| Abstract: Field-grown and potted trees of longan (Dimocarpus longana Lour.) cv. 'Shixia'were used as materials for studying the responses of longan to potassium chlorate applied by soil drench, foliar spray and branch injection. Changes in nitrogen metabolism, photosynthesis, carbon nutrition and leaf ultrastructure caused by chlorate treatments were studied in relation to their biological effects. Effects of foliar spray of ammonium sulphate or potassium nitrate on tree responses to chlorate treatment were examined in potted trees. In addition, chlorate metabolism in longan leaves was analyzed in respect of the toxicity mechanism of chlorate. The study yields the following results: 1. Treatment of potassium chlorate with various dosages and methods provoked a number of biological responses in longan trees, including acceleration of shoot growth, induction of panicle-like structures, inhibition of shoot growth, suppression of bud break, severe leaf drop and die back of shoots, but not complete flower induction. The different responses might be a result of the interaction between dosage effect, uptake ability, the internal physiological status that determined sensitivity to chlorate. 2. In potted longan, 10 to 20 g/tree of potassium chlorate caused severe leaf drop and leaf chlorosis. The treatments inhibited photosynthesis, reduced stomata conductance and transpiration while increased the CO2 concentration in mesophyll. Leaf chlorosis was accompanied by loss of chlorophyll and damage of chloroplast, where amylolysis, grana destruction and bleb formation was observed under electronic microscope. These demonstrate that photosynthesis inhibition was caused not only by stomata closure, but more importantly, by destruction of photosynthetic apparatus. In addition, chlorate treatment led to a reduction of starch content and an increase of soluble sugar, indicative of starch converted into soluble sugar in the leaves of treated trees. 3. Treatment of potassium chlorate did not cause significant difference in C/N ratio, although the total nitrogen tended to increase slightly in leaves of treated trees. In addition, the treatment lowered nitrate reductase activity (NR), provoked accumulation of soluble amino acids and soluble proteins in the leaf within 2 weeks after treatment. Soluble amino acids diminished thereafter. In contrast, soluble amino acids increased constantly in bud. Analysis of the amino acid composition showed that all proteineous amino acids were increased, most by folds. Proline was accumulated most prominently in leaf. The results suggested that the increase of soluble amino acids and soluble proteins was a result of de novo synthesis of amino acid (esp. proline) as well as breakdown of insoluble structural, functional or storage proteins. It seemed that the accumulated soluble amino acids in the leaves were mobilized to the bud. 4. Application of potassium chlorate resulted in a transient chlorate accumulation in the leaf. Severity of leaf drop was not significantly correlated to concentration of chlorate at 14 days after treatment, but instead, it showed a significant positive correlation with the concentration of non-toxic chlorine. This indicates that the toxicity effect of chlorate was not directly caused by chlorate itself, but by its chlorine-producing metabolism process. However, potassium nitrate considerably lowered the plant's sensitivity to chlorate.
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