Previous reviews of the effects of temperature on in vivo photosynthesis have mainly concerned the effects of temperature on light saturated rates. The quantum yield of photosynthesis (phi), as a measure of light limited photosynthesis, has generally been regarded as temperature insensitive. At temperatures close to the minima and maxima at which plants can sustain photosynthetic CO2 assimilation, light may damage the photosynthetic apparatus, an effect termed photoinhibition. A constant feature of photoinhibition is a reduction in phi. In maize, chilling-dependent photoinhibition reduces both phi and the light saturated rate of CO2 assimilation (Asat) and of O2 evolution. Analysis of recovery of CO2 uptake in these leaves suggests that whilst Asat recovers in a few hours, phi may not be fully restored for days. Examination of mature crop canopies shows that only a small proportion of the leaves are likely to become light saturated and then only for part of the day. The relative significance of temperature-induced changes in Asat and phi have also been tested in canopy models of maize crop photosynthesis. These suggest that whilst changes in either parameter will have similar effects on total canopy photosynthesis on the sunniest days of the year, for an average summer's day changes in phi will be of far greater importance. Consideration is therefore given to the factors associated with thylakoid membranes that may determine temperature-induced decreases in phi. Chilling of maize leaves under high light levels reduces the quantum yield of PSII and whole chain electron transport in concert with a decrease in the capacity of isolated thylakoids to bind atrazine, which is indicative of a loss or damage to the QB protein. Besides such classical symptoms of photoinhibition of PSII, chilling also induces the accumulation of a 31 kDa polypeptide in the thylakoids of maize leaves. This polypeptide fractionates with the light-harvesting chlorophyll a/b protein complex (LHCII) and has been tentatively identified as an unprocessed precursor of CP29 since it binds chlorophyll and is immunologically related to CP29. Accumulation of the 31 kDa polypeptide is associated with a modification in the energetics of LHCII, which may result in a decrease in excitation energy from LHCII to PSII and contribute to a decrease in phi. Examination is also made of how stress-induced modifications of interactions between PSII complexes, functioning of the cyt b6/f complex, the permeability of the thylakoid membrane to protons and the activity of the coupling factor may contribute to decreases in phi.