Seasonal Changes in the Physiology of S24 Perennial Ryegrass (Lolium perenne L.): 4. Comparison of the Carbon Balance of the Reproductive Crop in Spring and the Vegetative Crop in Autumn

Measurements of CO₂ exchange were used to construct a detailedaccount of the carbon economy of established simulated swardsof perennial ryegrass during 10 week periods in spring and autumn.Changes in sward dry weight estimated from gas exchange measurementsclosely matched observed changes in dry weight. In spring, light energy increased, the photosynthetic potentialof the canopy increased, and together these factors led to apattern of increasing photosynthetic uptake. In autumn, decreasinglight energy and decreasing canopy photosynthetic potentialled to decreasing photosynthetic uptake. During the periodsinvestigated, the changes in light energy receipt played themajor role in determining the pattern of photosynthetic uptake. A simple model of crop growth was used to illustrate the effectof such characteristic seasonal differences in the pattern ofphotosynthetic uptake on the subsequent loss of carbon duringrespiration and tissue death, and consequently on the productionof live tissue. The model describes how a reproductive cropin spring may accumulate more living dry matter than a vegetativecrop in autumn from the same total gross photosynthetic uptakeof carbon. Lolium perenne L., ryegrass, carbon economy, photosynthesis, respiration