Cycling of leaf conductance in citrus exposed to natural and controlled environments

Abstract

Three types of evidence indicate that stomatal cycling occurs in citrus under field conditions. First, dendrograph records collected from 1963 to 1965 on 30-year-old trees revealed daytime oscillations in trunk diameter, with a period of about 1 h. These oscillations occurred during winter and summer months at two southern California locations. Second, leaf conductance measurements were made on 12-year-old citrus in southern California during clear, mild days in March and April, 1975. Oscillations having a period of shorter than 1 h were observed. Third, oscillations in photosynthesis measured with a Shimshi apparatus were observed on 17-year-old citrus in Israel during clear days in November 1975. In controlled greenhouse experiments on 4-year-old citrus in large pots, cycling in leaf conductance, leaf water stress, and trunk diameter were observed, but the patterns differed with root temperatures of 5 and 25 °C. At 5 °C, cycling began early and decreased later in the day, and water stress and trunk diameter changes lagged behind conductance changes. At 25 °C, cycling became more prominent as the day progressed, and no lag was observed.