This Thesis studies alternate bearing behavior of Citrus, sweet orange, Clementine mandarin and hybrids, and the influence of fruit in the process.
 	The hypothesis is that, in Citrus, the fruit inhibits flowering through the synthesis of gibberellins and their transport to the buds, preventing them to sprout and altering apex differentiation into flower.
	Fruit exerted its inhibitory effect from the time it was close to reaching its maximum weight, namely 80% of its final size (BBCH stage 78). From then until the fruit completes the growth and colour break starts (BBCH stage 79; early November) flowering intensity is reduced by 6 times, from 43 flowers/100 nodes to 6.9 flowers/100 nodes. From then to bud sprouting (April) flowering intensity does not reduce significantly (2.9 flowers/100nodes). Experiments with tangor "Nadorcott" trees with similar crop load and differing in the harvesting date showed significant reduction of flowering, but since date of harvest and crop load interactions becomes statistically significant reveals that the effect is mediated by the number of fruits per tree. 
       The reduction in flowering paralleled the reduction in bud sprouting. This reduction was due to a decrease in the number of generative sprouted buds, whereas mixed-typed shoots were largely independent of the time of fruit removal, and vegetative shoots increased in frequency. The number of leaves and/or flowers per sprouted shoot was not significantly modified by fruit load. Fruit, however, did not alter the number of flowers per bud but rather the number of buds that eventually sprout.
       Neither soluble sugar content nor the accumulation of reserve carbohydrates seems to fulfil an inductive function, but our results show evidence that sprouting and/or flower formation may require a threshold level of carbohydrates as an energy source. Only some kind of imbalance in nitrogen metabolism was observed in trees tending to flower scarcely.
 	Changes in GA1 and GA4 gibberellins’ concentration in the bark tissue of the shoot feeding the fruit in navel sweet orange indicate transport from the fruit to the bud, explaining its flowering inhibition effect. Further, GA3 applied from BBCH stage 78 to whole maturation inhibits flowering significantly, and paclobutrazol and prohexadione-Ca, two inhibitor of the gibberellins synthesis, applied at the same dates promote it in all species and cultivar studied, although the fruit avoids the action of these inhibitors.
	Accordingly, fruit presence effect, high endogenous gibberellins concentrations in the bark tissues next to buds, and GA3 exogenously applied, coincide in time and way of action, what demonstrates gibberellins participate in the inhibitory effect of fruit on flowering.  


Abstract




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