Pg. 47-57 Effect of autumn water stress in combination with ethephon on flowering and fruiting of ‘Mauritius’ litchi in two different production areas of south Africa

Poor litchi flowering is a worldwide problem, especially in areas where temperatures during the flower induction period are not low enough. The effect of autumn water stress in combination with ethephon applications to control late autumn flush in order to improve flowering and yield was investigated over a four-year period. The study was carried out on the cultivar ‘Mauritius’ grown in a warm production area near Malalane (with moderate winters), and in a moderate production area near Hazyview (with cold winters). The trees were subjected to four irrigation treatments, which were two stress-induced irrigation treatments and two optimum irrigation treatments without water stress. The water stress treatments were applied during autumn, either between March and April (early stress), or between April and June (late stress). Up to 50% and 70% of the optimum tree water requirement was withheld for the early and late water stress periods, respectively. Ethephon was applied as spot sprays during April whenever young flush appeared to both water stress treatments as well as to one of the optimum irrigation treatments. The second optimum irrigation treatment was not treated with ethephon. At the Malalane trial site, late water stress in combination with ethephon applications successfully controlled vegetative flushing and increased flowering and yield, compared with early water stress and optimum irrigation in combination with ethephon application. Optimum irrigation without ethephon caused the trees to flush in all four trial years, reducing yields significantly. At the Hazyview site, flowering was clearly enhanced by ethephon applications with a small synergistic effect through water stress. Early water stress (March/April) appeared to be more beneficial for increasing flowering in most years, compared with late water stress, provided water stress was not interrupted by late rains. However, this effect was not carried through to yield due to climatic factors influencing tree phenology. There was little difference in yield between the treatments over the average of four years. Overall, the late water stress period was more beneficial in the warm production are (late flower induction) and the early water stress period more beneficial for the cooler production area (early flower induction). The use of ethephon with or without water stress remains an indispensable orchard practice for litchi production.

Summary:

• The study investigated how autumn water stress combined with ethephon applications affects flowering and fruit yield of Mauritius litchi in two South African production areas with different climates: a warm area near Malalane and a moderate-cool area near Hazyview.
• Four irrigation treatments were tested: two water stress treatments (early and late autumn stress withholding 50-70% of water) and two optimum irrigation treatments, with or without ethephon application to control late flush growth in April.
• At the warm Malalane site, late water stress combined with ethephon effectively controlled vegetative flushes, leading to increased flowering and yield.
• Optimum irrigation without ethephon caused excessive flushing and reduced yield.
• At cooler Hazyview, ethephon clearly enhanced flowering with minor additional benefit from early water stress.
• Early water stress was more beneficial here due to the earlier flower induction period, but yield differences among treatments were not significant due to climatic variability.
• Ethephon application to control flush growth remains essential in both areas to ensure good flowering and regulate alternate bearing, especially where weather conditions promote flush growth in late autumn.
• Water stress is more beneficial when timed correctly before and during flower induction one month before panicle emergence. Late water stress suits warm areas with late induction; early water stress suits cooler areas with early induction.
• Soil type influenced water stress effects: clay soils (Malalane) retained moisture longer allowing longer stress periods; sandy soils (Hazyview) had shorter, more pronounced stress effects.
• Starch content in branches showed no clear negative effects from water stress and correlated with growth phases rather than treatments.
• Recommendations include combining water stress and ethephon for better flowering and yield in warm climates, and using ethephon alone or with limited water stress in cooler climates with more variable rainfall.
• Prolonged water stress into flower panicle extension should be avoided to prevent negative impacts on fruit set and yield.
• Overall, both water stress and ethephon usage are indispensable orchard practices to stabilize litchi production under variable climatic conditions in South Africa.