Potassium (K) deficiency is one of the major abiotic stresses which has drastically influenced maize growth and yield around the world. However, the physiological mechanism of K deficiency tolerance is not yet fully understood. To identify the differences of root morphology, physiology and endogenous hormones at different growing stages, two maize inbred lines 90-21-3 (tolerance to K deficiency) and D937 (sensitive to K deficiency) were cultivated in the long-term K fertilizer experimental pool under high potassium (+K) and low potassium (–K) treatments. The results indicated that the root length, volume and surface area of 90-21-3 were significantly higher than those of D937 under –K treatment at different growing stages. It was noteworthy that the lateral roots of 90-21-3 were dramatically higher than those of D937 at tasselling and flowering stage under –K treatment. Meanwhile, the values of superoxide dismutase (SOD) and oxidizing force of 90-21-3 were apparently higher than those of D937, whereas malondialdehyde (MDA) content of D937 was obviously increased. Compared with +K treatment, the indole-3-acetic acid (IAA) content of 90-21-3 was largely increased under –K treatment, whereas it was sharply decreased in D937. On the contrary, abscisic acid (ABA) content of 90-21-3 was slightly increased, but that of D937 was significantly increased. The zeatin riboside (ZR) content of 90-21-3 was significantly decreased, while that of D937 was relatively increased. These results indicated that the endogenous hormones were stimulated in 90-21-3 to adjust lateral root development and to maintain the physiology function thereby alleviating K deficiency.

Potassium is an important nutrient element requiring high concentration for photosynthetic metabolism. The potassium deficiency in soil could inhibit soybean (Glycine max (L.) Merr.) photosynthesis and result in yield reduction. Research on the photosynthetic variations of the different tolerant soyben varieties should provide important information for high yield tolerant soybean breeding program. Two representative soybean varieties Tiefeng 40 (tolerance to K+ deficiency) and GD8521 (sensitive to K+ deficiency) were hydroponically grown to measure the photosynthesis, chlorophyll fluorescence parameters and Rubisco activity under different potassium conditions. With the K-deficiency stress time extending, the net photosynthetic rate (Pn), transpiration rate (Tr) and stomatal conductance (Gs) of GD8521 were significantly decreased under K-deficiency condition, whereas the intercellular CO2 concentration (Ci) was significantly increased. As a contrast, the variations of Tiefeng 40 were almost little under K-deficiency condition, which indicated tolerance to K+ deficiency variety could maintain higher efficient photosynthesis. On the 25th d after treatment, the minimal fluorescence (F0) of GD8521 was significantly increased and the maximal fluorescence (Fm), the maximum quantum efficiency of PSII photochemistry (Fv/ Fm), actual photochemical efficiency of PSII (ΦPSII), photochemical quenching (qP), and electron transport rate of PSII (ETR) were significantly decreased under K+ deficiency condition. In addition, the Rubisco content of GD8521 was significantly decreased in leaves. It is particularly noteworthy that the chlorophyll fluorescence parameters and Rubisco content of Tiefeng 40 were unaffected under K+ deficiency condition. On the other hand, the non-photochemical quenching (qN) of Tiefeng 40 was significantly increased. The dry matter weight of Tiefeng 40 was little affected under K+ deficiency condition. Results indicated that Tiefeng 40 could avoid or relieve the destruction of PSII caused by exceeded absorbed solar energy under K-deficiency condition and maintain natural photosynthesis and plant growth. It was an essential physiological mechanism for low-K-tolerant soybean under K-deficiency stress.