Citrus Tissue Culture

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Abstract
An in vitro bioassay for chemicals that affect Citrus abscission [from C. limon cv. Eureka, C. sinensis cultivars Washington Navel and Valencia, C. reticulata and C. paradisi] was used to identify 3 inhibitors of stylar abscission in lemon pistil explants incubated on defined nutrient media. The 3 inhibitors (picloram, 4-chlorophenoxyacetic acid and 3,5,6-trichloropyridine-2-oxyacetic acid) are all auxins and the most potent of them (i.e., pichloram) was at least 10 times more active in the bioassay than 2,4-dichlorophenoxyacetic acid. Picloram (2 .mu.M) also was effective in inhibiting stylar abscission in pistil explants from other Citrus cultivars such as mandarin, Valencia and Washington navel oranges and grapefruit. To study the physiology of auxins active as abscission inhibitors versus inactive auxins in lemon pistils, the transport and metabolism of [1-14C]-2,4-D was compared with that of [2-14C]IAA which is without effect in the bioassay over the range from 0.1-100 .mu.M. Insignificant quantities of labeled IAA and/or labeled derivatives reached the presumptive zone of stylar abscission under the test conditions. Labeled 2,4-D and/or labeled derivatives also were transported slowly through pistils but some radioactivity could be detected in the stylar abscission zone as early as 24 h after the start of incubation. Extensive conversion of [2-14C]IAA to labeled compounds tentatively considered to be glycoside and cellulosic glucan derivatives was found with the use of solvent extraction methodology. A significantly smaller percentage of the radioactivity in pistils incubated on [1-14C]-2,4D was found in fractions corresponding to these derivatives. Both transport and metabolism appear to be important factors affecting the activity of auxins as abscission inhibitors in the bioassay.