EFFECTS OF SOME PERIODS OF VARIABLE DAILY EXPOSURE TO TEMPERATURES OF5 AND 20 DEGREES-C ON PHOTOSYNTHESIS AND WATER RELATIONS IN MAIZE SEEDLINGS

The five different temperature regimes were used daily for several day s with the number of hours of 5 degrees C treatment (day/night): 15/9, 10/9, 5/9, 0/9, and 0/0 (nonchilled control plants). For the remainin g part of the day maize (Zea mays L.) seedlings were kept at 20 degree s C. Continuous chill caused visible injuries to the leaves, considera ble leakage of electrolytes and reduction of the content of fatty acid s. The ''warm'' periods during the day (20 degrees C), irrespective of the length of exposure, prevented the occurrence of these disadvantag eous symptoms. Tn a similar way, the ''warm'' periods diminished the i nhibition of photosynthesis and transpiration as well as the reduction of the leaves' osmotic potential, measured on the first day after the chilling had ceased, and on subsequent days they induced even a compl ete recovery of those processes to the level of control plants. An adv antageous effect of ''warm'' periods on gas exchange was maintained at different irradiances.