Hydration-dependent photosynthetic production of lichens: what do laboratory studies tell us about field performance?

Extensive investigations made in the past two decades on lichen photosynthe sis in relation to water content have shown two features of particular inte rest: first, the depression of net photosynthesis at high water contents, s uprasaturation (i.e. the lichen contains more water than necessary to satur ate photosynthesis), and, second, the ability of green algal lichens to reg ain photosynthetic activity by uptake of water from humid air. Evidence fro m several investigators is presented to confirm that both phenomena are now well substantiated through laboratory investigations. It has been question ed whether these features do actually occur in nature and, if they do, to w hat extent. Recent work is summarized that demonstrates that for many of th e lichens studied suprasaturation is of major importance and can result in depressed photosynthesis for around a third of the time that the lichens ar e photosynthetically active. Reactivation of photosynthesis of green algal lichens by high humidity is also, apparently, very common in some environme nts, for example, humid temperate rainforests, occurring almost every night . It is possible that the dominance of green algal lichens, rather than cya nobacteria[ species, in these habitats is a result of their ability to util ize water vapour. If so, then the phenomenon must have major ecological imp ortance for lichen productivity. In general, laboratory studies seem to be able to predict extremely well the behaviour of lichens in their natural ha bitat.