Chilling damage to photosynthesis in young zea mays: I. EFFECTS of LIGHT and TEMPERATURE variation ON photosynthetic co² assimilation

Carbon dioxide and water vapour exchanges of the second leaf of Zea mays in controlled environment cuvettes were measured in an open gas-exchange system, during and following subjection to low temperature stress. Photosynthetic CO2 assimilation (Fc) decreased markedly with decrease in leaf temperature so that Fc at 5 °C was c. 7% of Fc at 20 °C. Fc continued to decline if leaf temperature was maintained at 5 °C, and when returned to 20 °C the leaf could not regain its previous Fc. This chill-induced reduction in the capacity of the leaf to assimilate CO2 was proportional to the duration of the chill and increased with water vapour pressure deficit and photon flux density (I_n). Six hours at 5 °C decreased Fc on return to 20 °C, relative to Fc prior to treatment, by c. 10% in darkness and by c. 50% in a photon flux density approaching full-sunlight (I_p = 1.5 mmol m^-2 s^-1). The degree of reduction in Fc following chill treatment showed an almost linear dependence on both the length and temperature of the chill. Chill treatments resulted in a decrease in both stomatal and mesophyll conductances. Examination of the responses of F_c to light and CO₂ concentration suggested that chill damage to the capacity for CO₂ assimilation resulted from effects on both the light and CO₂ limited processes within photosynthesis.