CHARACTERIZATION OF CHILLING EFFECTS ON PHOTOSYNTHETIC PERFORMANCE OFMAIZE CROPS DURING EARLY-SEASON GROWTH USING CHLOROPHYLL FLUORESCENCE

Changes in a range of chlorophyll fluorescence parameters were monitor ed for leaves of crops of three Zea mays cultivars (MO17, CB3 and LG11 ) during early canopy development when large fluctuations in air tempe ratures occur. Crops were sown on 1 May 1993 and measurements made bet ween 17 May acid 7 June, Measurements of the ratio of variable to maxi mal fluorescence of dark-adapted leaves (Fv/Fm) and the quantum effici ency of photosystem II photochemistry (phi(PSII)), the efficiency of e xcitation-energy transfer to open photosystem II reaction centres (Fv' /Fm') and the photochemical quenching coefficient (q(P)) for leaves op erating at a photon flux density of 250 mu mol m(-2) s(-1) were made d uring the day at 3 h intervals, Fluctuations were observed in Fv/Fm, p hi(PSII) and Fv'/Fm' throughout the period and significant decreases o ccurred during periods of low temperatures and high photon flux densit ies, indicating chill-induced photoinhibition of photosynthesis. Chang es in Fv/Fm were highly correlated with changes in phi(PSII), for all three cultivars. Measurements of phi(PSII), Fv'/Fm', q(P) and the quan tum efficiency of CO2 assimilation (phi CO2) at a photon flux density of 1000 mu mol m(-2) s(-1) were made for leaves at 08.00 h each mornin g and demonstrated that changes in phi(PSII) were highly correlated wi th changes in CO2 assimilation. Chill-induced changes in phi(PSII) wer e shown to be correlated with changes in the quenching of excitation e nergy in the antennae of photosystem II and not with changes in the re dox state of the electron accepters of photosystem II. It is concluded that measurements of Fv/Fm and phi(PSII) provide useful, rapid and no n-invasive probes of the photosynthetic performance of maize leaves in the field during periods of chill-induced photoinhibition.