Characteristic Of Magnesium Nutrition And Interactive Effects Of Magnesium And Potassium In Rice

Magnesium (Mg) and potassium (K) are the essential nutrients for plant growth anddevelopment. They are the important components of plants, participate in variousphysiological and biochemical processess, and play an important role in promoting plantgrowth and development and metabolism. With the increase of crop production per unitarea, the increasing rate of nitrogen and phosphorus fertilization, the decrease of input bothin organic fertilizers and recycling crop residues and straw, there are large amounts of Mgand K extracted from the soil by high yield crops with little returning every year.Consequently, the original balance of among K, Mg and the other nutrients in soils isdestroyed. Mg has a high hydrated radius, it absorbs less strongly to soil colloids than othercations. It is therefore highly prone to be leached. In addition, the rate of Mg uptake can bestrongly depressed by other cations, such as K⁺, NH₄⁺, Ca²⁺, Mn²⁺, H⁺ and Al³⁺, whichresulted in decrease of Mg availability for roots.The deficiency of Mg and K has been increasing in many soils in China's southernareas, especially in the regions of red soils. Mg and K nutrients will become one of mainlimit factors in the further increase production of crops and the development of sustainableagriculture in ChinaCompared with N and P, relative little was reported on the effect of Mg nutrient andinteraction between Mg and K on plants. Although there are some reports the effect ofinteraction between Mg and K on the growth and nutrients uptake and translocation ofplants, very little information is known about the interactive effects of Mg and K on thecrop growth, grain yield and quality, and relative physiological metabolism. Rice (Oryzasativa L.), as one of the most important food crops in the world, is grown in flooded soils intropical and subtropical areas. Intensive weathering due to high rainfall and temperature inthe area may result in low available Mg, and particularly K deficiency for most crops.In the present experiments, rice (Oryza sativa L. ssp. Japonica cv. Wuyun]ing 7) wasused in a greenhouse with solution culture under different concentrations of K and Mg atNanjing Agricultural University. The aim of this study was to investigate effects of Mg nutrition and interaction between Mg and K on plant growth, nutrient uptake andtranslocation, and some relative physiological characteristics in rice.The results obtained are as in follows.Low Mg supply resulted in decrease of shoot biomass yield, increase of sugar contentsin roots, and then increase of its root to shoot ratio in order to adapt low Mg stress, beforethe appearance of Mg-deficient symptom. Moderate Mg supply was able to promote plantgrowth and development of rice, and resulted in increase of dry matter of yield. It alsoenhanced root vigor and bleeding sap rate and amino acid content. Root vigor wasrestrained to certain extent by both low and high levels of Mg. The rate of bleeding sap wascorrelated to the status of plant growth and root activity. At the 10-leaf stage whenconcentration of Mg was at 1.22 g kg^-1 in the leaves and 0.21 g kg^-1 in the roots, the averagediameter, volume, surface area, number of tips, total length and total length of 0.5-1.0 mmdiameter of root were increased by 10.9%, 14.3%, 7.2%, 14.9%, 17.1% and 5.5%respectively in comparison to normal supply of Mg in the rice.Mg deficiency in rice (less than 1.1 mg g^-1 DW in the shoot) resulted in significantreduction of shoot biomass, a decrease of total chlorophyll concentration, netphotosynthetic rate, as well as the reduction of activities of both nitrate reductase (NR; EC1.6.6.1) and glutamine synthetase (GS; EC 6.3.1.2) in the leaves. However, the Mgdeficient leaves contained higher starch, partitioned larger biomass into roots. Excess of Mg(larger than 3.0 mg g^-1 DW in the shoot), together with low K supply suppressed NRactivity and decreased concentration of soluble sugar in the leaves. There were greatantagonistic and moderate synergistic effects between K and Mg, but the K effects weremuch more significant than those of Mg effects on their uptake and translocation, NRactivity and net photosynthetic rate in the leaves. The optimum weight ratio of K to Mgranged between 22 and 25 in the leaves at tillering stage. Mg deficiency was notcompensated by moderate K supply but aggravated by excess K supply, suggesting specificroles of Mg in both dry matter production and partition of carbon assimilates in rice.At low Mg (withdrew from two-week 0.2 mM Mg supply) and high K (6 mM K) for21 days, the rice plants showed severe Mg deficiency and a significant decreases in the drymatter production, The Mg deficiency in leaves decreased chlorophyll concentrations,photosynthetic activity, and soluble protein, but significantly increased the concentrationsof soluble sugars and malondialdehyde (MDA), and the activity of superoxide dismutase(SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6) and peroxidase (POD, EC 1.11.1.7). In addition, Mg concentrations in the leaves and in the shoot biomass were negatively relatedto the activities of the three antioxidative enzymes and the content of MDA in leaves. Therewere very significant interactive effects between Mg and K supplied in the culture solutionon shoot biomass yield, chlorophyll content, photosynthesis rate, activity of SOD, CAT andPOD, and on MDA content in the leaves of rice. It is suggested that the high K level in thenutrient solution aggravated the effect of low Mg supply-induced Mg deficiency andcreated the oxidative damage in rice plants.The results in the present study showed that the interaction between Mg and K hadboth antagonistic effects and synergistic effects. The strong antagonistic effect between Mgand K was significant mainly at low Mg with high K rate of application, whereas thesynergistic effect was showed only at low rate of Mg and K supply, demonstrating theimportance of proportion and balance of Mg and K application.