An efficient water management for Citrus spp. trees in any cropping situation requires accurate quantitative information on water use. Interpretation of the water relations of most Citrus spp. cultivars is difficult due to the occurrence of stomatal oscillations whose origin is not well known and that cause sampling problems in irrigation management (Dzikiti et al. 2006 and 2008; Wright 2008).
Vegetative growth, and particularly leaf development and stem diameter, of orange trees (Citrus sinensis (L.) Osbeck) is particularly susceptible to water scarcity (Dzikiti et al. 2006; Aiyelaagbe and Orodele 2007), and plants respond to drought by changes in gas exchange, phytohormonal balance and polyamine contents (Wang and Liu 2009). Moreover, internal water storage contributed significantly to the daily total leaf transpiration in this species (Dzikiti et al. 2006 and 2008). García Petillo et al. (2004) compared the effects of different irrigation volumes on 'Washington Navel' orange yields during a five-year period (0, 50%, 100% and 150% ETc). To apply these treatments, one irrigation drip line per tree row, with drippers, of 2, 4 and 6 L h-1 capacity, separated 1 m apart were used for the 50%, 100% and 150% ETc treatments. Another treatment received the same amount of water as 100% ETc, but with two drip lines spaced 1 m apart per tree row and 2 L h-1 drippers, and showed significant increases in total fruit yield and fruit size if compared to 100% ETc. The application of the PRD irrigation method (50 and 100% ETc) to orange trees was evaluated over two growing seasons by Dzikiti et al. (2008b). The authors found that stomatal conductance in the PRD treatments was lower than in the control fully-watered treatment but no significant changes in average fruit yield were found between the two PRD treatments and the control plants. Regarding RDI, it has been demonstrated that the irrigation cut-off during the final fruit growth and maturity process (phase III) in orange (cv. 'Lane late' grafted on 'Carrizo' citrange) reduces midday SWP, does not reduce fruit yield, and increases total soluble solids and titrable acidity, without altering fruit quality and the final maturity index (Pérez-Pérez, Robles and Botía 2009). On the contrary, García-Tejero et al. (2010) found that RDI applied during the flowering and early fruit-growth phases (cv. 'Navelina' grafted onto 'Carrizo' citrange), both yield and fruit quality (in terms of total soluble solids and titrable acidity) were negatively affected.
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Other Citrus species appear to have responses against drought stress similar to those found in orange. Huang et al. (2000) examined the growth changes generated by mild drought stress on potted tangerine trees (Citrus reticulata Blanco, cv. 'Zhuju') by water withholding during early juice sac expansion stage. They observed that fruit growth was inhibited by drought stress but a greater water uptake was caused by a lower water potential in fruits of stressed plants, likely due to a higher water loss from fruit to transpiring leaves during water shortage and some active adaptive physiological responses (osmotic adjustment and cell wall loosening) of fruit to this stress. In satsuma mandarin trees (Citrus unshiu Marc.) a positive relationship between flower-bud induction and the level of endogenous plant hormones was found as a result of the application of mild (predawn LWP = from -0.5 to -1.0 MPa) and moderate drought stress (predawn LWP = from -1.5 to -2.0 MPa) (Yoshita and Takahara 2004). The results indicate that gibberellin levels were enhanced by severe drought stress, higher in the leaves from the branches that produce fewer flowers during flower-bud induction periods, whereas the levels of indole-3-acetic acid were higher in the leaves from the branches that produced more flowers during the season when flower-buds develop. As in C. sinensis, the measurement of maximum daily trunk shrinkage is a suitable and reliable indicator of the level water deficit reached by plants of other Citrus species, such as lemon (C. limon (L.) Burm. fil.) and sour orange (C. aurantium L.) (Ortuno et al. 2009).
