Pg. 38-40 Effect of Temperatures Above 20°C on Flowering in Lychee

Lychee (litchi) yields can be low in some locations in Australia, because the trees fail to flower. Flowering is promoted by low temperatures, but the critical level and duration of chilling required for successful induction are not known. The effects of maxima/minima temperatures of 15°/5°, 20°/5°, 20°/10°, 20°/15°, 25°/10°, 25°/15°, 25°/20° or 30°/20°C on vegetative growth and flowering of cvs Kwai May Pink and Casino were investigated. In a second experiment, plants of cv Wai Chee were given 0, 2, 4, 6, 8, 10 or 31 weeks of induction at 15°C before transfer to non-inductive conditions at 30°/20°C. In a third experiment, cv Wai Chee was grown at 15°C and given 0, 1, 2 or 8 h above 20°C (0, 2, 4 or 15 h above 15°C) per 24 h.

In Experiment 1 with cv Kwai May Pink, there was vegetative growth at 25°/20° and 30°/20°C, a mixture of vegetative growth and flowering at 25°/10° and 25°/15°C, and flowering at lower temperatures. Cultivar Casino responded similarly, except that there was no growth or flowering at 25°/10° and 25°/15°C. For both cultivars, nearly all branches flowered at 20°/15°C or lower. Flowering in Wai Chee occurred with four weeks at 15°C before transfer to warmer temperatures and was greatest with six weeks. There was also an effect of treatment on the type of inflorescence, with fewer leaves after ten weeks of induction. Plants of Wai Chee exposed to 2 h above 20°C per day were mainly floral, whereas those given eight or more hours were vegetative. It was concluded that flowering in lychee was best after ten weeks at 15°C, while periods of 8 h or more per 24 h above 20°C were detrimental. The subtropical adaptation of lychee is confirmed.

Summary:

• Lychee yields can be low in some Australian locations due to failure to flower, which is influenced by temperature.
• Flowering in lychee is promoted by low temperatures; the critical chilling level and duration needed were investigated.
• Experiments tested lychee cultivars Kwai May Pink, Casino, and Wai Chee under various temperature regimes with maxima and minima from 15/15°C to 30/20°C.
• Vegetative growth occurred at higher temperatures (25/20°C and 30/20°C); flowering was observed mainly at lower temperatures (20/15°C and below).
• Cultivar Kwai May Pink showed vegetative growth at higher temperatures and flowering at lower temperatures; cultivar Casino had similar response but no growth or flowering at some mid-range temps.
• For cultivar Wai Chee, four weeks at 15°C before transferring to warmer temperatures initiated flowering, with peak flowering after six weeks at 15°C.
• Exposure to temperatures above 20°C for 8 or more hours per day inhibited flowering and promoted vegetative growth. Short exposures (1-2 hours) above 20°C allowed flowering.
• The best flowering occurred after about 10 weeks at 15°C with fewer leaves on inflorescences, confirming lychee’s subtropical adaptation.
• Wide diurnal temperature fluctuations were not necessary for flowering; the mean temperature and duration of cooler temperatures were more critical.
• The study confirms that lychee flowering is best supported by several weeks of cool weather (around 15°C) and that prolonged exposure to above 20°C is detrimental to flower induction.
• These findings have important implications for lychee cultivation in subtropical regions, where occasional warm spells during the cold season may reduce flowering reliability.