Cold-induced photoinhibition and design of shelters for establishment of eucalypts in pasture

Juvenile Eucalyptus polyanthemos Schau. which had been established in an open pasture, were surrounded by individual shelters made of different materials:chicken wire, white translucent plastic, and three types of shade cloth transmitting 30, 50 or 70% of incident sunlight. Air temperatures within white plastic shelters were up to 6°C above ambient air temperatures, whereas air temperatures in other shelters differed little from ambient. Irradiance was the main factor which varied between shelters. Leaves were photoinhibited over winter as shown by depression in pre-dawn F_vF_m. The extent of that decrease in F_v/F_m was directly proportional to irradiance. Pre-dawn F_v/F_m recovered in all treatments during spring. Growth, as measured by stem elongation. occurred to a limited extent during winter, but primarily during spring. During both seasons, stem elongation was greatest in plants sheltered 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 absorption of sufficient light for growth during spring.