[Leaf photosynthetic potential in canopy layers of un-thinned and thinned apple orchards]

Through the comparison of leaf photosynthetic potential and of photosynthetically active radiation (PAR), leaf nitrogen concentration (Nl), and mass per unit leaf area (Ml) in different canopy layers of un-thinned orchard (UOD) and thinned orchard (TOD), this paper studied the effects of UOD rebuilding on the use efficiencies of PAR and nitrogen, and their relationships to the fruit yield and quality. Thinning obviously improved the radiation environment in canopy. The radiation distribution in TOD canopy was more uniformly than that in UOD canopy, and the invalid space with relative PAR (PARr) less than 30% in TOD approached to zero, while the minimum mean PARr in UOD was 17%, and the space under 0. 3 of relative canopy height was invalid. The leaf photosynthetic efficiency in TOD was notably improved. Comparing with that in UOD, the photsynthetic rate (Pn) at the middle and bottom of the canopy in TOD was increased by 7.8% and 10.2%, respectively. Meanwhile, the photosynthetic potential parameters such as maximum carboxylation rate (Vmax) and maximum electron transfer rate (Jmax) also increased remarkably in TOD. The leaf photosynthetic potential had significant correlation with Nl, and the Nl was strongly correlated with PARr. As a result, leaf photosynthetic potential and PARr could be estimated according to the spatial distribution of relative leaf nitrogen concentration (Nlr).