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

Plant growth depends upon the net fixation and transport of carbon from the chloroplast to inter-and intracellular sites of photosynthate demand. However, net photosynthate efflux from the chloroplast during photosynthesis may be 30 to 50% less than the CO2 fixation rate due to chloroplast starch formation from newly formed sugar phosphates within the chloroplast (3,8,12,22). The result is a linear increase in foliar starch concentration during illumination that may represent 10 to 30%o of the laminar dry weight by the end of each diurnal photosynthetic period (6,8,16,20).Foliar starch metabolism is the subject of an increasing number of physiological and biochemical studies (3,5,11,16,20) and reviews (13,14,22). However, the relationship between plant growth and this large energy reserve, which is unavailable for meristematic growth during the light period, is uncertain. The observations that diurnal declines in foliar CO2 exchange rates (CER)l have been correlated with elevated starch levels (4,20) support the hypothesis that starch accumulation may be an inefficient process in plant growth.

Starch accumulation rates increased up to five-fold in the leaves of five out of six species examined when plants were shifted from a long (12 or 14-h) to a short (7-h) photosynthetic period. The five species that responded were corn {Zea mays L,), pangola {Digitaria decumbens Stent,), soybean {Glycine max L,) Merr,), spinach {Spinacia oleracea L,), and sugarbeet {Beet vulgaris L,) However, the rate of starch accumulation in alfalfa {Medicago sativa L,) was unaltered following a shift from a long to a short photosynthetic period. We conclude that photosynthate partitioning into chloroplast starch, at least in a wide variety of species, is a programmable process that can be manipulated by altering the length of the daily photosynthetic period.

Photosynthetic oxygen evolutioni by isolated spinach (Spinacia oleracea L.) chloroplasts approached complete inhibition in the presence of a 5 mM concentration of sulfur dioxide. A similar inhibition was observed in the presence of equimolar concentrations of bisulfite ions, suggesting a parallel mode of action. In contrast, an equimolar concentration of sulfite ions was markedly less inhibitory and sulfate ions caused negligible inhibition of apparent photosynthesis. The mode of action of sulfur dioxide and related sulfur anions in inhibiting photosynthesis was found to be essentially independent of direct hydrogen-ion effects.

Two photosynthetic periods and photosynthetic photon flux densities (PPFD) were used to study the relationship between the rate of photosynthesis and starch accumulation in vegetative soybean leaves (Merr. cv Amsoy 71). Plants grown in short daily photosynthetic periods (7 hours) had higher rates of CO(2) fixation per unit leaf dry weight and of leaf starch accumulation than plants grown in long daily photosynthetic periods (14 hours) irrespective of PPFD. CO(2) fixation rates per unit leaf area were similar in 7-hour and 14-hour plants grown at low PPFD but were highest in 14-hour plants at the high PPFD. When single leaves of 14-hour plants were given 7-hour photosynthetic periods, their rates of starch accumulation remained unchanged. The programming of starch accumulation rate and possibly of photosynthetic rate by the length of the daily photosynthetic period is apparently a whole-plant, not an individual leaf, phenomenon. Programming of chloroplast starch accumulation rate by length of the daily photosynthetic and/or dark periods was independent of PPFD within the ranges used in this experiment.