Effects of gibberellic acid and paclobutrazol on growth and carbohydrate accumulation in shoots and roots of citrus rootstock seedlings

SummaryThe involvement of nonstructural sugars in shoot growth of citrus seedlings was studied in ‘Carrizo’ citrange (Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.) rootstocks by foliage applications of gibberellic acid (GA3) and paclobutrazol [(2RS, 3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-lyl)penta-3-ol] (PCB) and subsequent determinations of sucrose, glucose, fructose, and starch. PCB inhibited length (21%) and dry weight (19%) of the stem, whereas GA3 increased both (79 and 27%, respectively) in the absence or presence of PCB. On a dry weight basis, GA3 increased hexoses (22.5 mg g−1) and starch (48.4 mg g−1) in shoots (control values were 14.7 and 35.4 mg g−1, respectively), and PCB reduced sucrose (43%) and increased starch (36%). The sucrose reduction induced by PCB was effectively reversed by simultaneous GA3 applications. In roots, GA3 did not modify the levels of the nonstructural sugars, while PCB enhanced the amount of starch (38%). On a per plant basis, GA3 (38.8 mg) and GA3 + PCB-treated shoots (37.2 mg) contained higher total carbohydrates than controls (22.5 mg) and PCB-treated shoots (21.4 mg). There were high correlations between shoot growth (elongation or biomass) and the absolute levels of soluble sugars, suggesting that shoot growth is relying on the available amounts of soluble carbohydrates. However, sugar concentrations did not increase linearly with the growth of the developing seedling. These observations may suggest that at this early stage, growing seedlings are acting as utilization sinks rather than storage sinks. The data also indicated that GA3 altered the partitioning of assimilate between shoots and roots, increasing the percentage of soluble sugars and starch of the shoots, whereas PCB did not change the normal distribution of sugars. These results indicate that gibberellins stimulated growth and increased carbon supply in shoots, whereas PCB delayed growth, reduced sucrose, and enhanced storage sugars.