Tolerance to Drought, Low pH and Al Combined Stress in Tibetan Wild Barley Is Associated with Improvement of ATPase and Modulation of Antioxidant Defense System.

Aluminum (Al) toxicity and drought are two major constraints on plant growth in acidic soils, negatively affecting crop performance and yield. Genotypic differences in the effects of Al/low pH and polyethyleneglycol (PEG) induced drought stress, applied either individually or in combination, were studied in Tibetan wild (XZ5, drought-tolerant; XZ29, Al-tolerant) and cultivated barley (Al-tolerant Dayton; drought-tolerant Tadmor). Tibetan wild barley XZ5 and XZ29 had significantly higher H⁺-ATPase, Ca2+Mg2+-ATPase, and Na⁺K⁺-ATPase activities at pH 4.0+Al+PEG than Dayton and Tadmor. Moreover, XZ5 and XZ29 possessed increased levels in reduced ascorbate and glutathione under these conditions, and antioxidant enzyme activities were largely stimulated by exposure to pH 4.0+PEG, pH 4.0+Al, and pH 4.0+Al+PEG, compared to a control and to Dayton and Tadmor. The activity of methylglyoxal (MG) was negatively correlated with increased levels of glyoxalase (Gly) I and Gly II in wild barley. Microscopic imaging of each genotype revealed DNA damage and obvious ultrastructural alterations in leaf cells treated with drought or Al alone, and combined pH 4.0+Al+PEG stress; however, XZ29 and XZ5 were less affected than Dayton and Tadmor. Collectively, the authors findings indicated that the higher tolerance of the wild barley to combined pH 4.0+Al+PEG stress is associated with improved ATPase activities, increased glyoxalase activities, reduced MG, and lower reactive oxygen species levels (like O2- and H2O2) due to increased antioxidant enzyme activities. These results offer a broad comprehension of the mechanisms implicated in barley's tolerance to the combined stress of Al/low pH and drought, and may provide novel insights into the potential utilization of genetic resources, thereby facilitating the development of barley varieties tolerant to drought and Al/low pH stress.

Reactions to cadmium stress in a cadmium-tolerant variety of cabbage (Brassica oleracea L.): is cadmium tolerance necessarily desirable in food crops?

Cadmium is a cumulative, chronic toxicant in humans for which the main exposure pathway is via plant foods. Cadmium-tolerant plants may be used to create healthier food products, provided that the tolerance is associated with the exclusion of Cd from the edible portion of the plant. An earlier study identified the cabbage (Brassica oleracea L.) variety, Pluto, as relatively Cd tolerant. We exposed the roots of intact, 4-week-old seedlings of Pluto to Cd (control ∼1 mg L(-1) treatment 500 µg L(-1)) for 4 weeks in flowing nutrient solutions and observed plant responses. Exposure began when leaf 3 started to emerge, plants were harvested after 4 weeks of Cd exposure and the high Cd treatment affected all measured parameters. The elongation rate of leaves 4-8, but not the duration of elongation was reduced; consequently, individual leaf area was also reduced (P < 0.001) and total leaf area and dry weight were approximately halved. A/C i curves immediately before harvest showed that Cd depressed the photosynthetic capacity of the last fully expanded leaf (leaf 5). Despite such large impairments of the source and sink capacities, specific leaf weight and the partitioning of photosynthate between roots, stems and leaves were unaffected (P > 0.1). Phytochelatins (PCs) and glutathione (GSH) were present in the roots even at the lowest Cd concentration in the nutrient medium, i.e. ∼1 µg Cd L(-1), which would not be considered contaminated if it were a soil solution. The Cd concentration in these roots was unexpectedly high (5 mg kg(-1) DW) and the molar ratio of -SH (in PCs plus GSH) to Cd was large (>100:1). In these control plants, the Cd concentration in the leaves was 1.1 mg kg(-1) DW, and PCs were undetectable. For the high Cd treatment, the concentration of Cd in roots exceeded 680 mg kg(-1) DW and the molar -SH to Cd ratio fell to ∼1.5:1. For these plants, Cd flooded into the leaves (107 mg kg(-1) DW) where it probably induced synthesis of PCs, and the molar -SH to Cd ratio was ∼3:1. Nonetheless, this was insufficient to sequester all the Cd, as evidenced by the toxic effects on photosynthesis and growth noted above. Lastly, Cd accumulation in the leaves was associated with lowered concentrations of some trace elements, such as Zn, a combination of traits that is highly undesirable in food plants.