Chlorophyll fluorescence during drying and rehydration in the mosses Rhytidiadelphus loreus (Hedw.) Warnst., Anomodon viticulosus (Hedw.) Hook. & Tayl. and Grimmia pulvinata (Hedw.) Sm.

Chlorophyll-fluorescence parameters of three mosses, Rhytidiadelphus loreus (Hedw.) Warnst., Anomodon viticulosus (Hedw.) Hook & Tayl. and Grimmia pulvinata (Hedw.) Sm. were measured during drying, and following remoistening after a period of desiccation. Most changed little with drying until the full-turgor point was approached, when rapid changes accompanied loss of cell water. In R. loreus all parameters decreased together down to a water content of about 30 % d.wt (RWC approx. 0.15). At low water contents (5-16 % d.wt; RWC approx. 0.02-0.08) saturating pulses tended to suppress rather than stimulate fluorescence. In A. viticulosus F-v/F-m, q(P) and Phi PSII declined with loss of cell water, but NPQ peaked at approx. 70 % d.wt (RWC approx. 0.3). On remoistening after a few days air dry, F-v/F-m recovered very rapidly (< 1 min) in all three species to initial values of 0.6-0.7 in R. loreus, and approx. 0.7 in A. viticulosus and G. pulvinata. Phi PSII (Delta F/F'(m)) recovered to near-normal levels within 10-15 min (perhaps close to the attainable minimum time) in G. pulvinata and A. viticulosus. Recovery of Phi PSII was very variable but generally slower and less complete in R, loreus. NPQ peaked sharply in the first minutes of re-wetting in A. viticulosus, G. pulvinata, and the faster-recovering shoots of R, loreus, but rose only slowly in the shoots of R. loreus that showed slowest and least complete recovery, or in the other species after prolonged desiccation. The initial peak in NPQ was suppressed by dithiothreitol (DTT), suggesting that it is zeaxanthin dependent. After short or moderate periods of drying, A. viticulosus and G, pulvinata showed enhanced levels of NPQ for several hours, but returned to the predesiccation state within 24 h. Recovery from longer drying (or of more sensitive species, such as R. loreus) was slower, and effects of desiccation (including high NPQ) were still evident more than 24 h after remoistening. Slow-fluorescence parameters such as Rfd, and the widely used parameter F-v/F-m, are valuable in ecophysiology and stress physiology if their limitations are recognized, but even simple quenching analysis can yield valuable additional information. (C) 1999 Annals of Botany Company.