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The objective was to study the relationship between maize hybrids and their parents in potassium ion (K+)-uptake characteristics under the stress from a low amount of potassium. Superoxide dismutase (SOD) activity, the malondialdehyde (MDA) content, the potassium utilization index (KUI), root volume, total and active root uptake area, Michaelis-Menten coefficient (Km), and the maximum rate of uptake (Vm) of different maize (Zea maysL.) genotypes were determined. SOD activity and MDA content in the leaves of Si144, Xianyu335 and PH4CV were significantly higher than the control, whereas for Jidan27, Si-287 and PH6WC, no differences were detected. Jidan27 showed a higher KUI than Xianyu335 under 0.1 mM potassium chloride (KCl), but no differences were observed under 3 mM KCl. Jidan27, Si287, and PH6WC had a greater root volume, total and active uptake areas, higher Vm and smaller Km than the other genotypes under 0.1 mM KCl. The differences in the KUI among maize genotypes resulted from variation in the Km to a greater extent than the Vm. These results indicate that potassium absorption characteristics of the low-K+-tolerance genotype, Jidan27 resembled its female parent, Si-287, whereas that of the stress-sensitive genotype, Xianyu335 was more like its male parent, PH4CV. [ABSTRACT FROM PUBLISHER]

Eighteen soybean genotypes differing in aluminum (Al) tolerance were used to investigate genotypic differences in Al-induced citrate exudation and its role in Al tolerance. Aluminum accumulation and localization in soybean roots were examined by analysis of total Al and hematoxylin staining. Soybean genotypes exhibited a wide range of Al tolerance. Based on relative root elongation, several Al-tolerant genotypes from Brazil such as B 1, B 10, and B 15 were more tolerant than the Al-tolerant PT 416937 (PT) and Perry. All soybeans exuded citrate in response to Al stress, and some Al-sensitive genotypes secreted more citrate than tolerant ones, showing no correlation between the Al tolerance and Al- induced citrate exudation. Further study found that both copper (Cu) and cadmium (Cd) stimulated citrate and malate exudation in soybean, indicating that organic acid secretion is not specifically induced by Al. Aluminum concentrations were significantly higher in 2-3 and 3-4 cm of segments than that in 0-1 and 1-2 cm segments under 15 μM AlCl3. Both the root mature zone and apex were heavily stained by hematoxylin after exposure to 10, 15, or 20 μM AlCl3 (24 h), whereas root elongation zone was not stained. After exposure to 50 μM AlCl3 for 20 min, the Al-tolerant PI was less stained by hematoxylin than the Al-sensitive Young, suggesting that Al accumulation in root apices seem to be an immediate response to Al stress, and related to differential Al sensitivity. Present results suggest that citrate secretion induced by Al stress may not be a key mechanism responsible for the differential Al tolerance of some soybean genotypes and other mechanism(s) conferring Al exclusion should exist and operate immediately after exposure to Al stress. [ABSTRACT FROM AUTHOR]

In order to investigate the combined effect of drought stress and aluminum (Al) toxicity on citrate exudation in soybean, hydroponic cultivation with addition of Al and PEG-6000 was conducted to simulate Al-toxic dry soil. Results showed that 2-day exposure of soybeans to 5.5% (w/v) PEG-6000 or 100 µM AlCl3 mainly hindered root growth, while combined exposure to PEG and Al (PEG/Al) reduced both root and shoot growth. Photosynthetic rate of first trifoliolate for the Al-tolerant genotype PI 416937 (PI) was not affected by imposition of 5.5% PEG/100 µM AlCl3 (38–40 h), whereas photosynthetic rate for the Al-sensitive genotype YC was significantly reduced. Based on root fresh weight, Al-induced citrate exudation in the roots of soybean genotypes recovered from pre-treatment with 5.5 or 9% PEG was not altered, but was increased in the plants pretreated with 7% PEG without recovery. After 2 days of recovery from 2-day combined exposure to 5.5% PEG/100 µM AlCl3, the Al-tolerant PI exuded more citrate than its control, but the Al-sensitive YC exuded significantly less citrate than its corresponding control. Split root experiment revealed that Al-induced citrate exudation in one half of the root system was significantly reduced by exposing the other half of the root system to 0.5 mM CaCl2 solution containing 9% PEG or 9% PEG/50 µM AlCl3. However, organic acid secretion was not observed in the half of the root system in the Ca solution when the other part of the root system was exposed to the Ca solution containing 50 µM AlCl3, 9% PEG or 9% PEG/50 µM AlCl3. This suggests that no Al- or drought-induced signals such as ABA are involved in the citrate secretion in soybean. [ABSTRACT FROM AUTHOR]