TEMPERATURE IN RELATION TO PEACH CULTURE IN THE TROPICS

It has long been recognised that the major factor limiting stone-fruit production outside the Mediterranean climate of the Temperate Zone is the lack of winter chilling to break dormancy. Several developments have reduced this problem and made production possible in areas previously unsuitable:-

• Cultivars with low winter chilling requirements have been bred.
• Dormancy-breaking chemicals have been used to supplement chilling.
• Dormancy is avoided by defoliation-induced resumption of growth.

These approaches were combined at highland and lowland sites in the Philippines (Hermano et al., p. and Llanes et al., p.). No chilling is received at any of these test sites. However, dormancy was avoided by defoliation, supplemented at the highland site by dormancy-break treatments. Fruit set and maturation was achieved at the highland site but not at any lowland site for any cultivar.

Possible reasons for this lack of fruit set are:-

• Insufficient time for complete flower development due to renewed growth in response to defoliation.
• Insufficient cold for complete flower development viz. viable pollen and ovules.
• Too high temperatures at anthesis for effective pollination.

To pursue the last possibility, monthly mean minimum temperatures for the period of peach flowering were compared for the Okanagen Valley, British Columbia, 49°N; Loxton, South Australia, 35°S; Nambour, Queensland, 26°S; Inyanga, Zimbabwe, 18°S; Toviar; Venezuela, 10°N; Baguio, Philippines, 16°N, and General Santos, Philippines, 6°N (fig. 1).

The mean monthly minimum for the period of peach flowering and set in widely separated peach growing areas ranges from 6.8°C to 9.5°C. The mean monthly minimum at Baguio, Philippines for November when some fruit set occured is 14.9°C. The corresponding minimum for April when no set occurred is 15.5°C. At General Santos, Philippines where no fruit set has occurred, despite flowering at several times in the year, the mean minimum for the coolest month is 19°C and for the warmest month 21°C.

These comparisons suggest another limiting factor to peach production in the tropics. If this is established by direct experimentation eg. cooling at anthesis, growth chamber experiments, a new objective should be included in breeding programmes viz. tolerance to high temperatures at anthesis.