Effects of applying variable temperature conditions around inflorescences on fertilization and fruit set in date palms

• Temperature effects on date palm fertilization and fruit set were studied.
• Environment controlled units, “modular phytotrons”, were assembled on pollinated inflorescences in planta.
• Pollen-tube growth, fertilization, fruitlet formation and carpel degeneration were characterized by macro- and microscopic analyses.
• Low temperatures during fertilization decreased pollen germination rate.
• Low temperatures enhanced formation of parthenocarpic fruit and reduced normal fruit development.

Control of pollination and fertilization in date palm is essential for the development of high-quality fruit. Pollen is harvested from male trees and mechanically applied to inflorescences on female trees. An overly high rate of fruit set may cause excessive fruit load, requiring expensive fruit thinning to prevent reduction in fruit size and marketability. On the other hand, inefficient pollination results in lower yield. Although the female flower has three separate carpels, only one develops into a fruit; the others degenerate. Non-fertilized flowers may develop into parthenocarpic singlet or triplet fruit, which have no commercial value. Environmental conditions affect fertilization and fruit development. The current study characterizes fertilization and early fruit development in date palm under different temperature conditions. The date palm is a very large tree. To study environmental effects on its reproductive biology, special units, termed “modular phytotrons”, were designed for this research. They were assembled on pollinated inflorescences of whole ‘Medjoul’ date trees in the orchard, enabling modification of temperature regimes in planta. Pollen-tube growth, fertilization, fruitlet formation and carpel degeneration, as well as early development of parthenocarpic fruit were defined and characterized by macro- and microscopic analyses. Relatively low temperatures applied during plant fertilization significantly decreased pollen germination rate, enhanced formation of parthenocarpic fruit and reduced normal fruit development. The significance of these results to horticultural practices during fertilization and fruit thinning is discussed.