Carbohydrate reserve dynamics as influenced by shoot control strategies and climatic conditions prior to flowering in 'Mauritius' litchi

Changes in seasonal weather patterns due to climate change show an increasing impact on the phenological growth cycles of subtropical crops, including litchi. In the last decade, the onset of chilling temperatures in winter was reported to be later, with milder winter temperatures in general across the subtropical regions of South Africa. This change in environmental conditions results in continuous vegetative shoot growth during times of expected dormancy that is required for the accumulation of carbohydrate reserves to support flowering and fruiting of the next season. Non-structural carbohydrates, such as soluble sugars and starch, are important indicators of carbon source capacity of a plant and enable adaptation to environmental changes. In order to study carbohydrate levels and their effect on flowering in dormant versus vegetative growing trees during the pre-flowering period, experiments were conducted on 16-year-old 'Mauritius' trees in the north-east of South Africa using ethephon to inhibit shoot growth during autumn/early winter. Ethapon^® (a.i. 48% ethephon) was applied as a single full cover spray to dormant, hardened off trees at concentrations of 500 and 1000 mg L^‑1 to inhibit any further shoot growth. Control treatments either consisted of untreated trees or tree where ethephon was applied at 1000 mg L^‑1 as a spot spray to the young developing vegetative flushes, as is the current management strategy for flush control in South Africa. Wood samples of small branches were collected at various phenological stages and analysed for starch and soluble sugar content. Results indicated that starch and soluble sugars fluctuated in response to growth events. Shoot inhibition by ethephon preserved carbohydrates during the pre-flowering period and improved flowering. The relationship between growth phase, ethephon concentration and carbohydrate levels is discussed.