PACLOBUTRAZOL REDUCES PHOTOSYNTHETIC CARBON-DIOXIDE UPTAKE RATE IN GRAPEVINES

Paclobutrazol applied as a soil drench at 0, 1, 10, 100, or 1000 mug a .i./g soil reduced photosynthetic CO2 uptake rate of leaves formed bef ore paclobutrazol treatment within 3 to 5 days of treatment and the re ductions were maintained for 15 days after treatment. The percentage o f recently assimilated C-14 exported from the source leaf was reduced only at the highest paclobutrazol dose, and there was little effect of treatment on the partitioning of exported C-14 between the various si nks. In response to increasing doses of paclobutrazol, particularly at the higher doses, an increasing proportion of recent photoassimilates was maintained in a soluble form in all plant components. Reduced dem and for photoassimilates as a result of the inhibition of vegetative g rowth may have contributed to a reduction in photosynthetic CO2 uptake rate, but this reduction in photosynthesis rate could not be attribut ed to a feedback inhibition caused by a buildup of starch in the leave s. Paclobutrazol had only a minor effect, if any, on photosynthetic el ectron transport. Chemical name used: beta-[(4-chlorophenyl) ha-(1,1-d imethylethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).