Abstract. Paclobutrazol [(2RS,3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-l-yl)pentan-3-ol], formulated as GFU 265, applied at 100, 250, and 500 mg plant-~ to the soil of container-grown sweet orange [Citrus sinensis (L.) Osbeck cv. Valencia], suppressed plant weight, stem height, leaf size, and total leaf area. At the 500-mg dosage, total plant dry weight was reduced by 61%, stem height by 74%, and both leaf biomass and area by 80%, as compared to control plants. All paclobutrazol dosages induced fibrous root thickening and increased their soluble sugar and starch content. Fresh root biomass was 14 to 40% higher and root:shoot ratios were increased three-to sixfold for treated plants. Paclobutrazol applications of 250 and 500 mg plant-1 reduced leaf photosynthetic rate, ribulose bisphosphate carboxylase activity, total nonstructural carbohydrates, and dark respiration 70 to 80% of the control plants. Reductions of leaf photosynthetic rate, carboxylase activity, and photosynthate by paclobutrazol contributed to biomass reduction in treated sweet orange.Plant growth regulators have potential for manipulating growth of many agricultural crops. In citrus, compounds eliciting responses in tree growth and acclimatization to environmental stresses are of potential importance to commercial growers (Yelenosky 1985). Several plant growth regulators have been examined for their potential for controlling excessive shoot growth and for enhancing cold acclimation in citrus trees (Aron et al. 1985, Krezdorn Mention of a trademark, warranty, proprietary product, or vendor does not constitute a guarantee by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable.and Cohen 1962, Lima and Davies 1984, Yelenosky 1985. Such compounds can influence a variety of physiological and biochemical processes; however, there is limited information available on these processes in citrus.The present study was conducted to evaluate the responses of sweet orange plants to paclobutrazol [(2RS,3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-l-yl)pentan-3-ol], an inhibitor of gibberellin synthesis. We focus on growth and some critical physiological processes associated with the tolerance of sweet orange to soil-applied paclobutrazol. Reduction of vegetative growth by this compound has been reported in a number of fruit tree species (Davis et al. 1988).
Materials and Methods
Plant Materials and Growth ConditionsSeeds of sweet orange [Citrus sinensis (L.) Osbeck cv. Valencia] were germinated in 2.5-L plastic pots containing washed sand. Potted plants were grown in a greenhouse, watered daily, and fertilized biweekly with 500 ml of a 1.5% solution of 15-7-7 (N-P-K) liquid fertilizer per pot. During the study period, maximum day and minimum night temperatures inside the greenhouse were 32 and 20~ respectively, and relative humidity fluctuated from 50 (day) to 90% (night). The photosynthetic active radiation (PAR, 400-700 nm) inside the greenhouse was ab...