RESUME

    Most Prunus species have the tendency to set a large number of fruits, but it reduces the possibility of attaining commercial fruit size and quality at harvest. As a derived effect, alternate bearing cycles can be initiated since gibberellins produced in the seeds are the main cause of inhibition of floral bud initiation. To increase individual fruit size, growers thin fruit manually, but it represents a high cost that reduces growers’ returns. Paradoxically, the effect of gibberellins can be used agronomically to overcome these problems. 
    
    This PhD Thesis develops an easy agronomical technique to reduce flowering, and consequently the cost of hand-thinning of Prunus cultivars, by means of gibberellic acid application, exploring the implications in the floral process of fruit and inhibitors of gibberellins biosynthesis (paclobutrazol, prohexadione-Ca), through carbohydrates and nitrogen fractions content. We also studied the temperature requirements to sprout or bloom and their dependence on sugar’s content, using the ‘one node cutting’ technique.
    
    Experiments were carried out in three commercial orchards in the area of Valencia, Spain, and one in Esperanza, Argentina. The application of gibberellic acid during the floral induction period reduced flowering by 40%-50% in peaches and plums, being 50 mg l-1 of GA3 the most appropriate concentration. Gibberellic acid application also modified floral distribution along the shoot and some aspects of PSII efficiency. Our results evidenced an inhibitory effect of fruit on the flowering process, which acts directly, by reducing the number of floral buds, and indirectly, by reducing the number of developing shoots, their length, number of nodes and number of buds. We then hypothesize that flowering is not an inductive process but an inhibitory process. Neither soluble sugars in phloem sap of shoots nor carbohydrate reserves in fibrous roots, have been linked to the floral initiation process, but with energy supplied to sprout. However, we found evidences of some disturbances of the nitrate reduction process in trees tending to flower scarcely. 
    
    The experiments giving artificial chilling showed that chilling enhances leafing and blooming but with larger effect over the former. Excessive chilling diminished the percentage of flower budbreak in low chilling cultivars. Moreover, the mean time to budbreak (MTB) of leaf buds decreased faster with chilling, compared with flower buds. Experiments demonstrated a negative correlation between the number of chill hours accumulated till budbreak and the number of heat units accumulated from then to full bloom, and that chilling induces changes in carbohydrates and nitrogen metabolism in the adjacent tissues of buds. Finally, the results proved that budbreak starts when bud weight increases significantly with respect to its initial weight at leaf fall, in both floral and vegetative buds. Thus, results confirm the utility of the excised shoots method to complement field experiments in the search for a unique model of dormancy process.

Resume

Resume

2

Resumen

Resumen

3