PHOTOINHIBITION OF PHOTOSYNTHESIS CAUSES A REDUCTION IN VEGETATIVE GROWTH-RATES OF DWARF BEAN (PHASEOLUS-VULGARIS) PLANTS

Effects of photoinhibition of photosynthesis were assessed on growth rates of dwarf bean (Phaseolus vulgaris L.). Plants were grown at four photon flux densities (PFDs) ranging from 300 to 1000 mu mol m(-2) s(-1) and at 25/20 degrees C and 17.5/17.5 degrees C. Plants were also grown and transferred from 25 to 17.5 degrees C either at 500 and 1300 mu mol m(-2) s(-1) or from 500 to 1300 mu mol m(-2) s(-1). Relative growth rates (RGRs) were determined by destructive harvests and photoinhibition was assessed by fluorescence and photon yields of O-2 evolution. Carotenoid pigments were analysed by HPLC. The RGRs were PFD-dependent, increasing to a maximum at 430 mu mol m(-2) s(-1) at 17.5 degrees C and at 650 mu mol m(-2) s(-1) at 25 degrees C. At 25 degrees C, growth rates at 1300 mu mol m(-2) s(-1) were inhibited about 25% over that at light saturation. Transferring the plants from 25 to 17.5 degrees C at constant PFD reduced RGR by a similar amount. Concurrently increasing the PFD resulted in a further 9% reduction in RGR. These changes were consistent with photoinhibition-induced limitations on photosynthesis. Correlated with growth rate reductions were marked changes in both photon yield and F-y/F-m, revealing a causal link between photoinhibition of photosynthesis and inhibition of growth rates. However, acclimation occurred via zeaxanthin-mediated photoprotection during exposure to the photoinhibitory conditions, ameliorating the longer term effects of photoinhibition on growth.