The Genus Prunus

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The genus Prunus (family Rosaceae) comprises more than 400 species adapted to temperate areas and cultivated in Europe. In particular, stone fruit crops, such as peach (Prunus persica L.), plum (P cerasifera L. and P. domestica L.), almond (P. dulcis L.), apricot (P. armeniaca L.) and cherry tree (P. avium L.), are typical and economically important and mainly localized in Mediterranean regions. Productive stone fruit trees are usually grafted plants with a lower part, the rootstock and an upper grafted part, which is the genotype of the commercial variety. Rootstocks are important for agronomic purposes, as they have a different genetic background compared to the commercial varieties and can be used to confer various traits such as drought stress resistance.

A better understanding of the effects of water deficit on Prunus species has a primary importance for improved management practices (Girona et al. 2005b), breeding programmes (Rieger et al. 2003) and for predicting fruit growth and quality (Torrecillas et al. 1996; Besset et al. 2001; Esparza et al. 2001; Girona et al. 2002). During periods of water deficit, species of the genus Prunus show significant decrease in gas exchange (Ruiz-Sanchez et al. 2000a ; Besset et al. 2001; Klein et al. 2001). The decrease of soil humidity, together with high values of vapour pressure deficit (VPD), causes reduction in leaf water potential (LWP), carbon assimilation and transpiration in different species of Prunus (Rieger and Duemmel 1992; Rieger 1995; Berman and DeJong 1996; Alar on et al. 2000; Besset et al. 2001; Esparza et al. 2001; Klein et al. 2001; Rieger et al. 2003 ; Matos et al. 2004; Romero et al. 2004c; Gomes-Laranjo et al. 2006; Intrigliolo and Castel 2006; Dichio et al. 2007; Godini et al.

2008; Egea et al. 2010). Some studies also highlighted the activation of antioxidant defenses as a strategy to face drought-dependent oxidative stress in this genus (Scebba et al. 2001; Sofo et al. 2005; Sorkheh et al. 2011).

2.1 Peach and Apricot

The peach tree (Prunus persica L.) and the apricot tree (Prunus armeniaca L.) are two of the most common and economically important species of the Mediterranean basin (Grossman and DeJong 1998; Alar on et al. 2000; Besset et al. 2001; Girona et al. 2002). The drought tolerance of peach and apricot is mainly based on stomatal control (Arndt et al. 2000) and morphological characteristics (Rieger et al. 2003), together with some degree of osmotic adjustment (Alar on et al. 2000) ; Work in these two species covered subjects from the physiological processes adopted to regulate water status under drought conditions (Ruiz-Sanchez et al. 2000a; Rieger et al. 2003) to the biochemistry underlying plant response to water deficits and oxidative stress (Arndt and Wanek 2002; Sofo et al. 2005). Peach and apricot trees are highly sensitive to drought stress at particular phenological stages, such as flowering and fruiting, and during stem extension and fruit growth (Berman and DeJong 1997; Xiloyannis et al. 2005) ; Considering the sensitivity of these two species to water deficit, several authors highlighted the importance of considering wetting patterns, soil depth and root exploration in peach and apricot irrigation management (Ruiz-Sanchez et al. 2000a; Girona et al. 2002). Keeping in view an efficient use of water in peach and apricot orchards, it is of key importance to consider the type of training system and plant architecture, and particularly the distribution of light in the various parts of the canopy, as well as the system of irrigation and its management (Xiloyannis et al. 2010).

Among the indicators used for monitoring water status of peach and apricot trees, two of the most reliable were indicated by Alar on et al. (2000) and Arndt and Wanek (2002). The formers used foliar carbon isotope composition (#3C)

measured in leaves of peach under water deficit as a tracer to study whole plant carbon allocation patterns. In fact, it is known that foliar carbon isotope discrimination decreases in water-deficit situations as discrimination by the photosynthetic primary carboxylation reaction decreases. On the other side, Arndt and Wanek (2002) used sap flow measured with a heat-pulse technique as an indicator of transpiration and the water status of young apricot plants. They observed that when apricot trees are drought-stressed, measures of sap flow slightly underestimate actual transpiration, confirming an increasing hydraulic resistance under drought conditions. In apricot (cv. "Bulida"), a preconditioning treatment using a substantial reduction in the irrigation water (25% of crop evapotranspiration, ETc) promotes a better drought-hardening of the plants due to a greater osmotic adjustment (0.77 MPa) that prevents severe plant dehydration and leaf abscission (Ruiz-Sanchez et al. 2000a). This treatment may be valuable for young apricot plants in the nursery stage in order to improve their subsequent resistance to drought.

