CHANGES IN THE PHOTOSYNTHETIC LIGHT RESPONSE CURVE DURING LEAF DEVELOPMENT OF FIELD-GROWN MAIZE WITH IMPLICATIONS FOR MODELING CANOPY PHOTOSYNTHESIS

Changes in the photosynthetic light-response curve during leaf develop ment were determined for the fourth leaf of maize crops sown on 23 Apr il and 10 June. Temperatures were unusually mild during late spring/ea rly summer and neither crop experienced chilling damage. The concept o f thermal time was used to take into account the effects of different temperature regimes on developmental stage, thereby enabling photosynt hetic light-response data to be combined for both crops to describe th e general response. Large variations in the upper asymptote (A(sat)) a nd convexity (Theta) of the light-response curve occurred during leaf development, but the maximum quantum yield of CO2 assimilation remaine d relatively constant throughout. Dark respiration rates showed a smal l but significant decrease with leaf age and generally ranged between 5 and 10% of A(sat). A simple mathematical model was developed to asse ss the sensitivity of daily leaf photosynthesis (A(L)) to reductions i n the A(sat), Theta and the initial slope (Phi) of the light-response curve at different stages of leaf development. On bright sunny days, a nd at all developmental stages, A(L) was ca. twice as sensitive to red uctions in A(sat) than to reductions in Phi and Theta. In overcast con ditions, however, all three parameters contributed significantly to re ductions in leaf photosynthesis, although the contribution of Phi was greatest during early leaf growth, while older leaves were most sensit ive to depressions in A(sat). The implications of these results for mo delling the sensitivity of canopy photosynthesis to chill-induced phot oinhibition of the light-response curve are discussed.