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 pul vinata (Hedw.) Sm. were measured during drying, and following remoistening after a period of desiccation. Most changed little with drying until the fu ll-turgor point was approached, when rapid changes accompanied loss of cell water. In R. loreus all parameters decreased together down to a water cont ent 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 decli ned with loss of cell water, but NPQ peaked at approx. 70 % d.wt (RWC appro x. 0.3). On remoistening after a few days air dry, F-v/F-m recovered very r apidly (< 1 min) in all three species to initial values of 0.6-0.7 in R. lo reus, 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 o f Phi PSII was very variable but generally slower and less complete in R, l oreus. NPQ peaked sharply in the first minutes of re-wetting in A. viticulo sus, 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 comple te recovery, or in the other species after prolonged desiccation. The initi al peak in NPQ was suppressed by dithiothreitol (DTT), suggesting that it i s zeaxanthin dependent. After short or moderate periods of drying, A. vitic ulosus and G, pulvinata showed enhanced levels of NPQ for several hours, bu t returned to the predesiccation state within 24 h. Recovery from longer dr ying (or of more sensitive species, such as R. loreus) was slower, and effe cts of desiccation (including high NPQ) were still evident more than 24 h a fter remoistening. Slow-fluorescence parameters such as Rfd, and the widely used parameter F-v/F-m, are valuable in ecophysiology and stress physiolog y if their limitations are recognized, but even simple quenching analysis c an yield valuable additional information. (C) 1999 Annals of Botany Company .