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

Changes in a range of chlorophyll fluorescence parameters were monitored for leaves of crops of three Zea mays cultivars (MO17, CB3 and LG11) during early canopy development when large fluctuations in air temperatures occur. Crops were sown on 1 May 1993 and measurements made between 17 May acid 7 June, Measurements of the ratio of variable to maximal fluorescence of dark-adapted leaves (Fv/Fm) and the quantum efficiency of photosystem II photochemistry (phi(PSII)), the efficiency of excitation-energy transfer to open photosystem II reaction centres (Fv'/Fm') and the photochemical quenching coefficient (q(P)) for leaves operating at a photon flux density of 250 mu mol m(-2) s(-1) were made during the day at 3 h intervals, Fluctuations were observed in Fv/Fm, phi(PSII) and Fv'/Fm' throughout the period and significant decreases occurred during periods of low temperatures and high photon flux densities, indicating chill-induced photoinhibition of photosynthesis. Changes 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 quantum 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 morning and demonstrated that changes in phi(PSII) were highly correlated with changes in CO2 assimilation. Chill-induced changes in phi(PSII) were shown to be correlated with changes in the quenching of excitation energy in the antennae of photosystem II and not with changes in the redox state of the electron accepters of photosystem II. It is concluded that measurements of Fv/Fm and phi(PSII) provide useful, rapid and non-invasive probes of the photosynthetic performance of maize leaves in the field during periods of chill-induced photoinhibition.