Differences between barley and maize revealed in limitations of photosystems I and II under high temperature and low air humidity.

Non-photochemical quenching and limitations of the photosystem I and photosystem II activities were studied in C3 -plant barley and C4 -plant maize. Plants were exposed to prolonged heat stress under high and low air humidity. Both species decreased non-photochemical quenching at 37-42°C, which increased at 46°C. A decrease of photosystem II activity at 46°C in lower air humidity was achieved through different mechanisms. In barley, photosystem II was downregulated by the increase of non-photochemical quenching. In maize, photosystem II was downregulated by the increase of acceptor-side limitation. Analysis of transients also revealed differences between species. One second after a light induction, limitations flashes at the acceptor sides of both photosystems. Elevating the temperature decreased these flashes; acceptor-side limitations of both photosystems decreased proportional to each other. In maize, the size of flashes slightly diminished at 37°C and decreased more at 42-46°C. In barley, the size of flashes greatly decreased at 37°C and gradually returned to the control level under higher temperatures. Around photosystem II, the flash was quenched by a burst of non-photochemical quenching. In barley, the transient peaks of acceptor-side limitation and non-photochemical quenching were very similar at any temperature. This was not observed in maize.