ABSTRACT

The purpose of this thesis was to state the endogenous and exogenous factors involved in the regulation of citrus fruit external ripening (degreening). Hormonal and nutritional factors were studied: a) in cultivars of oranges and mandarins with early, intermediate and late season maturation; and b) modifying the external ripening before colour break, either by the application of growth regulators and nitrogen, or by interrupting the phloem connection between fruit and the rest of the plant (peduncle girdling). Exogenous factors (air and soil temperature) were evaluated in early ripening mandarins by lowering root temperature, shading the soil with a plastic net or a thick lime cover, and in relation to air temperature. 
The main findings were the followings:
- Gibberellins (GA1 y GA4) need to be exported from citrus flavedo to allow colour break.  
- The application of GA3 not only delays the loss of chlorophyll, but also reduces colored carotenoids concentration (™-cryptoxanthin, cis-violaxanthin y ™-citraurine), and increases typical green fruit carotenoids (™-carotene and lutein) during ripening. 
- ABA concentration in the flavedo increases during fruit degreening without having a target role.
- It is demonstrated in vivo that there is an inverse relationship between colour and nitrogen concentration in the flavedo. The evidence that GA increment is the primary endogenous signal controlling colour break, and that it promotes a rise in endogenous N, allow us to state that N is of secondary importance to citrus fruit external maturation.
- Fruit colour development is promoted by a drop in soil temperature below a certain threshold, associated to rootstock.
- The soil temperature fall, due to the use of the white plastic net or thick lime cover during two months before colour break, promotes a Clementine mandarin earlier degreening, and consecuently increases crop percentage at the first harvest time.
Considering the relationship between endgenous and exogenous factors, it is possible to establish that fruit degreening is caused by the soil temperature drop, which depends on the air temperature. This change in temperature leads to a cessation of both, vegetative and root growth. As a consequence of this, there is a reduction of GAs synthesis and transport from roots, and their exportation from fruit flavedo is commenced. Furthermore, N absortion and its concentration in the flavedo diminish, as well as reducing sugars and ABA increase.