Hardening by partial dehydration and ABA increase desiccation tolerance in the cyanobacterial lichen Peltigera polydactylon

* BACKGROUND AND AIMS: The ability of partial dehydration and abscisic acid pretreatments to increase desiccation tolerance in the cyanobacterial lichen Peltigera polydactylon was tested. * METHODS: Net photosynthesis and respiration were measured using infrared gas analysis during a drying and rehydration cycle. At the same time, the efficiency of photosystem two was measured using chlorophyll fluorescence, and the concentrations of chlorophyll a were spectrophotometrically assayed. Heat production was also measured during a shorter drying and rehydration cycle using differential dark microcalorimetry. * KEY RESULTS: Pretreating lichens by dehydrating them to a relative water content of approx. 0.65 for 3 d, followed by storing thalli hydrated for 1 d in the light, significantly improved their ability to recover net photosynthesis during rehydration after desiccation for 15 but not 30 d. Abscisic acid pretreatment could substitute for partial dehydration. The improved rates of photosynthesis during the rehydration of pretreated material were not accompanied by preservation of photosystem two activity or chlorophyll a concentrations compared with untreated lichens. Partial dehydration and ABA pretreatments appeared to have little direct effect on the desiccation tolerance of the mycobiont, because the bursts of respiration and heat production that occurred during rehydration were similar in control and pretreated lichens. * CONCLUSIONS: Results indicate that the photobiont of P. polydactylon possesses inducible tolerance mechanisms that reduce desiccation-induced damage to carbon fixation, and will therefore improve the supply of carbohydrates to the whole thallus following stress. In this lichen, ABA is involved in signal transduction pathways that increase tolerance of the photobiont.