Photoinhibition of Photosynthesis Causes a Reduction in Vegetative Growth Rates of Dwarf Bean (Phaseolus vulgaris) Plants

Abstract

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 pmol m^-2 s^-1 and at 25/20ºC and 17.5/17.5ºC. Plants were also grown and transferred from 25 to 17.5ºC either at 500 and 1300 μmol m^-2 s^-1 or from 500 to 1300 μmol m^-2 s^-1. Relative growth rates (RGRs) were determined by destructive harvests and photoinhibition was assessed by fluorescence and photon yields of O₂ evolution. Carotenoid pigments were analysed by HPLC. The RGRs were PFD-dependent, increasing to a maximum at 430 μmol mm^-2 s^-1 at 17.5ºC and at 650 μmol m^-2 s^-1 at 25ºC. At 25ºC, growth rates at 1300 μmol m^-2 s^-1 were inhibited about 25% over that at light saturation. Transferring the plants from 25 to 17.5ºC 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_vF_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.