Photosynthetic tolerances to desiccation of tropical intertidal seagrasses

The aim of the present study was to investigate whether tropical intertidal seagrasses were better adapted to tolerate desiccation than subtidally growing seagrasses. To do this, the photosynthetic performance of 8 seagrass species, growing from the upper intertidal to the shallow subtidal in Zanzibar, East Africa, was studied during the event of air exposure and the subsequent rehydration. Photosynthetic efficiencies were measured by pulse amplitude modulated (PAM) fluorometry as effective electron quantum yields of photosystem II (Y) since it had recently been shown that this measure parallels rates of O-2 evolution for several species under a defined irradiance. Contrary to our expectations, it was found that the shallow intertidal species were in general more sensitive to desiccation than the deeper species. This was expressed both as a faster decline in Y at decreasing water contents and as an inability to regain full photosynthetic rates during rehydration following even mild desiccation, as compared with the deeper-growing species. One exception was the subtidally growing Syringodium isoetifolium, which was very sensitive to desiccation. The 2 species which grow highest up in the intertidal zone, Halophila ovalis and Halodule wrightii, may not desiccate much in situ during low tide because the leaves Lie flat on the moist sand and, for the latter species, overlap one another so as to minimise water loss. Thus, it seems that desiccation tolerance is not a trait which determines the vertical zonation of tropical seagrasses. Rather, it is hypothesised that the ability to tolerate high irradiances, as well as to benefit from high nutrient inputs from the shore, allows the shallow species to occupy the uppermost intertidal zone.