Low Magnesium with High Potassium Supply Changes Sugar Partitioning and Root Growth Pattern Prior to Visible Magnesium Deficiency in Leaves of Rice (Oryza sativa L.)

This research was conducted to investigate the effects of low magnesium (Mg) with high potassium (K) supply on the Mg concentration, sugar partitioning and root growth of rice (Oryza Sativa L. cv. Wuyunjing 7) plants grown in hydroponics under greenhouse conditions, at Nanjing Agricultural University, China. The nutrient solution contained 0.01 and 1.0 mM Mg concentration, with K at 1.0 and 6.0 mM. Compared with the control (1mM Mg and K) treatment, the soluble sugar content at the treatment of low Mg (0.01 mM) with high K (6 mM) decreased by 35.7% in leaves, whereas increased 29.2% in roots at day 15 after treatment initiation. The shoot dry weight (DW) declined 12.9%, but root DW increased 12.1% leading to the significant increase in the root to shoot ratio at day 30. Furthermore, the total root length, total root surface area, root volume, average root diameter, total length of 0 0.5 mm and 0.5 1.0 mm diameter roots at day 30 significantly increased by 11.8%, 16.4%, 25.3%, 8.1%, 16.6% and 12.5%, respectively. Correlation analysis revealed the root to shoot ratio is closely related to the soluble sugar contents in roots and root morphological parameters of rice at day 15 and day 30. The typical visible symptoms of Mg deficiency in leaves of rice were obtained in the treatment of low Mg with high K at day 35. These findings indicated that low Mg with high K supply altered sugar partitioning and root morphological parameters, resulting in the increased root to shoot ratio prior to visible Mg deficiency symptoms in rice leaves. The increase in root to shoot ratio maybe an important adaptive mechanism for rice plants to respond to low-Mg stress during the early growth stage.