Photosynthetic tolerances to desiccation of the co-occurring seagrasses Halophila johnsonii and Halophila decipiens

Along the coastal lagoons of southeast Florida, Halophila johnsonii and Halophila decipiens co-occur subtidally, but only H. johnsonii occurs intertidally. Photosynthetic tolerances to desiccation in light and dark conditions were compared between H. johnsonii and H. decipiens using pulse amplitude-modulated (PAM) fluorescence measurements. Under light conditions (250 μmol photons m−2 s−1PAR), both species exhibited near-linear decreases in effective quantum yields (ΔF/F′m) with increasing desiccation. The most rapid decline in ΔF/F′m was observed in H. johnsonii. Under dark conditions, both seagrasses exhibited small decreases in maximum quantum yields (Fv/Fm) during desiccation until reaching a critical threshold of relative water content (RWCcritical). RWCcritical was significantly lower for H. johnsonii (0.21 ± 0.086) compared to H. decipiens (0.37 ± 0.15). However, the rate of decline in Fv/Fm below RWCcritical was greater in H. johnsonii than H. decipiens. Water loss rates, however, were not significantly different between these two species in either light or dark conditions (0.18 ± 0.047 g min−1 light, 0.12 ± 0.045 g min−1 dark versus 0.16 ± 0.047 g min−1 light, 0.099 ± 0.044 g min−1 dark for H. johnsonii and H. decipiens, respectively). Results from this study suggest that lower desiccation tolerance, at the leaf level, is not the primary factor precluding intertidal distribution of H. decipiens.