Fluorescence imaging application: effect of leaf age on seagrass photokinetics

Abstract

We used the Imaging-PAM fluorometer to map spatial variability of photosynthesis in three seagrass species, Halophila ovalis, Zostera capricorni and Posidonia australis. Photosynthesis was described by relative photosynthetic rate (PS/50), effective quantum yield (Φ PSII), non-photochemical quenching (NPQ and qN), electron transport rate (ETR) and leaf absorptivity. Photosynthetic patterns were linked to leaf age and light climate but patterns were not consistent across species. Longitudinal heterogeneity in photosynthesis was apparent along the leaves of all three species while lateral spatial heterogeneity was found only across Z. capricorni and H. ovalis leaves. Age of leaf tissue, determined by longitudinal location on the leaf, strongly influenced photosynthetic activity of Z. capricorni and P. australis. A comparison of H. ovalis leaves of differing maturity demonstrated the influence of leaf age on photosynthetic activity, yet a comparison of Z. capricorni leaves of differing maturity showed no leaf-age effects. Variations in stress-induced changes across a seagrass leaf can be used to identify areas or particular regions of the leaf, which are more susceptible to photodamage. Clear evidence of substantial within-leaf heterogeneity in photosynthetic activity (i.e., a two-fold variation in half saturation constant along a leaf of P. australis) has serious implications for use of small sections of leaf for photosynthetic incubations (such as O 2 or single-point chlorophyll a fluorescence measurements).