Chlorophyll-fluorescence measurements in bryophytes: evidence for three main types of light-curve response

Abstract

Diffusion theory predicts that, except in the lower part of the daylight range, carbon dioxide supply will always be limiting for photosynthesis in a unistratose leaf. We have used chlorophyll fluorometry to survey the photosynthetic responses of numerous bryophytes to a range of light intensities employing the ‘light curve’ approach. Initially, as light intensity is increased in a stepwise manner, electron transport rate (ETR) in bryophytes follows a saturation curve closely fitted by a negative exponential function, y = A(1 – e–kx), where y = ETR, x = light intensity (or photosynthetic photon flux density), A is the asymptote (ETR at infinitely high light intensity), k is a rate constant and e is the base of natural logarithms. The initial slope of the response curve, Ak, approximates maximum quantum efficiency (Fv/Fm) which is measured on dark-adapted plant material. However, at higher intensities ETR frequently veers away from the saturation curve owing to the onset of either photoinhibition or the dissipation of the excitation energy by a photoprotective mechanism, probably involving reduction of O2. In the latter case, the measurement of ETR significantly overestimates the rate of photosynthetic carbon fixation. We describe a simple approach that enables these instances of photoprotection and photoinhibition to be identified and discuss the wider significance of the results to the ecology of individual species.