Effects Of Different N And K Levels In Leaf On Photosynthetic And Growth Characteristics Of Newhall Navel Orange Budlings

Macroelement nitrogen (N) and potassium (K), are necessary for growth of citrus trees. The optimum N and K contents are beneficial to growth and development of citrus trees, but there would be negative effects when N and K contents are excessive. Newhall navel orange(Citrus sinensis Osbeck) is a major cultivar in China, and is popular with most consumers and growers for its good external appearance and superior interior quality. However, there are few reports about the suitable contents of nutrient elements for Newhall navel orange. This study was designed to investigate the effects of different N and K levels on photosynthetic and growth characteristics of young Newhall navel orange trees. The results could be used as a reference for determining of the optimum contents of N and K in leaf for Newhall navel orange production.One year old potted Newhall navel orange budlings on trifoliata orange(Poncirus trifoliata (L.) Raf.) rootstocks growing in purple soil, which were applied with different amounts of urea(CO(NH2)2), potassium sulfate(K2SO4) and potassium nitrate(KNO3) for inducing different N and K levels in leaf, were used to measure the nutrient element contents, chlorophyll(Chl) contents, net photosynthetic rate(Pn), stomatal conductance(Gs), intercellular CO2concentration(Ci) and transpiration rate(Tr) of leaves at regular intervals. Heights, stem diameters and biomasses of plants and the numbers of shoots, leaves and flower buds were recorded. Effects of different N and K levels in leaf on photosynthetic and growth characteristics of leaves on trees in this experiment were discussed and analyzed. Results are as follows.1. Contents of N and K in leaf changed as different amounts of N and K applications. As the amount of N applied increased, N contents in leaf increased from2.0%to3.3%gradually. K contents in leaf gradually rose from3.5%to5.0%with the increase of K application rate.2. N and K levels in leaf affected other nutrient element contents in leaf. Increasing leaf N levels promoted the absorption of Ca, Fe and Mn, but restrained the increase of P, K, Mg, S and Cu. High leaf K level was beneficial to the absorption of N, S and Zn, but decreased Ca, Mg and Mn absorption.3. There were significant positive correlations between leaf SPAD values and contents of chlorophyll a(Chla), chlorophyll b(Chlb), Ch1and carotenoid(Car) in the leaves of spring shoots, summer shoots and autumn shoots for all treated trees. High correlations between SPAD values and Ch1contents in leaves of unit area were also existed, which indicated that SPAD values can accurately reflect leaf Ch1contents of Newhall navel orange. Leaf Ch1contents increased linearly with the increase of N application rate, and leaf N level above2.9%were beneficial for maintaining the leaf Ch1contents in winter and early spring. But high leaf K level decreased the leaf Ch1contents greatly during winter-spring season.4. Pn, Gs and Tr of leaves on two-year-old autumn shoots and one-year-old spring shoots changed as a parabola with the increase of leaf N levels, but the variations of Ci showed no regularity. Pn, Gs and Tr of leaves on two-year-old autumn shoots and one-year-old spring shoots were highest when leaf N content was2.9%. Pn, Gs, Tr, and Ci changed irregularly when leaf K level increased. Increasing of leaf K levels induced the Pn decrease in leaves on two-year-old autumn shoots. When leaf K content was4.7%, Pn of leaves on one-year-old spring shoots was higher.5. Leaf N levels within2.2%-3.0%were suitable for Newhall navel orange growth. On this N level, plant biomasses increased significantly as the levels of leaf K increased. But on the basis of lower leaf N level, the plant growth was restrained as the leaf K level inrcesed. Leaf N level above3.0%promoted flower bud differentiation.In summary of the above, moderately increasing leaf N level promoted photosynthesis and growth of leaves on Newhall navel orange trees. On this basis, proper high leaf K level was benificial. But high leaf K level with lower leaf N level inhibited leaf photosynthesis and plant growth.