Regarding the relationship between drought and fruit yield, Berman and DeJong (1996) demonstrated that in well-watered peach plants (cv. "Elegant Lady"), tree water status is independent of crop load, whereas in trees receiving reduced irrigation, the degree of drought stress increased with increasing crop load. The same authors found that drought stress induces fruit fresh weight reductions at all crop loads, whereas fruit dry weight is not reduced by drought stress in trees having light to moderate crop loads. These results suggest that the degree of drought stress imposed did not affect the dry weight sink strength of peach fruit. On the other hand, drought-stressed trees with heavy crop loads had significantly reduced fruit dry weights, which were likely due to carbohydrate source limitations resulting from large crop carbon demands and drought stress limitations on photosynthesis. Crisosto et al. (1994) observed a higher density of trichomes and a continuous and much thicker cuticle on peaches (cv. "O'Henry") from the deficit and optimum irrigation treatments than from the excess irrigation treatment, indicating that a well-designed irrigation management can improve fruit quality and extend shelf life. In peach trees, there is a direct correlation between water availability and carbohydrate synthesis (Girona et al. 2002), and between photosynthetic rate and types of carbohydrates synthesised (Escobar-Gutiérrez et al. 1998). During fruit growth, high photosynthetic rates are necessary for growth requirements of peach (Besset et al. 2001). Sorbitol and sucrose are the two main photosynthetic carbohydrates of peach plants and their function depends on the organ of utilization and its developmental stage (Lo Bianco et al. 2000). In well-watered peach plants, these sugars are translocated from their sources, mainly mature leaves, and then absorbed by sink organs, such as shoot apices (Lo Bianco et al. 2000), developing fruits (Grossman and DeJong 1998) and buds during dormancy release (Marquat et al. 1998). Under drought stress, sucrose metabolism is only marginally reduced, whereas sorbitol accumulates in sinks and sources, contributing up to 80% to osmotic adjustment (Lo Bianco et al. 2000).

Regulated deficit irrigation (RDI), the practice of reducing applied water at selected phenologi-cal stages less sensitive to water deficit, was successfully applied to both peach and apricot (Ruiz-Sánchez et al. 2000b; Girona et al. 2005a; Dichio et al. 2007). The application of RDI, based on imposing plant drought stress in a controlled manner, is a feasible water-saving practice for Mediterranean arid areas. Moreover, RDI extended over a long period lead to adaptation of peach tree to dry conditions due to a better extraction of water from deeper soil. The success of RDI strongly depends on the appropriate use of localized irrigation techniques, which allows the control of soil water content (SWC) and plant water status. Moreover, an efficient use of irrigation water is particularly important for improving water uptake by root system. In peach, the application of RDI during the early stages of fruit growth until the end of shoot growth slightly influences fruit size and number (Boland et al. 2000) and a water deficit treatment during the final stage of rapid fruit growth causes decreases in fruit size but also significant increases in total fruit soluble solids (Crisosto et al. 1994; Besset et al. 2001; Naor et al. 2001). Thus, peach quality and taste can be considered as being improved by a water deficit applied in this phonological phase.

