Growth Kinetics, Carbohydrate, and Leaf Phosphate Content of Clover (Trifolium subterraneum L.) after Transfer to a High CO2 Atmosphere or to High Light and Ambient Air

Abstract

Intact air-grown (photosynthetic photon flux density, 400 microeinsteins per square meter per second) clover plants (Trifolium subterraneum L.) were transfered to high CO(2) (4000 microliters CO(2) per liter; photosynthetic photon flux density, 400 microeinsteins per square meter per second) or to high light (340 microliters CO(2) per liter; photosynthetic photon flux density, 800 microeinsteins per square meter per second) to similarly stimulate photosynthetic net CO(2) uptake. The daily increment of net CO(2) uptake declined transiently in high CO(2), but not in high light, below the values in air/standard light. After about 3 days in high CO(2), the daily increment of net CO(2) uptake increased but did not reach the high light values. Nightly CO(2) release increased immediately in high light, whereas there was a 3-day lag phase in high CO(2). During this time, starch accumulated to a high level, and leaf deterioration was observed only in high CO(2). After 12 days, starch was two- to threefold higher in high CO(2) than in high light, whereas sucrose was similar. Leaf carbohydrates were determined during the first and fourth day in high CO(2). Starch increased rapidly throughout the day. Early in the day, sucrose was low and similar in high CO(2) and ambient air (same light). Later, sucrose increased considerably in high CO(2). The findings that (a) much more photosynthetic carbon was partitioned into the leaf starch pool in high CO(2) than in high light, although net CO(2) uptake was similar, and that (b) rapid starch formation occurred in high CO(2) even when leaf sucrose was only slightly elevated suggest that low sink capacity was not the main constraint in high CO(2). It is proposed that carbon partitioning between starch (chloroplast) and sucrose (cytosol) was perturbed by high CO(2) because of the lack of photorespiration. Total phosphate pools were determined in leaves. Concentrations based on fresh weight of orthophosphate, soluble esterified phosphate, and total phosphate markedly declined during 13 days of exposure of the plants to high CO(2) but changed little in high light/ambient air. During this time, the ratio of orthophosphate to soluble esterified phosphate decreased considerably in high CO(2) and increased slightly in high light/ambient air. It appears that phosphate uptake and growth were similarly stimulated by high light, whereas the coordination was weak in high CO(2).