Critical Leaf Magnesium Thresholds for Growth, Chlorophyll, Leaf Area, and Photosynthesis in Rice (<i>Oryza sativa</i> L.) and Cucumber (<i>Cucumis sativus</i> L.)
Kailiu Xie | Yonghui Pan | Xusheng Meng | Min Wang | Shiwei Guo
Accurately understanding the critical threshold of leaf magnesium (Mg) concentration is crucial for rapid diagnosis of crop Mg status; however, little information is available on critical Mg concentration for different physiological processes in dicots and monocots. Here, we investigated the sensitivity of biomass, chlorophyll (Chl) at different leaf positions/ages, leaf area (LA), and photosynthesis (P<sub>n</sub>) to Mg deficiency between rice (<i>Oryza sativa</i> L.) and cucumber (<i>Cucumis sativus</i> L.). Plants were grown hydroponically under twelve Mg concentration gradients. Results showed reducing the external Mg supply to a certain level resulted in significant decline in biomass, Chl, LA, and P<sub>n</sub> in both plants. A leaf Mg threshold of 0.97 mg g<sup>−1</sup> DM (dry matter) for total biomass was found in rice, which was not identified in cucumber. Critical Mg thresholds for Chl a, b, and carotenoids (Car) showed a decreasing trend with leaf age, suggesting Chl in upper young leaves are more sensitive to Mg deficiency; however, visible Mg-deficiency symptoms were predominantly in mid-aged leaves with a higher rate of Mg remobilization, especially in cucumber. Leaf critical Mg concentrations for Chl a+b, P<sub>n</sub>, and LA were 1.22, 1.05, and 1.00 mg g<sup>−1</sup> DM in rice, respectively, which were lower than those of cucumber, 4.23, 4.09, and 3.55 mg g<sup>−1</sup> DM, implying that cucumber was more susceptible to low Mg stress; Chl a+b was the most sensitive indicator of Mg deficiency. Overall, Chl a+b of upper young mature leaves can be used as an early diagnostic index of Mg nutrition in crops, especially Mg-insensitive crops.
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