SOMCHAI ONGPRASERT1, WINAI WIRIYA-ALONGKORN2
1Mae Jo University, Department of Soil Resource and Environment, Thailand
2Mae Jo University, Department of Horticulture, Thailand
In 1998 it was found that potassium chlorate induced flowering in longan trees (Dimocarpus longan L.). Since then potassium chlorate has been used in most longan orchards in Thailand and some neighbouring countries. Recommended application rate in the early years following the discover was 500 g for a medium size tree. However, after repeated application for 4-5 years many farmers experienced unsatisfactory flower induction. Farmers responded to this phenomenon by increasing the application rates, in certain cases up to 2 kg. This practice did not solve the problem, instead created environmental problems and poluted longan orchards. This research aimed at studying the factors affecting longan flower induction by chlorate. The studied factors were concentration of the substance and plant nutrients, season, shade and the number of time the plants were subjected to chlorate. The study was conducted with 2 year"=old potted longan trees cultivated in sand in a plastic house at Mae Jo University.
A concentration of 200 ppm potassium chlorate in the standard nutrient solution was appropriate for inducing flower in the rainy and cool seasons and 400 ppm for the hot season. Toxic symptoms of excess chlorate concentration varied with the concentration, i.e., ranged from reduced flowering with extended flowering period without leaf or apex symptoms to no flowering with leaf burn. Excess concentration of nitrate induced leaf flush and inhibited flowering. Shading for 7 days after the application of chlorate, significantly reduced flowering. Increasing the chlorate concentration was not able to compensate the effect of shading. Repeatedly exposing to chlorate diminished the response of the trees. Split application of several low concentrations of chlorate resulted in comparable flowering as with a single application. The obtained result was able to explain existing phenomena on longan flowering after several years of repeatedly application of chlorate in orchards.
Keywords: Chlorate, flower induction, longan