SEASONAL ADAPTATION AND SHORT-TERM METABOLIC RESPONSES OF GELIDIUM-SESQUIPEDALE TO VARYING LIGHT AND TEMPERATURE

The responses of Gelidium sesquipedale productivity, photosynthetic ef ficiency and respiration to changes in light and temperature were meas ured on plants acclimated to different light/temperature combinations simulating average winter and summer conditions at 2 different depths. Photosynthetic efficiency was reported relative to frond surface area computed by a method described in this study. Algal pigment contents were monitored during acclimation. Results suggest that seasonal accli mation to light and temperature may have a significant effect on photo synthetic efficiency and on the dependence of productivity on light an d temperature. This effect may be the opposite of what is generally ex pected from acclimation to photon flux density alone but is in good ag reement with previously reported results indicating an increase in pho tosynthetic efficiency in plants grown at higher temperatures. There i s some evidence that temperature adaptation may compensate light adapt ation through changes at physiological levels other than the pigmentar y system. Significant seasonal effects on respiration, as well as a st rong temperature dependence, are apparent. Winter acclimated plants ha d higher respiratory rates than summer acclimated ones. Among the form er, plants acclimated to the greater depth exhibited lower respiratory rates than those acclimated to the shallower depth, which is an impor tant adaptation for surviving under low light conditions by reducing e nergetic losses. The compensation photon flux density values obtained in this work for G, sesquipedale suggest that during winter the averag e net photosynthesis may be negative for a significant part of its sub tidal populations.