Photosynthetic Strategies of Desiccation-Tolerant Organisms

Water restriction is one of the major world limitations for growth and survival of photosynthetic organisms, carbon assimilation and growth being the primary processes affected by water scarcity. Photosynthesis is very sensitively and negatively affected by the decrease in the availability of water within the cell for three major reasons: (1) water is essential to maintain cellular functions and integrity, (2) water molecules are the electron donors for the photosynthetic electron transport chain, and (3) in vascular plants, an additional side effect of water limitation is usually a restriction in CO₂ reaching the chloroplasts. To cope with these effects, photosynthetic organisms employ three different strategies: (1) escape, (2) avoidance, and (3) tolerance (Franks 2011). The first consists in the early completion of the life cycle, together with the formation of resistance propagules such as spores or seeds, before dehydration of tissues leads to irreversible damage. The second, widespread in most vascular plants (tracheophytes), consists in the delay of desiccation by imposing structural barriers (stomata, cuticle) to water loss. A side effect of this strategy is that it also imposes limitations to CO₂ availability in the chloroplasts, decreasing photosynthetic rates. These limitations are caused by stomatal closure, by changes in mesophyll conductance to CO₂ (Chaves et al. 2009), and at later stress stages, by metabolic restrictions such as those induced by decreases in Rubisco content and activity (Galmés et al. 2013). In parallel to these processes, a series of mechanisms acting on the maintenance of cell turgor and of suitable conditions for metabolic activity are activated.