Stress Effects of Chlorate on Longan (Dimocarpus longan Lour.) Trees: Changes in Nitrogen and Carbon Nutrition

Three-year-old potted longan (Dimocarpus longan Lour. cv. Shixia) trees were treated with potassium chlorate and effects on nitrogen and carbon nutrition were examined. The results showed that potassium chlorate at 10 and 20 g per pot failed to induce flower but suppressed shoot growth and caused leaf chlorosis and drop. The treatment significantly inhibited nitrate reductase but increased nitrogen concentration in the leaves and buds. Concentration of soluble amino acids in the leaves of treated trees increased within 14 days and then declined to the control level, while it increased constantly in buds. In both organs, the amino acid increase was an all-round one, with all the tested 21 amino acids increased. However, soluble proteins in the leaves were slightly increased by chlorate, indicating that de novo synthesis of amino acids was activated. Chlorate reduced photosynthetic rate and stomatal conductance but slightly increased CO2 concentration in the mesophyll, suggesting that chlorate treatment damaged photosynthetic apparatus. The damage was reflected by the destruction of thylakoids and grana in the chloroplasts. Chlorate also caused depletion of starch with significant accumulation of soluble sugars in the leaves. Accumulation of sugars and soluble amino acids indicates osmotic adjustment in response to the stress caused by chlorate treatment.