Dual effect of auxins on fruit growth in Satsuma mandarin

Abstract

Experiments were conducted to determine the direct effect of the synthetic auxins 2,4-D (2,4-dichlorophenoxyacetic acid), 2,4,5-T (2,4,5-trichlorophenoxyacetic acid), NAA (1-naphthaleneacetic acid), BNOA (2-naphthoxyacetic acid), 2,4,5-TP (2-(2',4',5'-trichlorophenoxy)-propionic acid), IZAA (ethyl 5-chloro-1H-3-indazolyl-acetate) and IAA (3-indolylacetic acid) on fruit growth and abscission in the ‘Satsuma’ mandarin. The compounds were applied locally to a small number of fruits, less than 3% of the total number of fruits on the tree, to obviate the effect of fruit thinning on fruit growth. All the compounds tested reduced the initial growth of the fruit; at the concentrations tested this effect was smaller for 2,4-D and 2,4,5-T than for the other compounds. Overall, there was a direct relationship between the reduction in early fruit growth and fruit thinning. Fruit growth after the June drop was increased by both 2,4,5-T and NAA. The effect of the other auxins on late fruit growth was not statistically significant. The effect of the auxins on final fruit size was the balance of both effects, the initial reduction of fruitlet growth and late growth stimulation. Under the conditions of our experiments only 2,4,5-T increased fruit size at maturity; this compound being capable of increasing fruit growth independently of any thinning effect.