Photosynthetic response to different phosphorus levels on young Newhall navel orange trees

Abstract：To explore the effect of different phosphorus levels on photosynthesis characteristics of young Newhall navel orange （Citrus sinensis Osbeck.） trees and provide scientific foundation for the phosphorus application in navel orange orchard, potted young Newhall navel orange trees grafted on trifoliate were used to study the photosynthesis rate （Pn） daily change law under the condition of different phosphorus levels. The relationship between phosphorus levels and Pn, transpiration rate （Tr） and water-use effi- ciency （WUE） was also observed. The effect of different phosphorus levels on the content of photosynthet- ic pigment was investigated as well. The results showed that Pn daily change law did not change with phos- phorus levels. Under 25 mg kg-t phosphorus level, Pn was always the highest on the whole day, and the contents of Chl.a, Chl.b and Car. were also the highest, so the level was the most favorable for photosyn- thesis in Newhall navel orange . In addition, there was extremely notable nonlinear quadratic regression relationship between phosphorus levels and Pn, Tr and WUE. Under lower or higher than 25 mg. L-1 phosphorus level, the parameters including Pn, Tr, WUE and the contents of photosynthetic pigments all decreased. The inhibition on the synthesis of Chl.a, Chl.b and Pn was much stronger under higher than 45 mg· L-1 phosphorus level than under lower than 25 mg L-l.