Other Mediterranean fruit species
Pomegranate trees (Punica granata L.) are considered as a crop with a high level of tolerance to soil water deficit. Pomegranate cultivation is mainly confined to the tropics and subtropics and it grows well in arid and semi-arid climates, but it is now widely cultivated in Mediterranean (Hepaksoy et al. 2009). In Spain, for example, its culture is concentrated in the south east, where fresh water available for agriculture is very scarce. The water relations of field grown pomegranate trees grown under different drip irrigation regimes were recently investigated by Intrigliolo et al. (2011). These authors observed that during spring and autumn, midday SWP was not significant different between irrigation treatments while there were considerable differences in leaf photosynthesis and stomatal conductance, suggesting a near-isohydric behaviour of pomegranate trees. This means that plants control gas exchange such that daytime water content is almost unaffected by soil water deficits, and that other mechanisms (e.g., ABA production and signaling) can be responsible for the regulation of plant water status. There is little knowledge about the response of pomegranate to drought, and in general to abiotic stresses. In one of the few researches, Bhantana and Lazarovitch (2010) studied the evapotranspiration, crop coefficient and growth of two young pomegranate varieties under salt stress, confirming that this species exhibits a high tolerance under adverse environmental conditions. If compared to other irrigation techniques, drip irrigation is the best way to increase fruit yield and plant growth of pomegranate, as its root system is particularly inhibited by water stagnation, whereas fruit yield is not significantly influenced by the level of irrigation (Sulochanamma, Yellamanda Reddy and Subbi Reddy 2005). Furthermore, irrigation of pomegranate trees is very important, as fruit splitting and cracking can occur, unless they are regularly irrigated. Excess watering or excessive rain during the maturation period may also cause similar damage to the fruits (Hepaksoy et al. 2009). Finally, vitamin C, reducing sugar and total sugar content were observed in fruit of drought-stressed plants (Lawand, Patil and Patil 1992).
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Pistachio is a crop indigenous to western and central Asia but its cultivation has spread to the Mediterranean region, which has become its second most important centre of diversity after Iran. The importance of Pistacia spp. is not limited to this product alone: the trees' great tolerance to drought and their ability to thrive in poor soil conditions make them particularly suitable for forestry programmes on marginal lands, where they can also represent a source of additional income for local farmers (Padulosi et al. 1998; Sedaghat 2008). Pistachio cultivation requires the use of rootstock because grafting is the only form of vegetative propagation, thus the choice of the most effective rootstocks plays a key role, as they determine the physiological and biochemical responses of the plants to drought (Ranjbarfordoei 2000, 2002; Gijón et al. 2010). The extreme drought resistance of Pistacia spp. enables farmers in arid and semi arid lands to grow this nut without irrigation (Kaska 2002). Despite the economic importance of edible pistachio (Pistacia vera L.), very little information is available on its nutrient requirements and water needs. Potassium (K) fertilization is found to be effective in increasing leaf K status, nut yield and quality in this species, and K uptake occurs mainly during the nut fill period (Zeng, Brown and Rosecrance 1998). Tajabadipour, Sepaskhah and Maftoun (2006), studied the effects of three irrigation frequency and five K levels on the plant water relations and growth of three pistachio cultivars ('Badami', 'Ghazvini', and 'Sarakhs'), founding that the dry weights of leaves, stems and roots significantly decreased with increasing irrigation intervals, whereas K application had no significant effect on LWP, osmotic potential, and turgor potential. From a molecular point of view, Yakubov et al. (2005) observed the accumulation of dehydrin-like proteins both in the inflorescence bud and in the bark of young pistachio stems, suggesting that they may have a role in drought and cold tolerances, as well as serving as storage proteins. An irrigation experiment involving pistachio (P. vera, cv. 'Kerman', on P. terebinthus rootstocks) was performed by Gijón et al. (2009) over a four-year period to determine the effect of RDI (at 65% and 50% of control irrigation) on nut yield and quality. The growth season was divided into three phenological stages: stage I - from sprouting until the end of rapid nut growth; stage II - from maximum nut size until the beginning of kernel growth; and stage III - from the beginning of kernel growth until harvest. The plants subjected to RDI were only significantly stressed during stage II, showing midday LWP of around -1.4 MPa. The application of RDI resulted in smaller nut diameter and lower total yield. Moreover, trees subjected to RDI had a total yield and percentage of split nuts similar to those of the controls, and did not show the normal alternate bearing pattern of this tree crop. The authors concluded that this rootstock-scion combination presents a high degree of drought-resistance and could be efficiently applied in pistachio cultivation.
Opuntia, also known as 'nopales' or 'paddle cactus', is a genus in the family Cactaceae. The most commonly culinary species belonging to this genus is the Indian fig Opuntia ficus-indica (L.) Miller, commonly known as 'prickly pear'. This species is native to Mexico but it is also found in southern Europe and northern Africa, where it contributes, like olive tree, to the typical Mediterranean landscape. The prickly pear tree is able to store high water amounts in its succulent organs and it has a very wide (even though not deep) root system with dense and rapidly regenerating root hairs, that allow plants to efficiently use extremely low rainfall (Mulas and Mulas 2004). Prickly pear has a CAM photosynthesis and thus maintains the stomata of mature cladodes open only in the night, but, under extremely severe water deficits, the stomata remain closed all day long and so the plants use to photosynthesize only the CO2 deriving from the respiration (Nieddu et al. 1997). Pimienta-Barrios et al. (2000) evaluated the effects of seasonal variation in temperature, irradiation, and soil moisture content on the photosynthetic rates of prickly pear. They demonstrated that this species is strongly adapted to arid climates and that stem photosynthesis by cladodes (stem modified for photosynthesis that looks like leaves) allows plants to fix carbon to be used during the periods when soil water content is very low. Drought significantly affects cladode morphology and inhibits new cladode production, as these latter have a C4-photosynthesis and open the stomata during the day, with consequent water losses (Nieddu et al. 1997). Furthermore, cladodes can reach temperatures 15°C higher than the environmental ones, maintaining their enzymatic activities up to 60°C.
