THE OCCURRENCE OF PHOTOINHIBITION IN AN OVER-WINTERING CROP OF OILSEED RAPE (BRASSICA-NAPUS L) AND ITS CORRELATION WITH CHANGES IN CROP GROWTH

The maximum quantum yield of CO2 uptake (PHI), 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, PHI 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-degrees-C showed significant reductions in PHI. 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 PHI 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 PHI 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.