Photoinhibition of photosynthesis in dwarf bean (Phaseolus vulgaris L.) leaves: effect of sink-limitations induced by changes in daily photon receipt

Bean (Phaseolus vulgaris L. cv. Long John) plants were grown with photoperiods of 6 and 16 h at constant photon flux density (PFD), giving a daily photon receipt (DPR) of 17 and 48 mol.m-2 respectively. Vegetative growth was determined at regular intervals and diurnal whole-plant photosynthesis measured. Intact trifoliate leaves were exposed to photoinhibitory treatments at PFDs of 800 and 1400 micromoles.m-2.s-1 at temperatures of 14 and 20 degrees C, both in the absence and presence of the inhibitors chloramphenicol and dithiothreitol. Fluorescence and photon yields were determined at regular intervals throughout each treatment. Plants grown with photoperiods of 6 h had significantly lower growth rates than those grown with 16-h photoperiod but no difference in net photosynthetic rates or photon yields were found. Carbohydrate analyses confirmed short-day plants were strongly sink-limited. Long-day plants were slightly sink-limited, with a high proportion of starch in the leaves and reduced photosynthesis between 13 and 16 h. Plants grown in low DPR were more susceptible to photoinhibition, from sustained closure of some photosystem II reaction centres, than plants grown in high DPR. Capacity for thermal dissipation appeared dependent on PFD while photochemical capacity was more dependent on DPR.