Recent Findings on the Mechanism of Action of the Synthetic Auxins Used to Improve Fruit Size of Citrus

Among the existing techniques used to enhance final fruit size in citrus, the use of synthetic auxins gives good results. The effect and mode of action of these substances has been studied over the last eight years and reproducible results are obtained. These substances act through an indirect effect, reducing fruitlet growth rate and promoting abscission, thus reducing competition; and through a direct effect, affecting mostly endocarp growth. The latter is due to an increase of vesicle size that, in turn, is due to cell expansion rather than cell division. The comparison of fruits of the same size from control and treated trees suggests that fruit sink strength is enhanced as a result of the auxin treatments. Moreover, cell size of the vascular tissues of the peduncles is also increased. Fruit quality is, together with yield, the most important factor of an orchards profitability. Quality includes several internal and external characteristics; but, in the Mediterranean basin, since the crop is mainly for fresh consumption, fruit size represents one of the main quality factors. Factors controlling citrus fruit size are well known (Goldschmidt and Monselise, 1977) and techniques such as girdling (Cohen, 1984) and the application of fruit thinning agents (Wheaton, 1981; Gallasch, 1988) have been successfully used to improve size. For a long time there was an interest in separating the indirect effect of fruit abscission and the direct effect on size, in order to make proper use of the synthetic auxins as fruit size enhancers without reducing the crop. In recent years, some results of these studies have been 'reported (Guardiola eta!., 1988; Agusti eta!., 1995b). In this paper we summarize our recent findings on the mechanism of action of the synthetic auxins as fruit enhancers, and their effect on the development of fruit tissues. Materials and Methods Experiments were run for 8 years (1988-1995). Trials were conducted in commercial plantations of the Valencia citrus area in Spain, using the most relevant mandarin and orange varieties, grafted onto sour orange (Citrus aurantium L.), citrange Carrizo (Poncirus 922 trifoliata [L.] Raf x Citrus sinensis [L.] Osb.) or Cleopatra mandarin (Citrus reshni Hort. ex Tan.) rootstocks. The following synthetic auxins were included: ethyl 5chloro-1H-3-indazolylacetate (IZAA, ethylchlozate); 1naphthaleneacetic acid (NAA), ethyl ester; 3chlorophenoxyacetic acid (3-CPA); 4chlorophenoxyacetic acid ( 4-CPA); 2,4-dichlorophenoxyacetic acid (2,4-D), isopropanol ester; 2,4,5trichlorophenoxyacetic acid (2,4,5-T), isobutyl ester; 2,4dichlorophenoxypropionic acid (2,4-DP), butylglycol ester; 4-chloro-o-tolyloxyacetic acid, thioethyl ester (MCPA-thioethyl; phenothiol); and 3,5,6-trichloro-2pyridyloxyacetic acid (3,5,6-TPA), free acid and isopropyJ:..ester. See tables and figures for concentrations applied. Applications were made to entire trees as a cover spray to the point of run off, and locally to tagged 15-20 romdiameter fruitlets from single-flowered leafy inflorescences, by dipping the fruitlets in the auxin solutions for 3 sees. Five fruitlets per tree with ten trees, at least, were treated with each auxin and concentration. A non-ionic wetting agent (nonylphenyl-polyethylene· glycol ether) was added in all cases. Treatments were applied in two dates for 'Clementine' mandarins (Citrus reticulata Blanco): during the natural physiological fruit drop, when the mean fruit diame~er was 11-15 mm (Date I), or after the fruit drop, with fru!IS 15-20 mm in diameter (Date II). Proc. Int. Soc. Citriculture, 1996, VoL 2