Lower air humidity reduced both the plant growth and activities of photosystems I and II under prolonged heat stress.

The warming is global problem. In natural environments, a heat stress is accompanied with a drought usually. The effect of lower air humidity remains obscure. Maize and barley plants were supplied with an unlimited source of water for the root uptake and undergone to heat stress for 48 h at contrast conditions of air humidity. The lower air humidity decreased photochemical activities of photosystem I and photosystem II. The small effect was revealed in control. The temperature elevation to 37{degrees}C and 42{degrees}C increased relative activities of the both photosystems; the photosystem II was activated more. The effect of air humidity disappeared at 37{degrees}C; at 42{degrees}C, the effect was small. At 46{degrees}C, lower air humidity magnified substantially the inhibitory effect of heat. Consequently, the maximal and relative activities of the both photosystems were decreased in maize and barley; the plant growth was reduced greatly. The photosystem II was inhibited more. At 46{degrees}C, maize plants increased water uptake by roots at lower air humidity and survived; barley plants were unable to increase water uptake and died. Therefore, air humidity is the important component of environmental heat stress influencing activities of photosystem I and photosystem II and thereby plant growth and viability. HighlightThe effect of severe heat stress was magnified with lower air humidity. At 46{degrees}C, their mutual action inhibited photosystem I and II and reduced greatly the plant growth and viability.