PHOTOSYNTHESIS AFTER FREEZING STRESS IN PLANTS WITH VARIOUS DEGREES OF FREEZING TOLERANCE

The aim of this study was to clarify whether there are general differe nces between the woody and herbaceous plants in their responses to a s ingle non-lethal freezing stress, when measured under comparable condi tions. Activity of net photosynthesis (P(N33)), the curves of P(N) ver sus photosynthetic photon fluence density and P(N) versus intercellula r partial pressure of CO2, and ratio of variable to maximal fluorescen ce (F(v)/F(m)) were measured at optimal temperatures in the leaves of six woody and six herbaceous plant species prior to frost-hardening, a fter frost-hardening at low positive temperatures and after a single o ver-night freezing stress (-10-degrees-C, 12 h) of the shoots. Recover y of P(N) at 20/15-degrees-C was also followed. There were no general differences between woody and herbaceous plants in their photosyntheti c responses to freezing stress. In most plants, P(N33) Was strongly re duced after freezing mainly due to the disturbances in functioning of the chloroplasts, but it recovered with a few days. However, P(N) of s ome herbaceous plants (Cochlearia, one cultivar of spinach) was insens itive to non-lethal freezing. Furthermore, stomatal opening of a few h erbaceous plants (Cochlearia, Valerianella) was disturbed after freezi ng without any sign of an impairment in functioning of the chloroplast s.