In vivo Changes in Chloroplast ThylakoidMembrane Activity during Viable and Non-viable Dehydration of a Drought-tolerant Plant, Borya nitida

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Abstract
Expanded leaves of Borya nitida Labill. were detached and placed in atmospheres of 100, 96, 88, 53 or 13% relative humidity at 25°C. In vivo chlorophyll fluorescence induction kinetics and the photooxidation of cytochrome f were measured before and after dehydration and again after rehydration by floating on water. In contrast to slow dehydration at 96% R.H., which resulted in an initial stimulation of photosystem II activity, the slow development of a partial block on the photoreducing side of photosystem II and the eventual degreening of leaves to a yellow viable condition, fast dehydration at 13% R.H. induced rapid loss of photosystem I activity and a slower development of a block on the photooxidizing side of photosystem . Although the leaves remained green, photo- system activity did not recover upon rehydration. Charge separation in the photosystem II complex still took place in the dehydrated leaves but upon rehydration the photosystem II reaction centre appeared to be disassociated from the light harvesting system. Dehydration at intermediate relative humidities (88 and 53%) led to a partial block on the photoreducing side of photosystem II and, more slowly, inhibition of photosynthetic electron transfer on the photooxidizing side of photosystem II. Following rehydration there was partial recovery of reactions associated with photosystem I but the leaves were unable to return to their original condition.