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 gro wing seagrasses. To do this, the photosynthetic performance of 8 seagrass s pecies, growing from the upper intertidal to the shallow subtidal in Zanzib ar, East Africa, was studied during the event of air exposure and the subse quent rehydration. Photosynthetic efficiencies were measured by pulse ampli tude modulated (PAM) fluorometry as effective electron quantum yields of ph otosystem II (Y) since it had recently been shown that this measure paralle ls rates of O-2 evolution for several species under a defined irradiance. C ontrary to our expectations, it was found that the shallow intertidal speci es were in general more sensitive to desiccation than the deeper species. T his was expressed both as a faster decline in Y at decreasing water content s and as an inability to regain full photosynthetic rates during rehydratio n following even mild desiccation, as compared with the deeper-growing spec ies. One exception was the subtidally growing Syringodium isoetifolium, whi ch was very sensitive to desiccation. The 2 species which grow highest up i n the intertidal zone, Halophila ovalis and Halodule wrightii, may not desi ccate 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 wa ter loss. Thus, it seems that desiccation tolerance is not a trait which de termines the vertical zonation of tropical seagrasses. Rather, it is hypoth esised that the ability to tolerate high irradiances, as well as to benefit from high nutrient inputs from the shore, allows the shallow species to oc cupy the uppermost intertidal zone.