PHOTOSYNTHETIC PRODUCTIVITY OF AN IMMATURE MAIZE CROP - CHANGES IN QUANTUM YIELD OF CO2 ASSIMILATION, CONVERSION EFFICIENCY AND THYLAKOID PROTEINS

The effect of growth temperatures on quantum yield (phi) was examined for leaves at different stages of development within the immature canopies of two crops of field grown maize (Zea mays cv. LG11) sown on 3 May and 20 June 1990. During the period of 23 to 49 d after sowing, the crop sown on the 3 May experienced temperatures below 10-degrees-C on 19 occasions compared with only two for the crop sown on 20 June. A period of severe chilling at the end of May and the beginning of June was associated with a marked reduction in phi for all leaves in the early-sown crop. This chill-induced depression in phi was greater in recently emerged than more mature leaves in the canopy and was found to be accompanied by modifications in the polypeptide profiles of thylakoids isolated from the leaves. During the chilling period, decreases in some polypeptides, notably in the range of 41-42 and 20 kDa apparent molecular size, and increases of polypeptides of c. 15-16 kDa were observed compared with leaves developing at warmer temperatures in July. The efficiency of converting intercepted radiation into dry matter (conversion efficiency) was 42% lower in the early- than late-sown crop, but no significant relationship between conversion efficiency and quantum yield was found in either treatment.