Regulation of Photosynthetic Rate of Two Sunflower Hybrids under Water Stress

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Abstract
A single pulse of O3 (0.15 microliter per liter, 5 hours) induced β-1,3-glucanase and chitinase activities in O3-sensitive and -tolerant tobacco (Nicotiana tabacum L.) cultivars. In the O3-sensitive cultivar Bel W3, the response was rapid (maximum after 5 to 10 hours) and was far more pronounced for β-1,3-glucanase (40- to 75-fold) than for chitinase (4-fold). In the O3-tolerant cultivar Bel B, β-1,3-glucanase was induced up to 30-fold and chitinase up to 3-fold under O3 concentrations that did not lead to visible damage. Northern blot hybridization showed a marked increase in β-1,3-glucanase mRNA in cultivar Bel W3 between 3 and 24 hours following O3 treatment, a transient induction in cultivar Bel B, and no change in control plants. The induction of β-1,3-glucanase and chitinase activities following O3 treatment occurred within the leaf cells and was not found in the intercellular wash fluids. In addition, O3 treatment increased the amount of the β-1,3-glucan callose, which accumulated predominantly around the necrotic spots in cultivar Bel W3. The results demonstrate that near-ambient O3 levels can induce pathogenesis-related proteins and may thereby alter the disposition of plants toward pathogen attack.