Pg. 6-10 Effect of the time of bud-break in winter, as manipulated by the commencement of irrigation, on flowering intensity of ‘Mauritius’ litchi trees

In ‘Mauritius’ litchi, country yield and the tree flowering intensity are strongly correlated. General failure to flower is associated with poor years, and intense flowering, with exceptional years. Pronounced seasonal variability in cropping is recognized as a significant problem. It was observed that the time of the commencement of irrigation can markedly influence the time of bud development during the winter months in ‘Mauritius’ litchi. Timely release from a relative drought stress may therefore effect general bud development when conditions are ideally inductive. The aim of the present study was to assess the effect of the release from drought stress on various dates during winter, after its early imposition, on the intensity of flowering, as well as on fruit retention, in ‘Mauritius’.

Trees were pruned (30 to 40 cm heading back of all branches) on February 8, 9 and 10, 2005, at the time when general postharvest flushing was beginning. Pruning was effective in causing a uniform flush response, bud development having occurred generally by February, 2005, in the pruned trees. To determine the effect of release from drought stress after its imposition, the microsprinklers irrigating the trees were blocked on April 15, after the post-harvest flush had generally materialized. Irrigation was reinstated on a number of dates, namely, June 9 (L₂), July 4 (L₃ ), and July 15 (L₄, L₅ , L₆ ) 2005. Ethrel (150 ml Ethrel per 100 l water, dates April 15 and May 5) was sprayed on the trees in April and May, 2005, to prevent new flush from developing during these months.

Flowering intensity or percentage of inflorescences bearing fruits was not markedly affected by the treatments, flowering intensity being greatest in the untreated and trees released latest from drought stress (July 15, 2005). Drought imposition was apparently effective to an extent in delaying bud development. Trees pruned and released from drought stress on July 15, 2005, were generally as productive as, or more productive than, the untreated trees. Inflorescences developing later in the season tended to set fruits, whereas those developing earlier were prone to fruitlessness. Correlative analysis relating stage of growth and development in July or September 2005, to flowering intensity or inflorescence fruit setting ability as seen in late October 2005, confirms the supposition that the time of winter bud development as it relates to winter temperatures, directly determines whether inflorescences develop or not, as well as the ability of the inflorescences to set fruits. Future research is to be aimed at assessing the date of post-harvest pruning on flowering intensity and inflorescence fruit setting capacity in view of the effect of pruning on flushing time, and shoot maturation stage when conditions are maximally inductive for flowering.

Summary:

• Flowering intensity and crop yield in Mauritius litchi are strongly correlated; poor flowering leads to low yields and pronounced seasonal variability is a significant issue.
• The timing of irrigation commencement in winter markedly affects the timing of bud development in Mauritius litchi, with potential to manipulate general bud-break at inductive periods.
• Sixty bearing Mauritius litchi trees (plus ten controls) were pruned after harvest, with drought stress imposed by withholding irrigation and then released on different winter dates (June 9, July 4, July 15).
• Pruning produced a uniform flush response, while unpruned trees had non-uniform, prolonged shoot growth.
• Flowering intensity was highest in unpruned trees and in trees where drought stress was released latest (July 15).
• Drought stress effectively delayed bud development, but did not significantly improve flowering intensity or fruit set compared to controls.
• Terminal shoots need to be adequately mature during the optimally inductive period for successful bud-break and flowering; late or early bud-break outside this period results in a vegetative response.
• Stage of bud-development during July or September was closely linked to flowering intensity and fruit-setting ability observed in October.
• Inflorescences developing later in the season typically set more fruit; earlier developing inflorescences tend to be fruitless.
• The results support that the timing of bud-break relative to the inductive temperature window in winter determines inflorescence formation and fruit set.
• Future research will focus on the timing of post-harvest pruning to optimize flowering intensity and fruit setting through shoot maturity management.
• The study acknowledges support from Hamawasha Farm and highlights the need for ongoing investigation into pruning dates to maximize crop yield in Mauritius litchi.