COLD-INDUCED PHOTOINHIBITION AND DESIGN OF SHELTERS FOR ESTABLISHMENTOF EUCALYPTS IN PASTURE

Juvenile Eucalyptus polyanthemos Schau. which had been established in an open pasture, were surrounded by individual shelters made of differ ent materials: chicken wire, white translucent plastic, and three type s of shade cloth transmitting 30, 50 or 70% of incident sunlight. Air temperatures within white plastic shelters were up to 6 degrees C abov e ambient air temperatures, whereas air temperatures in other shelters differed little from ambient. Irradiance was the main factor which va ried between shelters. Leaves were photoinhibited over winter as shown by depression in pre-dawn F-v/F-m. The extent of that decrease in FvF m was directly proportional to irradiance. Pre-dawn F-v/F-m recovered in all treatments during spring. Growth, as measured by stem elongatio n. occurred to a limited extent during winter, but primarily during sp ring. During both seasons, stem elongation was greatest in plants shel tered by 50% shadecloth. However, treatment-induced variation in stem elongation during spring was correlated with pre-dawn values for F-v/F -m measured during the previous winter. Reducing sunlight by 50% thus appeared to effect the best compromise between conflicting demands for protection from cold-induced photoinhibition during winter and for ab sorption of sufficient light for growth during spring.