Pg. 33-39 How important is timing of water stress for consistent litchi yields?

Various studies in literature indicate that flower induction and initiation in litchi is solely triggered by temperature differences. However, reducing soil moisture by withholding water in autumn and early winter to levels below the water requirements of the trees, can assist them to enter a rest period and indirectly influence flower induction positively.

Four treatments were applied to litchi trees in the Malelane and Hazyview areas under different soil (clay and sand) and irrigation systems (micro and drip). Water stress was applied either between March and April, between April and May or not at all. Furthermore, Ethapon was applied to three of the treatments during autumn when late flush appeared. One wet treatment was not treated with Ethapon at all.

Although late rains interfered with the trial, results indicate that water stress put a hold on trees, whereas trees with no water stress and no Ethapon reacted faster to temperature changes that are unfavourable for flower induction/initiation, leading to lower yields. When the wet and dry treatments with Ethapon application were compared, it was clear in both locations that water stress increased flowering and yield compared to optimum irrigation. Whether earlier or later water stress would be more beneficial could not be established after the first year. However, the success of correct timing for water stress appears to be closely related to flower induction time. Such a relationship as well as induction periods of the various production areas still need to be investigated further.

Summary:

• Flower induction and initiation in litchi is primarily triggered by temperature differences, with ideal maximum temperatures below 20°C and minimum below 15°C.
• Applying water stress (reduced irrigation) in autumn and early winter to below tree water requirements helps trees enter a rest period and indirectly supports flower induction.
• The study tested four treatments on litchi trees in two South African regions with differing soils and irrigation systems: water stress between March-April, April-May, no water stress, and Ethapon application to control late flush.
• Results indicate that water stress slows tree activity, while no water stress leads to rapid response to unfavorable temperature changes, reducing yield.
• Water stress combined with Ethapon treatment increased flowering and yield compared to optimum irrigation without stress.
• It was unclear whether earlier or later water stress periods are more beneficial; success likely depends on aligning water stress timing with flower induction periods.
• Water stress did not prevent all vegetative flushes, but reduced them compared to well-irrigated conditions.
• Temperature and soil type influenced the effectiveness of water stress; for example, clay soils dry slower, impacting timing.
• Yield and flowering percentage were higher in water-stressed treatments, with no significant difference in fruit size.
• Starch reserves during flower induction appear important for flowering success; low starch during March-April may reduce yields even under good temperatures.
• Ethapon effectively controlled vegetative flush during April, but water stress still conferred benefits beyond Ethapon use alone.
• Water stress did not directly influence starch content, but starch content varied with tree phenology.
• The study suggests re-evaluating traditional South African water stress timing to better coincide with flower induction, likely in March through May.
• Further research is needed on induction periods in different regions, timing of flower initiation, and underlying physiological processes improved by water stress.
• Overall, water stress during flower induction and initiation improves flower formation and yield compared to optimum irrigation, even with use of Ethapon to control flushing.