Effect of different types of fruiting shoots on organic acid content in citrus fruit grown in a solar greenhouse

Abstract：【Objective】In order to provide a reference for pruning of fruiting branches in greenhousegrown citrus, the experiment was carried out to explore the influence of different types of fruiting branches on the content of organic acids in citrus fruit.【Methods】The experiment used‘Nanfeng tangerine' (Citrus reticulate Blanco‘Kinokuni') grafted on the rootstock of trifoliate orange [Poncirus trifoliate (L.) Raf.] as material. Gas chromatography was used to measure the contents of sugars and organic acids. The activities of enzymes involved in tricarboxylic acid cycle (TCA) were analyzed, and key genes related to organic acid metabolism were analyzed by real-time quantitative reverse transcription polymerase chain reaction.【Results】Sucrose contents in fruits from both the leafy and leafless bearing shoots showed an increasing trend throughout the whole development period, and the accumulation of sucrose was accelerated from 120 days after flowering (DAF) . Quinic acid content presented a decreasing trend during fruit development in both shoot types, and the decrease amplitude was largest from 30 DAF to 90 DAF, after which quinic acid content became relatively constant as fruit matured.Glucose and fructose levels dropped sharply from 30 DAF, and began to rise slowly from 60 DAF. Thechange trends of glucose and fructose were similar with slight fluctuation during the whole development period. Sucrose content was much higher than those of glucose and fructose when the fruit ripened. During the whole development period, changes in citric acid content in fruit from the two types of bearing shoots were similar but relatively complicated with fluctuations. The differences in citric acid content between the two types fruiting shoots were not significant. The change in malic acid content in fruit from the leafy and leafless shoots was great from 30 DAF to 120 DAF, and leveled off from 120 DAF. The change pattern of malic acid in the two shoot types was similar, but malic acid content in fruit from the leafless shoots was 2.53 mg·g-1 at 30 DAF, which was significantly higher than that from the leafy shoots (1.51 mg·g-1) . Malate dehydrogenase (MDH) activity in the fruit from the two shoot types showed a relatively constant increasing pattern. The activities of citrate synthase (CS) and aconitase (ACO) fluctuated significantly bud displayed an increasing trend during the whole development period.Phosphoenolpyruvate carboxylase (PEPC) activity declined generally with some fluctuation at certain period. However, the change trends of the four important enzymes were consistent in the two shoot types, which had no significant difference in enzyme activities. The expression trends of CS and ACO were similar to that of citric acid content. The expression of the two genes in the leafy shoots was higher than in the leafless shoots in the early stages of development, but an opposite pattern was found in the late stages of development. The expression levels of SDH and FH in fruit from the leafless shoots were generally significantly higher than those from the leafy shoots, but the change trends of gene expression were of no defined pattern. The expression of MDH encoding the mitochondrion malate dehydrogenase in fruit from the leafy shoots was significantly higher than that from the leafless shoot at 30 DAF, after which it became significantly lower in the leafy shoot than that in the leafless shoots until the fruit became ripe at 210 DAF, when the expression of MDH became similar in the two shoot types.The expression of MDH encoding malate dehydrogenase in the cytoplasm had significant difference between the two shoot types at several developmental stages. The expressions of the above genes presented a pattern of declining at first and increasing later, and were similar in the two shoot types. The expression level of ME (malic enzyme) in the cytoplasm was significantly different between the two shoot types during the whole development period. ME expression in fruit from the leafy shoots increased slightly from 30 DAF to 60 DAF, then decreased until 150 DAF and rose again towards full maturation, while in the leafless shoots, it had two peaks throughout the whole development period.【Conclusion】In the case of‘Nanfeng tangerine'grown in greenhouses, the types of fruiting shoots affect the fruit setting rate and yield, but not the contents of sugars and organic acids and the activities of key enzymes related to organic acid metabolism in fruit. Therefore, when we prune the citrus trees, we need to consider its impact on yield.