THE FACTORS AFFECTING LONGAN FLOWER INDUCTION BY CHLORATE

It was found in 1998 that potassium chlorate was able to induce the flower in longan trees (Dimocarpus longan L.). Since then potassium chlorate has been used in most longan orchards in Thailand. The recommended application rate in the early years was 500 g for a medium size tree. However, after repeated application many farmers have experienced satisfactory flower induction. The farmers have responded to this phenomenon by increasing the application rate, up to 2,000 g in certain cases. This practice did not solve the problem, and may create more problems on environment. This research was aimed at studying the factors affecting longan flower induction by chlorate. The factors studied were concentration of KClO₃, plant nutrients, time of application, shade and the number of times that the plants subjected to chlorate. The study was done with 2-year-old longan trees grown in pots with sand culturing technique in a plastic house at Maejo University, Chiang Mai, Thailand. The results indicate that a concentration of 200 mg of potassium chlorate per liter of standard nutrient solution was appropriate for inducing flower in the rainy and cool seasons and 400 mg/L for the hot season. Toxic symptoms of over chlorate concentration varied with the concentration, ranging from less flowering with extended flowering, to no flowering with leaf burn. Over concentration of nitrate induced leaf flush and inhibited flowering. Shading for 7 days after the application of chlorate significantly reduced flowering. Increasing of chlorate concentration was not able to compensate for the effect of shading. Repeatedly exposing to chlorate diminished the response. Split application of several low concentrations of chlorate resulted in the comparable flowering as that of the single application of appropriate concentration. The obtained result was able to explain existing phenomena on longan flowering after several years of repeated application of chlorate in orchards.