The control of leaf photosynthesis rate by the level of assimilate concentration in the leaf: A review of the hypothesis

1. Radioactive products in detached leaf segments were examined after periods of steady-state photosynthesis in (14)CO(2). 2. After exposure to (14)CO(2) for approx. 1sec. more than 93% of the fixed radioactivity was located in malate, aspartate and oxaloacetate. After longer periods large proportions of the radioactivity appeared in 3-phosphoglycerate, hexose monophosphates and sucrose. Similar results were obtained with leaves still attached to the plant. 3. Radioactivity appeared first in C-4 of the dicarboxylic acids and C-1 of 3-phosphoglycerate. The labelling pattern in hexoses was consistent with their formation from 3-phosphoglycerate. 4. The reaction giving rise to C(4) dicarboxylic acid appears to be the only quantitatively significant carboxylation reaction. 5. Evidence is provided that the radioactivity incorporated into the C(4) dicarboxylic acid pool is transferred to sugars via 3-phosphoglycerate. A scheme is proposed to account for these observations.

Light integrators with a linear response are not suitable for measuring the light climates of plants because plants are not linear integrators. It should be possible to make a quantitative allowance for this nonlinearity by using the CO(2) uptake curve of the plant. To test this, we have subjected white clover plants to different levels of constant light, comparing the rate of increase of total dry matter with the net rate of uptake of CO(2) per day. Temperature, humidity, daylength and nutrient supply were kept constant. The growth rate calculated from CO(2) uptake agreed well with the observed rate over the light levels tested (3.7-88 w.m(-2), 0.4-0.7 micron: 1 w.m(-2) = 10(3) erg . sec(-1) cm(-2)). All plants put on weight over the few days of the experiment, even those placed at light levels below their compensation point. The plants adapted their respiration rates to be a constant proportion of their growth rates. Most of the adaptation occurred within 24 hours of the light change. The adaptation of respiration has implications for models of light/growth relations in plant communities, almost all of which assume that respiration is proportional to leaf area and independent of growth rate or light level. The only model which does not is that of de Wit, and this gave good agreement with our results.