Effect of low magnesium stress on mineral element contents in various organs, yield and fruit quality in different lemon varieties

Abstract: 【Objective】The soil type in the south of China is mainly red soil, and magnesium (Mg) deficiency is a common problem in citrus production. Mg deficiency leads to the decrease of fruit yield and quality in lemon-producing areas of Yunnan province. Different lemon varieties have various characteristics of tolerance to Mg deficiency. In this study, the variety that could tolerate Mg deficiency was screened out by comparing the response of mineral nutrient contents in various organs, yield, and fruit quality to low Mg supply in different lemon varieties. Hopefully, the study will provide a theoretical ba- sis for the proper lemon cultivar regionization and fertilization management.【Methods】The grafted seedlings were selected from three main lemon varieties, Femminello, Yunning No.1 and Tahiti, which were relatively uniform in stem thickness and height, and grew well. After the attached soil was washed out and main roots were pruned, the grafted lemon seedlings were transplanted into a plastic pot filled with 28 kg of silver sand, and cultivated in a greenhouse. And then, the seedlings were supplied with distilled water only, and watered with 1/2 concentration of Hoagland's nutrient solution and Mg-free Aron’s full concentration nutrient solution at the stage of shoot growing. Mg nutrient gradient stress was carried out when 7-8 new leaves emerged, and the treated period was one year. Three Mg concentra- tion gradients were set for each lemon variety, which were deficient magnesium (Mg0: 0 mg·L-1), insuf- ficient magnesium (Mg6: 6 mg·L-1) and normal magnesium level (Mg24: 24 mg·L-1). There were three biological repetitions for each treatment with five plants in each repetition. At the end of the pot trial ex- periment, the mineral nutrient contents including nitrogen, phosphorus, potassium, zinc, iron, manga- nese, copper, calcium, and magnesium in root, stem, leaf and fruit were determined, and the fruit quality including fruit size, TSS, total acid content, and vitamin C contents was determined【. Results】Under low magnesium stress, the highest declines of Mg contents presented in the leaves and roots of Femmi- nello, the fruits of Yunning No.1 and the stems of Tahiti, which were 29.96%, 43.97%, 21.13%, and 26.87%, respectively. With the same Mg concentration treatment, the Mg content in the leaves of Yun- ning No.1 was significantly higher than that of two other varieties, while the Mg content in the stem of Tahiti was significantly higher than that of two other varieties, and the Mg content of the fruit was as follows: Yunning No.1 > Femminello > Tahiti. Under low magnesium stress, the trend of mineral nutri- ent changed in a similar way as underground parts of different lemon varieties, but there were differenc- es in the aboveground parts. The contents of P, K, Ca, Mn in various parts of the lemon plants increased under the low magnesium stress conditions, and the contents of Zn in fruit, stem and root decreased sig- nificantly. The contents of copper and iron varied in various parts of different lemon varieties. The exter- nal fruit quality was almost unaltered, but the juice yield, total soluble solid, total acid and vitamin C contents decreased significantly under low magnesium stress conditions. The effect of low magnesium stress on different varieties was as follows: Tahiti > Yunning No.1 > Femminello. The total soluble solid, total acid, and vitamin C contents in the fruit were significantly correlated with Mg content, which were significantly negatively correlated with K, Ca and Mn contents. Under low magnesium stress con- ditions, the fruit number and the yield per plant decreased significantly, with Yunning No.1 decreasing the most, the treatments of Mg deficiency and Mg insufficiency decreasing by 67.62% and 44.14% re- garding fruit number, respectively, and decreasing by 67.07% and 46.17% regarding yield, respectively, while Tahiti declined the least. Difference in magnesium content and individual plant yield between treatments resulted in significant difference in the amount of magnesium contained in the fruit. Under the condition of normal magnesium content, the fruit yield, Mg content, and Mg load of Yunning No.1 were significantly higher than two other varieties, and it required more Mg to maintain normal growth of lemon, resulting in greater sensitivity to Mg deficiency. Under low Mg conditions, there was no sig- nificant difference in yield between Tahiti and two other varieties, but its Mg content and fruit’s Mg load were significantly lower than those of two other varieties, As a result, magnesium- requiring amount was less than two other varieties, showing certain tolerance to Mg deficiency.【Conclusion】In summary, low magnesium stress displayed the least decrease of magnesium content and yield reduction and influence on other elements in Tahiti. These results suggested that Tahiti was the most resistant to magnesium deficiency among the three varieties. Thus lemon varieties like Tahiti should be chosen to plant in areas with severe Mg deficiency. It is also important to make reasonable Mg fertilizer supply in each growing area with different varieties.