Carbon Assimilation and Translocation in Soybean Leaves at Different Stages of Development

Abstract

Carbon assimilation, translocation, and associated biochemical characteristics of the second trifoiolate leaf (numbered acropetally) of chambergrown soybean, Glycine max (L.) Meff., plants were studied at selected stages of leaf development during the period from 10 to 25 days postemergence. Leaves of uniform age were selected on the basis of leaf plastochron index (LPI).The test leaf reached ful expansion (A..) and maximum CO2 exchange rates on a leaf area basis at 17 days postemergence (LPI 4.1). Maximum carbon exchange rates per unit dry weight of lamina were attained several days earlier and declined as specific leaf weight increased. Chlorophyl and soluble protein continued to increase beyond the attainment of A.., but were not accompanied by further increases in photosynthetic rates. All observations were made on the second trifoliolate leaf (T2) numbered acropetally at six stages of development from 11 to 25 days postemergence. In order to reduce ontogenetic variability, plants were selected for each stage on the basis of plastochron index (Pl)2 (7). The PI system was devised by Erickson and Michelini (7) to provide a precise numerical indication of the stage of development of a vegetative shoot and individual leaves thereon. Briefly, the PI of soybean was calculated as follows: the parameter chosen to indicate leaf size was the midvein length of the terminal leaflet. A reference length of 20 mm was arbitrarily selected. Then, the node number (n) having the youngest leaf with a reference length exceeding 20 mm was determined by counting acropetally from the cotyledonary node. The midvein length of this leaf (Ln), and that of the leaf at the adjacent node above (L+,I) were measured and the PI of the plant calculated from the following relationship: Much of the fixed carbon in leaves isThe leaf plastochron index (LPI) of a leaf at any given node, n, equals PI minus n.Carbon

Effects of Water Stress on Photosynthesis and Carbon Partitioning in Soybean (Glycine max [L.] Merr.) Plants Grown in the Field at Different CO2 Levels

“…Spinach leaf SPS is activated by glucose-6-P and inhibited by Pi (6). In studies with spinach leaf protoplasts, Stitt et al (25) reported that cytosolic [glucose-6-P] decreased as CER was reduced. It is likely that cytosolic [Pi] increased as the concentration of P-esters such as glucose-6-P decreased.…”

“…Thus, metabolic regulation of SPS may coordinate the rate of sucrose formation in the cytosol with the rate of C-assimilation in the chloroplast when CERs are low. However, when CER is high (light-saturated), sucrose formation may be limited by the maximum activity of SPS (12,13,24,25). Consequently, short-term CO2 enrichment of soybean plants did not result in increased rates of export even though CER was increased (14).…”

Effect of Photoperiod on Photosynthate Partitioning and Diurnal Rhythms in Sucrose Phosphate Synthase Activity in Leaves of Soybean (Glycine max L. [Merr.]) and Tobacco (Nicotiana tabacum L.)

“…SPS is subject to metabolic regulation (1,7,10), and furthermore, its maximum activity, as measured in leaf extracts, appears to reflect the capacity of the sucrose biosynthetic path- ' way. For example, the activity of SPS increases during leaf expansion concurrent with increased rates of sucrose formation and export (9,16,21). In fully expanded leaves, SPS activity varies among different genotypes and in general, higher activities of SPS in leaf extracts have been associated with increased partitioning of carbon into sucrose and decreased formation of starch (11-13, 20, 21).…”

Biochemical Basis for Effects of K-Deficiency on Assimilate Export Rate and Accumulation of Soluble Sugars in Soybean Leaves

“…Translocation in K-deficient leaves may be restricted by reduction in the rate of synthetic metabolism in general (5) or sucrose formation in particular (27); entry ofsucrose into the transport pool (1); or inhibition ofsome step involved in phloem loading (16,30). In unstressed soybean leaves, several studies have suggested that the rate of assimilate export may be related to the activity of SPS (7,10,11 25,26). However, studies have not been conducted with soybean plants to determine the relative impact of K-nutrition on export rate with respect to CER, or SPS activity.…”