Developmental problems in over-winter off-season longan fruit. I: Effect of temperatures

Fruit development was investigated in the field in over-winter off-season (bloomed in late November) and on-season (bloomed in mid April) longans (Dimocarpus longan Lour. cv. Chuliang) in 2004–2005 and 2007–2008 crops, and in potted trees grown in phytotrons set at cold (15/10 °C, day/night, simulating winter) or warm temperatures (28/23 °C, day/night) in 2008. Development of fruit in both on-season and off-season longans could be divided into two stages based on the pattern of fresh weight increase. Stage I was characterized by the slow fruit growth, and Stage II by rapid fruit growth with aril expansion. Off-season longan had a longer period of fruit development than on-season longan, chiefly due to a longer Stage I. The development of off-season fruit, especially in Stage I, was exposed to low and abruptly fluctuating temperatures. In contrast, the temperatures during on-season fruit growth were warmer and less fluctuating. Off-season fruit were smaller with a significantly higher fruit drop. Intensive fruit drop did not occur during cold period until temperature had risen. Fruit cracking in off-season longan was severe (27.6%) in 2004–2005 with a dry and cold winter, but negligible in 2007–2008 with a wet and cold winter. In phytotron experiment, trees exposed to the cold temperature regime during early fruit development produced significantly smaller fruit compared to those exposed to the warm temperature regime (6.2 g vs 7.3 g), while there was no significant difference in fruit drop rate and cracking incidence. The results suggested temperatures lower than 15 °C were stressful for the growth of young fruit and reduced the growth potential and thus fruit size. Severe fruit drop in over-winter off-season longan might be associated with stressful cold plus abrupt temperature fluctuations, while severe fruit cracking in over-winter off-season longan might be related to cold and dry weather in the winter.

Research highlights▶ Over-winter longan has such developmental problems as small fruit size, severe fruit drop and cracking. ▶ Stressful temperatures of <15 °C in young fruit stage reduce fruit growth potential and final size. ▶ Stressful cold plus abrupt temperature fluctuations induces excessive fruit drop. ▶ Severe fruit cracking is related to cold and dry weather in young fruit stage. ▶ Long-term exposure to cold in Stage I might reduce heat requirement of Stage II.