Rising CO₂ levels and their potential significance for carbon flow in photosynthetic cells

Abstract

Abstract. In the first part of this review, I discuss how we can predict the direct short‐term effect of enhanced CO2 on photosynthetic rate in C3 terrestrial plants. To do this, I consider: (1) to what extent enhanced CO2 will stimulate or relieve demand on partial processes like carboxylation, light harvesting and electron transport, the Calvin cycle, and end‐product synthesis; and (2) the extent to which these various processes actually control the rate of photosynthesis. I conclude that control is usually shared between Rubisco (which responds sensitively to CO2) and other components (which respond less sensitively), and that photosynthesis will be stimulated by 25–75% when the CO2 concentration is doubled from 35 to 70 Pa. This is in good agreement with the published responses. In the next part of the review, I discuss the evidence that most plants undergo a gradual inhibition of photosynthesis during acclimation to enhanced CO2. I argue that this is related to an inadequate demand for carbohydrate in the remainder of the plant. Differences in the long‐term response to CO2 may be explained by differences in the sink‐source status of plants, depending upon the species, the developmental stage, and the developmental conditions. In the third part of the review, I consider the biochemical mechanisms which are involved in ‘sink’ regulation of photosynthesis. Accumulating carbohydrate could lead to a direct inhibition of photosynthesis, involving mechanical damage by large starch grains or Pi‐limitation due to inhibition of sucrose synthesis. I argue that Pi is important in the short‐term regulation of partitioning to sucrose and starch, but that its contribution to ‘sink’ regulation has not yet been conclusively demonstrated. Indirect or ‘adaptive’ regulation of photosynthesis is probably more important, involving decreases in amounts of key photosynthetic enzymes, including Rubisco. This decreases the rate of photosynthesis, and potentially would allow resources (e.g. amino acids) to be remobilized from the leaves and reinvested in sink growth to readjust the sink‐source balance. In the final part of the review, I argue that similar changes of Rubisco and, possibly, other proteins are probably also involved during acclimation to high CO2.

The growth and gas exchange response of soil‐planted Norway spruce [Picea abies (L.) Karst.] and red oak (Quercus rubra L.) exposed to elevated CO2 and to naturally occurring drought

“…Considering that this was the second year of growth in elevated COj, the red oak trees appear to have kept the enhanced levels of photosynthesis, that have often been regarded as transitory (e.g. Stitt, 1991;Ryle, Powell & Tewson, 1992;Johnsen, 1993). Wolfe (1994) has discussed how when plants are grown in the field with unrestricted rooting volume, the increased photosynthetic capacity is more likely to be maintained.…”

Hexokinase as a sugar sensor in higher plants.

“…Elevated sugar levels cause stunted growth, reduced photosynthesis, enhanced anthocyanin accumulation, and curled, chlorotic, and necrotic leaves (Casper et al, 1986;von Schaewen et al, 1990;Dickinson et al, 1991;Sonnewald et al, 1991Tsukaya et al, 1991;Huber and Hanson, 1992). In addition, environmental factors, such as elevated C02 (Stitt, 1991;Stitt et al, 1991;van Oosten et al, 1994;Nie To whom correspondence should be addressed at Department of Molecular Biology, Massachusetts General Hospital, Wellman 11 17, Boston, MA 021 14. et al, 1995), and intrinsic genetic variations, such as different invertase levels (Goldschmidt and Huber, 1992), have been proposed to affect photosynthetic capacity through sugar regulation.…”

An Arabidopsis mutant showing reduced feedback inhibition of photosynthesis

“…*Current address: Department of Molecular Genetics, John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK. plant (for review see Sheen, 1994;Stitt, 1991). In particular, photosynthesis is inhibited when the production of photosynthates exceeds the rate of utilization by sinks, a sink being an organ which utilizes or stores carbohydrate generated by other parts of the plant.…”

“…Such plants have a reduced export capacity and the leaves of these plants have high levels of soluble carbohydrates which correlated with a reduction in photosynthetic capacity. Photosynthetic acclimatization to elevated CO2 leading to a loss of photosynthesis can also be understood in terms of carbohydrate accumulation under sink limitation (Stitt, 1991;Van Oosten and Besford, 1996).…”

“…This non-destructive screening procedure allowed the isolation of a number of potential sun (sucrose-uncoupled) mutants. However, it has been shown previously that the feedback regulation of photosynthesis by soluble carbohydrate is dependent on the developmental stage of the tissue studied (Stitt, 1991). These mutants were selected in a screen on dark-germinating seedlings.…”