| The main objective of this Doctoral Thesis was to evaluate the agronomic and physiologic responses of both young and adult almond trees to distinct deficit irrigation strategies, as well as to assess the applicability and utility of several plant water status indicators for irrigation scheduling. To this aim, three experiments were planned on two different experimental plots: (a) one of them carried out on adult 'Colorada' almond trees irrigated with highly saline water, where three irrigation treatments were established: (i) TCTL, irrigated at 125% ETC throughout all growing season; (ii) TRDC, receiving 100% TCTL until the fruit reached its definitive external size, then 70% TCTL until the beginning of stage IV, 30% during stage IV and 70% TCTL in postharvest; and (iii) TSCO, rainfed treatment. (b) two of them carried out on young 'Marta' almond trees, where five irrigation treatments were defined (i, ii, iii) TCTL, TRD80 and TRD60, irrigated at 120, 80 and 60% ETC all growing season and (iv, v) TRDC40 and T RDC20, irrigated at 100% ETC during all year except during stage IV, when 40 and 20% ETc was supplied, respectively.;The most significant mechanisms of adult almond trees under both water and salt stresses (Experiment 1) were: (i) a decrease in predawn leaf water potential, which allows the plant to preserve a water potential gradient between soil and leaves in order to maintain root water uptake; (ii) an early stomatal regulation, evidenced since the first hours of the day, that maximizes CO 2 assimilation efficiency with respect to water losses; (iii) osmotic adjustment which took place during most of the growing season, and allowed the plant to maintain cell turgor and thus compensating leaf water potential decline; and (iv) leaf area index (LAI) decrease, which supposed an additional contribution to maintain cell turgor, since it reduces shoot/root ratio and therefore the transpiration rate for a similar evaporative demand.;With respect to the productive response, both control and regulated deficit irrigation (RDI) treatments showed higher values than expected taking into account the salinity level of irrigation water. On the other hand, RDI treatment, which saved 50% of the irrigation water volume, presented similar yield to control treatment. These results evidence a good degree of adaptation of this autochthon variety to both water and salt stresses, leading to a more efficient use of soil available water.;From the results of Experiments 2 and 3, vegetative growth indicators were the most sensitive to water stress, in particular trunk diameter. Trees growing under the most severe water stress conditions (TRD60 and TRDC40) were the most affected in terms of tree growth, particularly those trees that suffered water restriction since the beginning of the plantation. The analysis of the productive response showed no significant changes between water restricted and control treatments, except for TRD60 which presented lower yield values than control treatment in the third year of study. The general tree growth decrease observed in water-restricted young almond trees when compared to fully irrigated trees lead to smaller tree crowns, and is likely to reduce the number of fruit positions and thus crop load once the trees reach the productive period. Consequently, it results highly convenient to establish irrigation scheduling criteria aimed to allow young trees to reach the productive period as fast as possible.;Among all the tree water status indicators considered in this study, maximum daily shrinkage (MDS) presented the best correlation with predawn leaf water potential (Psia) and midday stem water potential (Psi t), indicators traditionally considered as reference in this kind of studies. A similar result was found when analyzing the correlations with crop reference evapotranspiration (ETo) and vapour pressure deficit (VPD). In addition, we obtained a close relationship between MDS and VPD on both seasonal and monthly basis, which allowed establishing reference baselines that might be used for irrigation scheduling. These results, joined to the reliability of this indicator to detect soil moisture changes as well as the possibility of continuously monitoring tree water status, makes MDS a pertinent and useful plant state variable to be used in almond irrigation scheduling, provided reference baselines are available. |
