The occurrence of photoinhibition in an over-wintering crop of oil-seed rape (Brassica napus L.) and its correlation with changes in crop growth.

The maximum quantum yield of CO2 uptake (Φ), as a measure of light-limited photosynthetic efficiency, of a Brassica napus crop was measured on most days from mid-October until mid-April. During the winter, Φ was decreased by up to 50%. From January to March, leaves exposed to direct sunlight on days with minimum air temperatures near or below 0° C showed significant reductions in Φ. However, control leaves, artificially shaded from direct sunlight on these days, did not show any decrease. This provides statistical evidence for a light-dependent inhibition of CO2 uptake in the field, termed here photoinhibition. Recovery of Φ during warmer interludes was slow, requiring approx. 2-3 d. Concurrent measurements of light interception by the crop canopy and dry-matter accumulation showed that the efficiency with which intercepted light was converted into dry matter varied, declin between January and March to 33% of the value recorded in the warmer autumn months. Conversion efficiency was significantly and positively correlated with quantum yield. In a closed crop canopy during winter, light will be limiting for photosynthesis for much of the time. Under these conditions depression of Φ at the leaf level may contribute significantly to decreased dry-matter accumulation at the crop level, since the light-limited rate of CO2 uptake is likely to govern canopy photosynthetic rate.