Abstract
Potassium (K) nutrition plays a key role in alleviating a variety of peel disorders in tree fruit, but the effect of this nutrient on the physiological and metabolic profiles involved in the fruit splitting of citrus remains unclear. Three levels of K were used to treat citrus ‘Ehime Kashi 34’ (Citrus Nishinoka × C. Shiranui), a hybrid cultivar with fruit that easily split. The results showed that the roots of the treatment with K fertilizer increased the contents of calcium (Ca2+), nitrogen (N), and K in the skin and flesh, the fruit firmness ratio of the peel to the flesh, photosynthetic rate, stomatal conductance, and concentration of intercellular CO2. In contrast, it decreases the relative chlorophyll index and content of Ca2+ in the leaves. Simultaneously, 59 and 13 differentially expressed metabolites (DEMs) were detected in the peel and flesh, respectively, after treatment with K. Of them, five compounds were upregulated, including the synthesis of various amino acids in the peel and the accumulation of various glycoside metabolites in the flesh which were upregulated. The accumulation of levels of gibberellin and glycoside were downregulated. That could be the main reason why potassium alleviates fruit splitting.
Highlights
Late-maturing citrus varieties, such as ‘Ehime Kashi No 28’ and ‘Ehime Kashi No 34’, are increasing in economic importance owing to their preferable flavor and maturity period close to the Chinese New Year [1,2], these varieties are susceptible to a disorder of the fruit peel, termed fruit splitting, which seriously affects the quality of fruit and causes substantial economic losses
This study shows that the roots of the treatment with K fertilizer can effectively alleviate the incidence of citrus fruit splitting
High K can promote the metabolism of amino acids in the peel of citrus fruit and accelerate the consumption of gibberellin, which may result in less fruit splitting
Summary
Late-maturing citrus varieties, such as ‘Ehime Kashi No 28’ and ‘Ehime Kashi No 34’, are increasing in economic importance owing to their preferable flavor and maturity period close to the Chinese New Year [1,2], these varieties are susceptible to a disorder of the fruit peel, termed fruit splitting, which seriously affects the quality of fruit and causes substantial economic losses. It is worth noting that if K is augmented to cracked citrus cultivars to achieve a level of 1.0% to 1.5% of K in the leaves, the peel will be thicker and not split [3] This could be owing to the ability of K to promote fruit cell division and root proliferation, increase the efficiency of root system to absorb nutrients important for plant growth, and increase the thickness and strength of the peel, thereby reducing fruit splitting [8,9]. A deficiency in K can synthetically constrain growth owing to its effects on the diffusion of CO2 and assimilation products that will result in an impact on photosynthesis [17] This results in a large accumulation of carbohydrates in the source leaves and limits the translocation of sucrose from buds to sink tissues, including fruit. K plays an important role during the process of regulating plant metabolites, but it is not enough to explain the specific effect of K on fruit splitting
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