Photoinhibition of Photosynthesis and Its Recovery in Two Maize Hybrids Varying in Low Temperature Tolerance

Photoinhibition of photosynthesis was induced in intact leaves of two maize hybrids that differed in low temperature tolerance by exposing the leaves to a photon irradiance (PI) of 1500 �mol m-2s-1 at temperatures of 10, 15 and 20°C. Hybrid A665 × H99 (CBD) is adapted to the U.S.A. Corn Belt and is intolerant of cool conditions, while NZlA × 5-113 (CT) is of high-altitude tropical origin and tolerant of low temperatures. Each hybrid was grown at two temperature regimes of 22/18 and 16/14°C at a PI of 700 �mol m-2s-1. Recovery from photoinhibition was followed at the same temperatures and at a PI of 20�mol m-2s-1. In either case the time-courses of photoinhibition and recovery were monitored by measuring chlorophyll fluorescence at 692 nm and 77K and the photon yield of photosynthetic O2 evolution. Chronic photoinhibition resulting from growth at the moderate PI and at 16/ 14°C was apparent in both hybrids but was more severe in the CBD hybrid. Additional photoinhibition, induced by subsequent exposure to high light, was highly temperature-dependent with increasing severity at lower temperatures. The CT hybrid was more susceptible to this photoinhibition but photoinhibition in the CBD hybrid was more temperature-dependent than in the CT hybrid. Susceptibility to photoinhibition was, therefore, not apparently related to the degree of low temperature tolerance in these maize hybrids. Induced photoinhibition was readily reversible though recovery was temperature-dependent with the most recovery at the higher temperature. Recovery in these hybrids was not impeded by temperatures as low as 10°C as it is in other species and there was no difference between the hybrids in their ability to recover. However, differences in chronic rather than induced photoinhibition conformed to a greater extent with the ability of these maize hybrids to tolerate low temperature.