Desiccation-time limits of photosynthetic recovery in Equisetum hyemale (Equisetaceae) spores

The chlorophyllous spores of Equisetum survive desiccation, yet cannot tolerate this quiescent state for more than ~2 wk. The hypothesis that spore viability of Equisetum hyemale L. is limited by inhibition of photosynthetic recovery was tested using chlorophyll a fluorescence and oxygen-exchange analyses. Experimental spores were desiccated at 2% relative humidity and 25C for time periods of 24 h, 1 wk, and 2 wk, and then rehydrated at 200 mmol photons/m2s (PAR) and 25C for up to 24 h. Spores desiccated for 24 h recovered photosynthetic competence very rapidly during rehydration, reaching the O2 compensation point in 6.3 ~ 0.3 (mean +/- SE) min. Recovery of photosynthetic performance of spores desiccated for 1 wk was slower, as judged by significantly slower increases of (1) photochemical efficiency of photosystem (PS) II, (2) PS II quinoneB-reducing center concentration, (3) quinoneB concentration, (4) water-oxidation activity, (5) rate of light-induced O2 evolution, and (6) apparent quantum yield of net O2 exchange. Photosystem-II and whole-spore photosynthetic competence of 2-wk desiccated spores was increasingly impaired, and did not recover during rehydration. Origin fluorescence yield and dark respiration were not affected by desiccation time following rehydration. The results suggest that the extremely short viability of disseminated spores of Equisetum hyemale is due to the inability to recover losses of water oxidation and photosystem II-core function following 2 wk of desiccation.