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The fruit yield of prickly pear is quite low, likely due to limiting environmental factor (low water amounts, soils with low levels of organic matter), but a fertilization up to 160 kg ha-1 determines a yield increase and a high fruit quality (Mulas and Mulas 2004). Mulas and D'hallewin (1997) estimated that fruit yield in irrigated plants is at least two folds higher than that of un-watered plants, due to higher fruit number per cladode and not to increases in fruit weight. On the other hand, irrigated plants presented an increase in fruit peel thickness, that reduced the juice percentage, and in seed weight (Mulas and D'hallewin 1997). Snyman (2006) aimed at quantifying the effects of drought stress on the growth of tap roots, side roots and rain roots of the species Opuntia ficus-indica (L.) (cv. 'Morado', with green cladodes) and O. robusta Wendl. (cv. 'Monterey', with blue cladodes), both having edible fruits. They planted one-year-old cladodes in root boxes and pots in a greenhouse. Placing the cladodes flat on the soil, more areoles came in contact with the soil, and each areole complex formed on average three roots. From the analysis of the growth of tap roots, side roots and rain roots, and from the data on root size and density O. robusta appeared to be less sensitive to drought than O. ficus-indica.
Loquat (Eriobotrya japonica Lindl.), also called Japanese medlar or Japanese plum, is a subtropical evergreen tree crop indigenous to southeastern China but very well adapted to mild-winter areas of the Mediterranean basin (Hueso and Cuevas 2008). Drought stresses causes significant decreases in leaf expansion rate, area and photosynthetic pigments, and in stomata size, and increases in stomata density (Luo et al. 2007). Both deficit irrigation during the entire season and post-harvest RDI from mid-May through the end of August (reduction of 20% water needs, with water savings established around 1450 m3 ha-1 yr-1) were successfully applied in this species (Hueso and Cuevas 2008). It was observed that post-harvest RDI usually advances full bloom 10-20 days, allowing to obtain a more precocious and valuable yield, whereas the effects of continuous deficit irrigation is less noticeable (Hueso and Cuevas 2008; Fernández, Hueso and Cuevas 2010). On the contrary, yield and fruit quality are not affected for the different deficit irrigation strategies. The optimal month for the application of post-harvest RDI in loquat seem to be July, due to the positive effects on the advancement of bloom and harvest date and its harmlessness for flower development, even thought every RDI applied in the period June-August (with a water reduction up to 75%) does not influence negatively fruit set, size and yield (Cuevas et al. 2007).
Conclusions and perspectives
A new approach in fruit orchard management is imposed by the environmental emergencies that are marking this recent period (e.g., soil degradation as a result of erosion and desertification, water shortage, greenhouse effect). In semi-arid Mediterranean lands, the adoption of agricultural systems by means of conventional, non-sustainable techniques causes the reduction of soil organic matter, groundwater contamination, soil deficiency of mineral elements (in particular phosphorus and nitrogen), alkalinization/salinization, and nutritional imbalances in plants. On the other hand, the recent researches on the physiology of fruit trees and on soil chemical and biological fertility in fruit orchards have revealed that sustainable and innovative soil management systems, with a particular emphasis on irrigation, allow to obtain an optimal plant nutritional equilibrium, avoid nutrients accumulation and leaching risks, improve irrigation efficiency, and prevent soil erosion and root asphyxia. As highlighted in this chapter, the definition of appropriate irrigation techniques (e.g., SDI, RDI, PRD) and soil management in Mediterranean fruit orchards are indispensable requisites for preserving soil quality, positively affecting soil microbial activity and fertility, and maintaining top yields of high quality. Considering the scientific, practical and socio-economical importance of these topics, and the increasing environmental emergencies related to water scarcity, a conspicuous number of studies is expected in the next years. At the moment, it is clear that the application, optimization, innovation of sustainable agricultural techniques with a low negative environmental impact can allow to recover or increase the normal levels of total fertility in agro-ecosystems, with positive effects on both soil and yield quality.