INFLUENCE OF TEMPERATURE AND LIGHT ON GROWTH AND PHOTOSYNTHETIC PHYSIOLOGY OF FUCUS-EVANESCENS (PHAEOPHYTA) EMBRYOS

The influence of temperature and light on growth and photosynthetic ph ysiology were investigated in embryos of Fucus evanescens grown at 5 o r 20 degrees C under irradiances of 15 or 150 mu mol photons m(-2) s(- 1) for 7-10 days. Growth was light-independent, but high-temperature e mbryos were always significantly larger than those grown at low temper ature. Photosynthesis-irradiance responses were measured at growth tem perature and a standard temperature (20 OC) to isolate instantaneous e ffects of temperature from acclimation responses. Our data indicate th at growth and photosynthesis are uncoupled during the early developmen t of Fucus, and that acclimation of the photosynthetic light-harvestin g apparatus occurred. Light-limited net photosynthesis (Psub-sat) resp onded similarly to high temperature and low light. Rates of Psub-sat w ere similar in embryos grown at 20 degrees C (regardless of light) and at 5 degrees C in low (c. 1.2 nmol O-2 mm(-3) min(-1)), whereas those of 5 degrees C high-light embryos were lower (c. - 0.04 nmol O-2 mm(- 3) min(-1)). Changes in Psub-sat were associated with changes in initi al slope of the photosynthesis-irradiance curve (a) and dark respirati on. Differences in ct were attributed to increased absorption due to i ncreased chlorophyll a content and PSII reaction centre densities. Cha nges in or were also correlated with changes in fluorescence induction kinetics, with high-temperature and/or low-light embryos exhibiting h igher ratios of variable:maximum fluorescence (F-v/F-m) than 5 OC high -light embryos (c. 0.5 vs. 0.19). In contrast to Psub-sat, changes in light-saturated photosynthesis (P-max) in response to growth under dif ferent temperature/light regimes did not confer metabolic compensation . Rates of P-max were highest in 20 degrees C high-light embryos (7.3 nmol O-2 mm(-3) min(-1)), lower in 20 degrees C low-light and 5 degree s C low-light embryos (c. 2.6 nmol O-2 mm(-3) min(-1)) and lowest in 5 degrees C high-light embryos (2.3 nmol O-2 mm(-3) min(-1)). We sugges t that the ability to achieve temperature-independent rates of Psub-sa t may be important for fucoid embryos that recruit in intertidal micro habitats where photosynthesis is often light-limited.