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

Syringodium filiforme does not exhibit physiological compensation as a func tion of season. This has important implications with respect to the diversi ty and spatial distribution of seagrass species, with Syringodium-like phys iological 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 s ite in Lower Laguna Madre, Texas with a mean depth of similar to 1.2 m. Pho tosynthesis-irradiance (PI) experiments were conducted at ambient temperatu res, in conjunction with measurements of reaction center density and size, to characterize photosynthetic apparatus structure and performance. Values for relative quantum yield (alpha; O-2 evolved per incident photon), compen sation point (I-c), saturation point (I-k), dark respiration (R-d) and ligh t-saturated photosynthesis (P-max), collected during late summer 1996, exhi bited 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. Densit ies for photosystem I (PSI) were highest during winter months (ca. 0.5 pmol mm(-3)); densities for photosystem II (PSII) exhibited no seasonal trend, ranging from 0.2 to 0.7 pmol mm(-3) Little variation was noted regarding th e size of PSI (PSUP700), whereas, size estimates for PSII (PSUO2) were larg est during winter and early-spring (ca. 5400 Chl-a P680(-1)). These data in dicate that S. filiforme may have some capacity for altering photosynthetic apparatus structure. However, since alpha, I-c and I-k did not change with season, the benefit of such adjustments is not obvious. We suggest photoph ysiological parameters may not be reliable indicators of environmental stre ss in Syringodium filiforme, as this species does not exhibit a great poten tial for phenotypic plasticity. This work serves as a basis for future stud ies designed to address physiological compensation in seagrasses and the ab ility of coastal macrophytes to respond to environmental change (e.g. reduc tions in light availability). (C) 2000 Elsevier Science B.V. All rights res erved.