The use of growth regulators is not uncommon in fruit cultivation. Plant regulators are used to increase the fruit size, maturity rate, and quality characteristics.
Preharvest application of naphthalene acetic acid (NAA) at 10, 20, 30, 40, and 60 ppm to immature date fruits of the Zahdi date palm 15–16 weeks after pollination (i.e., late in the kimri stage) influences fruit size, fruit weight and volume, pulp–seed ratio, and moisture content (Mohammed and Shabana, 1980).
The highest increase in fruit weight (39%) can be obtained with the 60-ppm application ofNAA. Total solids are not changed with the 40- to 60-ppm application of NAA but fruit ripening is delayed by about a month. Ethrel significantly increases the length, diameter, fresh weight, volume, and pulp– seed ratio of 2,4,5-trichlorophenoxy-propionic-acidtreated fruits but fails to show such effects in NAAtreated fruits (Mohammed et al., 1980). Ethrel increases the moisture content of NAA-treated date fruits and reduces the total soluble solid contents of both types of auxin-treated fruits. Gibberellic acid (GA) can be applied to unpollinated date flowers to produce seedless fruits, but the fruit yield decreases and fruit ripening is retarded.
Additional application of 200 ppm of ethephon improves the fruit quality in terms of color, total soluble solids, and titratable acidity (Maximos et al., 1980). However, the higher dosages of ethephon (0, 125, 250, 500, 1000, or 2000 ppm) applied to Shahani cultivar date fruits, from Iran, during harvest showminor increases in the dry weight percentage of the pulp–seed ratio, titratable acidity, soluble solids, and respiration rates; and a minor decrease in pH, firmness, and astringency (Rouhani and Bassiri, 1977). The effect of GA treatment of date fruits on date palm trees depends on the level and rate of application. Mohammed et al.
(1986) sprayed the date bunches of three Zahdi and three Sayer cultivars with 0, 50, 100, or 150 ppm GA during the slow period of growth (i.e., 12–14 weeks after full bloom and pollination).
About 18 weeks after the treatment, fruit bunches were harvested and the fruits were analyzed for physical parameters and chemical composition. GA had no pronounced effect on total soluble solids, except at the highest level applied to Zahdi cultivar, in which the sucrose content was higher.
Total and reducing sugars increased with GA application but varied with the cultivar and the rate of GA application.
The natural plant hormone, indole-3-acetic acid (IAA) is suggested as being involved in the control of date fruit development (Abbas et al., 2000).
The female flowers of the date tree are very rich in IAA, and if not pollinated, this leads to the setting of parthenocarpic fruit (i.e., fruit which fails to ripen fully, due to the absence of a climacteric rise in respiration). The concentration of IAA declines 2 weeks after pollination (at fruit set) probably because it is used in cell division.
The IAA concentration rises again in 2 weeks, possibly due to the embryo development but remains high up to 8 weeks after pollination, before it declines to a minimum in fully ripe fruits, i.e., around 18 weeks after pollination. The fall in IAA is accompanied by a marked increase in gibberellins in date plants. The application of plant growth regulators, alone or in combinations, is known to produce varied results in date fruit characteristics and in the productivity of the date palm tree (Al-Juburi et al., 2001). Application of 150 ppm of GA or 1000 ppm of ethephon on flower clusters of the Barhee date palm tree showed no consistent effect on fruit characteristics or productivity.
However, 100 ppm of 2-(1-naphthyl) acetic acid (NAPA) or the above growth regulator mixtures reduced fruit dry matter and fruit ripening percentage, but increased the fruit weight per bunch per tree. NAPA, when applied to Barhee date palm flowers 20 days after pollination, resulted in the increase of the fruit flesh percentage and the date palm yield.