Photosynthetic Carbon Metabolism in Leaves and Isolated Chloroplasts from Spinach Plants Grown under Short and Intermediate Photosynthetic Periods

Abstract

ABSTRACIResponses of folar and isolted intact chloroplast photosynthetic carbon metabolism observed in spinach (Spinacia okracea cv Wisconsin Bloomsdal) plants exposed to a shortened photosynthetic peried (7-hour light/17-hour dark cycle), were used as probes to examine in vivo metabolic factors that exerted rate determination on photosynthesis (PS) and on starch synthesis. Compared with control plants propated contiuosly on a 12-hour light/12-bour dark cycle, 14 to 15 days were requied, subsequent to a shift from 12 to 7 honrs dayength, for 7-howr plants to begin to grow at rates comparable to those of 12-hour daylegth plants. Became of shorter daily dtions of PS, daily demand for photosynthate by growth processes appeared to be greater in the 7-hour thano in the 12-how plants. The resut was that 7-hour plants established a 1.5-to 2.0-fold higer total PS rate than 12-hour phats.Inact chioroplasts isolted from the baves of7-hour plants (7-h PLD) displayed 1.5-to 2.0-fold higher PS rates than plastids isolated from 12-hoor plants . Plastid lamellae prep from 7-and 12-h PID isolates displayed equivalnt rates of ferredoxin-dependent Al? and NADPH photoformation d ng that eler transport processes were not factors in the establishment of higher 7-h PLD PS rates. Analyses, both in kaves as well as intact PID isolates, of dark to light transitional inceases in Calvin cycle intermediates, eg., ribulose-1,5-bisphosphate (RuBP) and 3-phospboglycerate (3-PGA), as well as estimations of activities of RuBP carboxylase and fructose-1,6-bispbosphate phosphatase, indcated that 7-hour plant leaves displayed higher PS rates (tha 12-how plants), became there was a higher maitude of activity of the Calvin cycle.Alhough both the folar level of starch and sucrose, as well as starch synthesis rate, often was higher in 7-howr compared with 12-howr plant folg, the higher 7-hour plt total PS rates incated that maximal suose and starch leels did not mediate any 'feedback' inhibition of PS. The higber 7-hour plant foliar and PLD PS rates resulted in higher gluose-l-P leels as well as a higher ratio of 3-PGA:Pi, both factors of which would enance the activity of chloroplast ADP-glcose pyrophospboryblse, and which were attributed to be casal to the highe starch synthesis rates observed in 7-hour plant foliage and PLD isolates.Leaf photosynthetic rate may be regulated by the requirement for photosynthate, i.e. the 'demand' for photoassimilate, exerted by the growth and development of sink organs in the higher plant (8, 11). Silvius et al. (28)

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Alterations in Growth, Photosynthesis, and Respiration in a Starchless Mutant of Arabidopsis thaliana (L.) Deficient in Chloroplast Phosphoglucomutase Activity

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Photosynthate Metabolism in the Source Leaves of N2-Fixing Soybean Plants

“…Released glucose from starch (amyloglucosidase) or from sucrose (invertase) fractions were converted enzymically to glucose-6-P to be estimated with glucose-6-P dehydrogenase (4,22). Consequently, starch content is expressed in terms of glucose equivalents (4).…”

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