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- Author or Editor: Penelope A. Rousos x
The identification of exceptional genotypes in a breeding program can involve evaluating the responses of large numbers of plants to environmental stress (2). Major hydroponic systems include aerated standing or flowing-nutrient solution, mist, and nutrient film cultures. Flowing solution culture currently provides the most consistent environment for roots, but it is costly and difficult to maintain (1, 3).
Eighty-four cabbage cultivars Brassica oleracea L., Capitata group, (tested in groups of nine) were germinated and supported on a fabric-lined growing panel over Hoagland's solution (0.03 mg·liter−1 Cu++ as CuSO4) for 2 weeks and then transferred intact to treatments (1.5 and 2.0 mg·liter−1 Cu++) and control 0.03 mg·liter−1 Cu++) vats for 1 week. Based on relative growth (percent of control), ‘Wisconsin All Seasons’ was identified as more tolerant to high Cu++ levels than ‘Globe King’. The two cultivars were compared for growth and Cu concentration in roots and shoots at 0-, 2-, 4-, and 6-day intervals during exposure to Cu++ levels of 0.03, 1.5, and 2.0 mg·liter−1. Although there were no cultivar differences, Cu treatments did significantly affect root Cu concentrations. Copper uptake into the shoots of both cultivars was linear over time. However, the less-tolerant cultivar Globe King had significantly higher shoot Cu levels than the more-tolerant cultivar Wisconsin All Seasons. Although in both cultivars Cu concentrations were lower in the shoot than roots, the higher Cu concentration in the shoot of the less-tolerant cultivar Globe King may be responsible for its reduced growth.
Cabbage leaf disks (Brassica oleracea L., Capitata group) were floated (adaxial side up) in Cu2+ solutions (0, 0.16, and 0.40 mm Cu2+) for 1–4 days. The experiments were conducted in both light and dark environments. In light, total chlorophyll, chlorophyll a, chlorophyll b, and chlorophyll a/b ratios declined linearly with increasing exposure duration and Cu2+ concentrations. The rate and magnitude of these declines were unaffected by the addition of 0.5 mm CaCl2. Between the 2 cultivars tested, the relative chlorophyll contents in ‘Market Prize’ declined faster and reached lower levels than ‘Resistant Danish’, suggesting that ‘Market Prize’ is more susceptible to Cu2+stress. In the absence of light, there was little difference between the chlorophyll loss in the controls and Cu2+-treated tissue. For light and dark experiments, loss of chlorophyll a was primarily responsible for reductions in total chlorophyll content and in chlorophyll a/b ratios.
Three-week-old seedlings of the cabbage (Brassica oleracea L. Capitata group) cultivars Wisconsin All Seasons and Globe King were exposed for 3 weeks to Cu2+ concentrations ranging from 1.2 to 2.5 mg Cu2+/liter. Plants were harvested after 14, 16, 19, or 22 days of exposure. During Cu2+ treatment, shoots appeared healthy, but roots turned golden, with increased lateral branching and tip browning. Cultivars responded identically for all characteristics measured. Root Cu concentrations were >3000 μg·g−1 or 300 times the control after Cu2+ treatment. Shoot Cu concentrations (13 μg·g−1), while significantly higher than the control, were not above critical values associated with yield reductions. The high root Cu concentrations were not associated with lower shoot or root yields. The initial symptoms of Cu2+ toxicity in the first fully expanded leaves were elevated leaf Cu concentrations and lower total chlorophyll and chlorophyll a:b ratios. There was no effect on leaf fresh weight per unit area, transpiration, respiration, net photosynthesis, or apparent photosynthetic quantum yields.