplast lamellae, possibly resulting from the apparent increase in starch deposition. Also, large numbers of osmiophilic granules appeared within the chloroplasts.These changes were evident prior to the time the seed entered its most rapid period of growth which was 4 to 7 weeks after flowering, The initial decline in photosynthesis did not appear to be due to an increase in leaf resistance or a decline in RuBPCase activity or level. The decline in protein levels began between 5 and 6 weeks after flowering and was paralleled by the decline in carboxylase activity and level. Associated with these changes were an increase in the size of the osmiophilic granules within the chloroplasts, a decrease in the number of chloroplasts with a corresponding increase in the apparent cellular breakdown products, and a dissolution of the vacuoles. No large increase in leaf resistance or change in specific activity of carboxylase was observed until late in senescence.

Phosphorus Nutrition Influence on Leaf Senescence in Soybean

“…Leaf P concentration did not increase until the soil P level exceeded that required for maximum growth. Leaf senescence is often characterized by a large decrease in leaf N during the seed filling period (2,3,7,19,28,29). For all soil P treatments used in this study, there was a large decline in N concentration during the seed filling period (Fig.…”

Characterization of Amino Acid Efflux from Isolated Soybean Cells

“…We observed that net amino acid efflux was generally about 0.250 nmol amino-N.min-'. 10 (15,18,20,28). Thus, our experimental system seems to be physiologically relevant to whole-plant studies.…”

Programmed cell death during plant growth and development

“…Naturally occurring leaf senescence is an orderly, and sometimes slow, process. For example, the near complete loss of chlorophyll from senescing soybean leaves, which begins approximately 4 weeks after flowering, is accomplished over a 6 week period (Wittenbach et al, 1980). During soybean leaf senescence, approximately 80% of starch and 90% of soluble protein are lost from the leaves.…”

“…Numerous biochemical characterizations of increased proteolytic activity during senescence have been presented (Matile and Winkenbach, 1971;Martin and Thimann, 1972;Drivdahl and Thimann, 1977;Wittenbach et al, 1980Wittenbach et al, , 1982Miller and Huffaker, 1982;and reviewed by Feller, 1986), including evidence for activity of both thiol (Miller and Huffaker, 1982;Drivdhal and Thimann, 1977) and serine proteases (Miller and Huffaker, 1982). Further support for an important role for proteases in leaf senescence has come from the identification of senescence-related genes.…”