EFFECTS OF GIBBERELLIC-ACID AND PACLOBUTRAZOL ON GROWTH AND CARBOHYDRATE ACCUMULATION IN SHOOTS AND ROOTS OF CITRUS ROOTSTOCK SEEDLINGS

The involvement of nonstructural sugars in shoot growth of citrus seed lings was studied in 'Carrizo' citrange (Cirrus sinensis (L.) Osb. x P oncirus trifoliata (L.) Raf.) rootstocks by foliage applications of gi bberellic acid (GA(3)) and paclobutrazol [(2RS, nyl)-4,4-dimethyl-2-(1 ,2,4-triazol-lyl)penta-3-ol] (PCB) and subsequent determinations of su crose, glucose, fructose, and starch. PCB inhibited length (21%) and d ry weight (19%) of the stem, whereas GA(3) increased both (79 and 27%, respectively) in the absence or presence of PCB. On a dry weight basi s, GA(3) 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 GA(3 ) applications. In roots, GA(3) did not modify the levels of the nonst ructural sugars, while PCB enhanced the amount of starch (38%). On a p er plant basis, GA(3) (38.8 mg) and GA(3) + PCB-treated shoots (37.2 m g) contained higher total carbohydrates than controls (22.5 mg) and PC B-treated shoots (21.4 mg). There were high correlations between shoot growth (elongation or biomass) and the absolute levels of soluble sug ars, suggesting that shoot growth is relying on the available amounts of soluble carbohydrates. However, sugar concentrations did not increa se linearly with the growth of the developing seedling. These observat ions may suggest that at this early stage, growing seedlings are actin g as utilization sinks rather than storage sinks. The data also indica ted that GA(3) altered the partitioning of assimilate between shoots a nd roots, increasing the percentage of soluble sugars and starch of th e shoots, whereas PCB did not change the normal distribution of sugars . These results indicate that gibberellins stimulated growth and incre ased carbon supply in shoots, whereas PCB delayed growth, reduced sucr ose, and enhanced storage sugars.