Magnesium Effects on Carbohydrate Characters in Leaves, Phloem Sap and Mesocarp in Wax Gourd (Benincasa hispida (Thunb.) Cogn.)

Abstract

Magnesium (Mg) is critical for agricultural production and human health. The wax gourd yield was greatly affected by Mg fertilizer and it serves as an excellent crop to study Mg functioning in sink growth; however, as a Cucurbitaceae plant which is called a raffinose family oligosaccharides (RFOs)-transporting plant, its adaptive mechanism of photoassimilates transportation and distribution to Mg nutrients remains unknown. Herein, we used two Mg treatments (+Mg 90 kg ha−1; CK as control 0 kg ha−1) to observe the effects of Mg on the photoassimilates status in the leaves, phloem sap and fruit of wax gourd grown across the entire growth stage under a field condition. For the first time, we confirmed that stachyose and raffinose, which accounted for 54.9–78.0% of the total carbohydrates across the whole growth phase in the phloem sap, were the most predominant sugars used for the long-distance transport of wax gourds. They were strongly increased by Mg application which started from the seedling stage to the end of growing season. This response was earlier and more dramatic than the over-accumulation of sucrose in leaves regardless of positions. Hexoses (glucose plus fructose) were the main soluble sugars in the source leaves as well as in the mesocarp tissues, and their responses to Mg varied with temporal and spatial differences. The difference in the sugar status in the leaves between the upper, middle and lower positions is closely related to the concentration of Mg ions. Compared with Mg deficiency (CK), Mg application stimulated sugar accumulation in the leaves at the seedling and flowering stages by 10.6–24.5%. On the contrary, after fruit set, Mg supplementation significantly reduced the soluble sugar concentration in leaves by 33.5–67.9%. Mg treatment also spiked the fructose and glucose in fruit at the fruit expansion stage by 26.7% and 16.4%, respectively. Taken together, our results showed that Mg tailors the carbohydrate status in the source (leaves), flow (phloem sap) and sink (mesocarp tissues) in wax gourds, especially during the fruit growth period. The lower stachyose in the phloem at the beginning of fruit setting may be an early indication of a curtailed sink process in wax gourds grown in Mg deficient soil.