Starch, soluble sugars and chlorophyll measurements
Soluble sugars and starch were determined by the anthrone method (Yemm and Willis 1954). Pigments were extracted and determined according to the method described by Markwell et al. (1986). The contents of chlorophylls a  and b  were calculated using the formulas described previously (Porra et al ., 1989) and expressed as μg pigment g–1 fresh weight (FW).