Photosynthetic performance in Syringodium filiforme: seasonal variation in light-harvesting characteristics

Syringodium filiforme does not exhibit physiological compensation as a function of season. This has important implications with respect to the diversity and spatial distribution of seagrass species, with Syringodium-like physiological characteristics, that are exposed to chronic low light conditions common to estuarine environments. Adult Syringodium filiforme plants were sampled at bimonthly intervals between August 1996 and August 1997 from a site in Lower Laguna Madre, Texas with a mean depth of ~1.2 m. Photosynthesis-irradiance (PI) experiments were conducted at ambient temperatures, in conjunction with measurements of reaction center density and size, to characterize photosynthetic apparatus structure and performance. Values for relative quantum yield (; O2 evolved per incident photon), compensation point (Ic), saturation point (Ik), dark respiration (Rd) and light-saturated photosynthesis (Pmax), collected during late summer 1996, exhibited no significant differences from those collected in late spring 1997. Changes in pigment concentration, exhibiting no distinct seasonal pattern, were manifested in adjustments of both photosystem density and size. Densities for photosystem I (PSI) were highest during winter months (ca. 0.5 pmol mm3); densities for photosystem II (PSII) exhibited no seasonal trend, ranging from 0.2 to 0.7 pmol mm-3. Little variation was noted regarding the size of PSI (PSUP700), whereas, size estimates for PSII (PSUO2) were largest during winter and early-spring (ca. 5400 Chl-a P680-1). These data indicate that S. filiforme may have some capacity for altering photosynthetic apparatus structure. However, since , Ic and Ik did not change with season, the benefit of such adjustments is not obvious. We suggest photophysiological parameters may not be reliable indicators of environmental stress in Syringodium filiforme, as this species does not exhibit a great potential for phenotypic plasticity. This work serves as a basis for future studies designed to address physiological compensation in seagrasses and the ability of coastal macrophytes to respond to environmental change (e.g. reductions in light availability).