A study was conducted on the diurnal changes of
photosynthetic rate and fluorescence parameters of maize(C_4 crop) and peanut(C_3
crop) under field conditions, The results show there were obvious differences between the sensitivities of maize and peanut to photon flux density (PFD). The diurnal changes of Pn in maize was single peaked, consistent with that of PFD; however the curve of peanut was double peaked with an evident decline at noon. Maximal
photochemical efficiency of PSⅡ in the darkness (Fv/Fm), actual photochemical efficiency of PSⅡ in the light (ΦPSⅡ), photochemical quenching co-efficient (qP) and non-photochemical quenching (NPQ) all presented apparent diurnal changes. The declines in Fv/Fm, ΦPSⅡ, qP were observed, but NPQ amounted to maximum value at midday. All the 5 indexes(Pn,Fv/Fm,ΦPSⅡ,qP,NPQ) showed obuious difference between two crops. Maize(C_4 crop) had higher Pn,ΦPSⅡ,NPQ and lower Fv/Fm,qP than peanut(C_3 crop). These differences in parameters between maize and peanut was especially obvious at midday.