DROUGHT TOLERANCE AND LIGHT REQUIREMENTS OF HIGH AND LOW SUBLITTORAL SPECIES OF MEDITERRANEAN MACROALGAE OF THE GENUS CYSTOSEIRA C-AGARDH (FUCALES, PHAEOPHYCEAE)

A comparative study of two Mediterranean macroalgae, the high growing sublittoral Cystoseira mediterranea and the low growing sublittoral Cy stoseira zosteroides was made to evaluate their ability to resist desi ccation, their drought tolerance and their photosynthetic light requir ements. Desiccation resistance and drought tolerance were markedly hig her for the high sublittoral C. mediterranaea. Cystoseira mediterranea lost 50% of its relative water content (RWC) after 3.3 h, whereas C. zosteroides lost the same amount of moisture in only 1.1 h of exposure to air in controlled laboratory conditions (20 degrees C and 75% of r elative air humidity). After complete desiccation, only the high subli ttoral species was able to rehydrate fully by re-wetting. Cystoseira m editerranea was able to maintain its full photosynthetic rate to a RWC as low as 44%, whereas photosynthesis of C. zosteroides was reduced i n plants with an 80% RWC. Photosynthetic rate was zero in plants of C. mediterranea and C. zosteroides with a RWC of 18% and 34%, respective ly. Photosynthesis-irradiance (P/I) curves of C. mediterranea were cha racteristic of sun adapted species, with high saturating irradiance (I -k > 1600 mu mol photon m(-2) s(-1)), high irradiance at the compensat ion point (I-c = 21 mu mol photon m(-2) s(-1)), and high maximum photo synthetic rate (P-max = 12.3 mg C g ash free dw(-1) h(-1)). Conversely , C. zosteroides photosynthetic response to light was typical of shade adapted species, exhibiting photoinhibition at irradiances above 800 mu mol photon m(-2) s(-1), and having comparatively lower I-k (375 mu mol photon m(-2) s(-1)), I-c (9 mu mol photon m(-2) s(-1)), and P-max (6.0 mg C g ash free dw(-1) h(-1)). Our results show that taxonomicall y and morphologically related plants display marked differences in des iccation resistance, drought tolerance and light requirements, in acco rdance with the environmental conditions prevailing in their respectiv e habitats.