Resumen de Relaciones hídricas del olivo (cv. Morisca) y el ciruelo japonés (cvs. Red beaut y angeleno) en Extremadura
In semiarid regions such as Extremadura, the rational use of irrigation to ensure the development of a modern agriculture, respecting the environment it is important. For the development of different irrigation strategies it is important to understand water relations of crops and their response to different water availability to maximize the benefits of irrigation, avoiding the waste of water.
In the last 20 years, irrigation has been introduced in traditional rainfed crop, such as olives; while new irrigated crops have been introduced in this region having a growing economic importance, as in the case of Japanese plum trees. Both have a great economic and social development in the area, due to the high crop surface, the economical profits and the high labour requirements.
With the achievement of this work, the objective is to move forward in the knowledge of water relations of both species and find out solutions to characteristic agronomic behaviours of specific cultivars growth in the zone.
In the case of olive, cultivar Morisca was chosen for being an important cultivar in the region about which little information is available. This cultivar has a marked alternate bearing and excessive vegetative growth, which requires severe pruning operations, increasing cultivation costs. Accurate information on the physiological response of the tree to water deficit, at different growth stages and crop load levels, is essential for designing irrigation strategies for optimizing resources and maximizing farmers' profits. For this purpose two field experiments of three years each, between 2002 and 2007 were performed. The objective was to understand the individual and interactive effect that crop load level and different water supply have on vegetative growth, water relations, water use, and olive and oil yield of olive trees.
In Experiment 1, carried out between 2002 and 2004, four irrigation treatments and three crop load levels were established. Three of the irrigation levels were based on the application of different percentages of evapotranspiration, ETc (125%, 100% and 75%) following the FAO method (Doorenbos and Pruitt, 1974) and a forth treatment was kept under rainfed conditions. Each irrigation treatment was studied at three crop load levels: On (100%), Medium (50%) and Off (0%).
For the second experiment, 2005-2007, three irrigation treatments were established and crop load was not manipulated with the aim of studying natural alternate bearing. As far as irrigation s concerned, an excess treatment was kept (applying 115% of ETc), another treatment was watering at 100% of the ETc and the final treatment was to keep the trees under severe water deficit (with water applications of 65% of the ETc). The first year was Off, however during the following years there were On and Medium loads in 2006 and 2007 respectively.
Water deficit had a marked effect on vegetative growth of olive trees in respect to the best irrigated treatment. Reductions at canopy scale (about 20% in shaded soil), at trunk scale (up to 54% in the early years) and at branch scale with reductions between 51% and 75% were observed Moreover, for the same irrigation treatment, On trees had lower growth rates than Off trees . Irrigation had higher incidence than crop load over vigor control, also notable was the reduction in growth rates as tree age increased. Both variables also impacted on the water relations of olives; well-watered trees showed the highest values of stem water potential and greater stomatal conductance (gl), although both values decreased as water supply was reduced. On trees presented less stem water potential and greater gl, but there were no differences between medium and high crop loads. In terms of yield, the annual yield was a function of the number of olives per tree, thus crop load was essential to get maximum yield. Water status and the competition between fruits, caused by crop load, had influenced the individual fruit weight of olives, having less effect on well-water trees. However no significant differences were found on the effect of crop load in olive oil content but low load trees had olives with greater oil content due to a higher fruit weight, this is because different crop load levels had the same oil percentage per olive.
Differences in water relations among load levels implied a greater water need in On olive trees by extracting more volume of water from the soil in these trees.
Water relations differences were depicted between On and Off trees, which also agrees with the greater soil water extraction observed in On trees respect to the Off ones.
This work has highlighted an important thing, FAO method (Doorenbos and Pruitt, 1974), using the crop coefficients proposed by Pastor et al. (1998) and the reduce coefficient proposed by Fereres et al. (1982), underestimates the actual water needs of these trees. This is due to the lower values of stem water potential observed in control treatment, irrigated at 100% ETc calculated by this method in respect to the most irrigated trees. In addition, the results obtained in other parameters such as vegetative and reproductive growth clearly indicated water stress in trees withtin control treatment.
The application of different water doses and its effect on tree water status meant a reduction on transpiration as water stress increase. A linear reduction of yield (y = 1.2302x-21.15, R2 = 0.89) was presented as tree transpiration decrease, which shows the high sensitivity of cultivar Morisca to water deficit.
Taking into account the vigorous pattern and the marked alternate bearing of this cultivar, it is recommended to apply different irrigation schedules in On and Off years. The sensitivity of cv. Morisca to water deficits is high, for this reason it is advisable to apply 100% of water needs of the olives during On years. However, irrigation schedules which implies a slight water deficit in Off years may help to control the excessive vegetative growth and reduce pruning cost, without yield loss. In this work, the most advisable treatment for Off years is the treatment which received 65% of ETc along the season.
Respecting to the other studied crop, the Japanese plum, little information is known about their physiology related to water availability. The first step that was proposed is to know their water needs, as well as the differential response to irrigation. Two of the most representative cultivars of the zone differing on time to maturing, Red Beaut and Angeleno, were chosen for this study. To get the first objective, a monthly estimation of evapotranspiration (ET) was done during three years (between 2007-2009) by water balance using the gravimetric method for both cultivars. In addition, the evolution of vegetative growth, water relations and the impact that these differences had on water use, were studied.
Water restrictions were avoided on water supply, with the aim of maintaining both cultivars well watered, and the results highlight cultivar differences in vegetative growth patterns and physiological behaviour. The early maturing cultivar, Red Beaut, presented greater vegetative growth than Angeleno, with higher ground cover percent, and bigger trunks and leaves, in spite of having the same annual branch growth and the same internode length.
The early maturing cultivar, Red Beaut, had higher vigor than Angeleno, expressed by greater ground cover, trunk growth and bigger leaf area, despite having the same branch and internode length. Also there were differences in water relations between cultivars: Red Beaut depicted lower gl and net assimilation rate than Angeleno. These differences meant warmer crown temperature and higher stem water potential.
These cultivar differences had impact both in the seasonal water use and in the annual one. It was observed that the early maturing cultivar, Red Beaut, had greater ET only early during spring until it reached the peak in early summer. The same happened in the late maturing cultivar, Angeleno, which was less vigorous, with greater gas exchange and higher water needs since early summer, so more water supply is required to supply the ET losses. The annual ET, shows that this cultivar has greater water needs than Red beaut, especially when ET is calculated per unit of ground cover.
These results also reveal the cultivar influence on water use, water relations and vegetative growth of Japanese plum trees and suggest an approach towards cultivar-specific irrigation scheduling adapted to the local conditions.
