The dwarfing mechanism of citrus rootstocks F&A 418 and #23 is related to competition between vegetative and reproductive growth
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The annual development of Navelina (Citrus sinensis (L.) Osbeck) trees budded on three hybrid citrus rootstocks was studied. Two rootstocks, named #23 and #24, were obtained from the cross of Troyer citrange (C. sinensis x Poncirus trifoliata (L.) Raf.) x Cleopatra mandarin (C. reshni Hort. ex Tan.). The third rootstock, named F&A 418, came from a cross of Troyer citrange x common mandarin (C. deliciosa Ten.). Rootstocks #23 and F&A 418 are dwarfing rootstocks and reduce the size of the scion by about 75%. Rootstock #24 yields a standard size scion. Major growth differences that influenced tree size were apparent during the first summer after grafting and appeared to be related to fruit productivity, because defruiting the dwarfed scions caused a significant increase in vegetative shoot development, including summer sprouting. The reduced growth of the dwarfed scions was not restored by hormone application, indicating that a hormonal deficiency is unlikely to be the primary reason for scion dwarfing, although differences in gibberellin concentrations were found in actively growing shoots. Leaf photosynthesis was similar in scions on all three rootstocks, but the carbohydrate accumulation in fruits and fibrous roots during the summer sprouting period was significantly greater in the dwarfed trees than in the standard trees. Our results suggest that the dwarfing mechanism induced by the F&A 418 and #23 rootstocks is mediated by enhanced reproductive development and fruit growth, resulting in reduced vegetative development in the summer. Thus, a change in the pattern of assimilate distribution appears to be one of the main components of the dwarfing mechanism.