Nitrate reduces the detrimental effect of potassium chlorate on longan (Dimocarpus Longan Lour.) trees

A recent finding that potassium chlorate induces longan (Dimocarpus Longan Lour.) flowering at any seasons has led to wide use of this chemical in longan industry for off-season harvest, despite the risk of injury to the tree. In this paper, we examined the influences of pretreatment of nitrate by foliar spray with 3% potassium nitrate on the responses of potted longan (Dimocarpus longan Lour.) cv. Shixia (a subtropical ecotype) trees to soil applications of potassium chlorate at 10 and 20 g plant−1. Chlorate treatments caused severe leaf drop and chlorophyll breakdown and suppressed budbreak, flowering and shoot growth. Pretreatment of potassium nitrate reduced the severity of leaf drop and chlorophyll breakdown caused by chlorate but the extend was less significant in high dosage chlorate (20 g plant−1) treatment than low dosage (10 g plant−1) chlorate treatment. Potassium nitrate did not alleviate the suppression effect of chlorate on budbreak, flowering and shoot growth, although it promoted these processes per se. Chlorate treatment resulted in only a transient accumulation of chlorate in the leaves, which peaked around 14 days after chlorate treatment (DACT), while chloride accumulated constantly within 28 DACT. Pretreatment of potassium nitrate tended to promote chlorate accumulation but did not cause a significant change in chloride accumulation. Severity of leaf drop was not significantly correlated to chlorate accumulation but significantly to chloride accumulation, suggesting that toxicity of chlorate to longan tree was realized during the chloride-producing process of chlorate reduction. Nitrate influenced the correlations between severity of leaf drop and chloride accumulation by reducing the slopes of the regressed lines of leaf drop versus chloride, indicating that nitrate reduced the sensitivity of longan trees to the toxic intermediates produced during chlorate reduction. The results indicate that the application rates were excessive for potted trees. The potential benefit of potassium nitrate in a longan production system based on application of chlorate needs to be evaluated in field trees.