Interrelationships of vegetative growth and assimilate distribution of Citrus limettioides seedlings in response to root-applied GA3 and SADH

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Abstract
The effects of root-applied GA3 or SADH on vegetative growth, distribution of 14C-labelled assimilates, and starch content of various organs of Citrus limettioides seedlings were investigated. In contrast with the uniform elongation rate of control seedlings, GA3 induced an initial high elongation rate which later dropped to the level of the control, resulting in considerably longer seedlings. Final leaf area was drastically reduced by GA3 root treatment, which also induced both earlier appearance of thorns and more and shorter thorns per seedlings. Elongation rate of SADH-treated seedlings was initially retarded, but vigorous growth was later resumed for a short period, causing treated seedlings to be eventually similar to, or slightly longer than, controls. SADH did not affect the number and timing of thorn emergence but considerably promoted their elongation.The effect of GA3 and SADH on the vegetative growth was closely related to their differential effects on distribution of assimilates and starch in shoot and root tissues. A high growth rate was characterized by an increased supply of 14C-labelled assimilates to shoot organs and especially to buds as well as by a greater utilization of starch. The opposite was found in the root system, resulting in a significant modification of shoot–root ratios of both 14C-Iabelled assimilates and starch. The changes in 14C-labelled assimilates and starch usually preceded changes in growth, thus suggesting that GA3 and SADH effects on growth are mediated via their effects on the activity of apical sinks.