Figure 5.

Long-term acclimation of mature leaves of shade-grown V. surinamensis seedlings to daily direct sun exposure in the presence (black bars, Aclar) and absence (white bars, Mylar) of ambient UV-B light. Plants were sun-exposed for 30 min (between 11 and 14 h, local time) on 8 consecutive d (d 1–8) and again on d 14. Between exposure periods, the plants were kept under shade conditions (maximum PAR, 110 μmol m⁻² s⁻¹). A, F_v/F_m before and after sun exposure on d 1 at a PAR dose of 3.4 mol m⁻² (left panel) and d 14 at a PAR dose of 3.6 mol m⁻² (right panel), recorded subsequent to 10 min of dark adaptation. Means ± se of three leaves from different plants are given. B, Relative contents of UV-absorbing substances on d 17 (absorbance measured at λ_max in the UV region, 282 nm). Data are means ± se from two control plants (six leaf segments) and three sun-exposed plants. Control plants were not exposed to direct sunlight, but otherwise kept until d 17 under the same conditions as sun-exposed plants.