On the other hand, withholding irrigation applied after harvest reduces vegetative growth of early-maturing peach trees (Johnson et al. 1992; Ghrab et al. 1998; Girona et al. 2005a) and can improve fruit quality (Gelly et al. 2004; Dichio et al. 2007) . It is important to note that RDI, though applied during the post-harvest stage, has to be performed avoiding high levels of drought stress, which could negatively influence the accumulation of reserve carbohydrates, flower development and thus, indirectly, crop yield of the following year (Xiloyannis et al. 2005). Dichio et al. (2007) evaluated the effects of RDI applied in the post-harvest stage of mature peach plants (cv. "Springcrest") trained to transverse Y in an experimental field located in Southern Italy. These authors confirmed the possibility to reduce the irrigation water by applying RDI during phe-nological stages less sensitive to water deficit without negatively affecting peach growth and yield. In their experiment, from bud break to harvest, irrigation was carried out by applying 100% ETc. while from harvest to early autumn, plants were separated into three groups and subjected to different irrigation treatments (100%, 57% and 34% ET ). RDI determined the reduction in the growth of waterspouts and lateral shoots but did not influence the growth of fruiting shoots. No significant reductions in crop yield and quality were observed in the 57% ET treatment, whereas c about 1,100, 1,800 and 2,500 m3 ha-1 of water were saved in the first, the second and the third year, respectively. In the second year of the trial, the use of RDI in the post-harvest stage determined carbohydrate and nitrogen (N) accumulation in roots, branches, shoots and floral buds. Therefore, the results of Dichio et al. .2007) demonstrate that, under scarce water supply conditions, a clear benefit for both vegetative growth, and carbon and N allocation of peach trees can be obtained through the use of RDI during the post-harvest stage. In another study, Ruiz-Sanchez et al. (2000b) evaluated the response of apricot trees (cv. "Bulida") to RDI under Mediterranean climate. The authors applied an RDI treatment irrigated at 100% ETc during the critical periods (second rapid fruit growth period and 2 months after harvest) and with a reduction of 40% ETc during the other periods. An average water saving of 34% was achieved in the fourth year RDI treatment and apricot quality was not modified by RDI treatment. Furthermore, when irrigation water saving was around 25%, the yield obtained was similar to that of the control treatment. It is noteworthy that apricot fruit growth showed few differences between the control and the RDI treatment during the deficit irrigation period, but an accelerated rate of growth was noted when irrigation was increased to 100% ETc. In conclusion, the satisfactory yield obtained with RDI in Mediterranean peach and apricot orchards suggests to adopt it for early ripening cultivars grown in semi-arid areas with limited water resources in order to improve irrigation efficiency and save water while maintaining top yields of high quality.

With the adoption of a sustainable management under semi-arid climatic conditions (an example is reported in Fig. 6.1), peach and apricot yield can be enhanced up to 25-30%, and the amounts of water and of N, P, K and soil carbon inputs annually incorporated into the soil increase significantly if compared to a non-sustainable orchard (Xiloyannis et al. 2010). Among the sustainable practices, cover crops are of key importance, as in Mediterranean apricot groves the use of mixture of herbaceous species with a high biomass production, such as Viciafaba/Avena sativa, can produce approximately 1.0 ton ha-1 of humus, with clear benefits for soil fertility (Celano et al. 2002). Moreover, in rain-fed conditions, it is beneficial to sow cover crops in autumn and sow them just before spring in order to avoid water and nutrient competition (Xiloyannis et al. 2005). Sustainable practices also have positive effects on soil microbiota, that influences soil fertility and plant growth by regulating nutrient availability and increasing their turnover (Kushwaha et al. 2000; Borken et al. 2002; Widmer et al. 2006; Govaerts et al. 2008). A molecular approach was often used to reveal qualitative changes in the structure of soil bacterial and fungal communities in various Mediterranean agro-ecosystems

Fig. 6.1 Comparison between a sustainable and a conventional management of a peach orchard (Prunus persica (L.) Batsch Nectarine, cv. "Supercrimson" grafted on GF677) located in Southern Italy, under a semi-arid

climate with an average annual rainfall of 525 mm. Peach trees were trained to vase (500 plants ha-1) with a north-south row orientation (data from Sofo et al. 2010a)

(Bending et al. 2002; Marschner et al. 2003) but little is known on the molecular and metabolic aspects of soil microbial community at orchard level. One of the few researches on this subject was carried out by Sofo et al. (2010a), that examined the short-time effects (after 4 years) of two different management (sustainable and non-sustainable) systems on microbial genetic, functional and metabolic diversity of a Mediterranean peach orchard (cv. "Supercrimson"), evaluated by a combination of culture-dependent and culture-independent techniques. They revealed qualitative and quantitative changes of soil microbial communities (different electrophoretic patterns of bacterial 16S ribosomal and fungal 18S ribo-somal RNA genes and higher indexes of microbiological diversity) in response to a sustainable soil management.

2.2 Almond

Almond (Prunus dulcis L.) is the most important tree nut produced on a global basis, and its limited gene pool limits the cultivation to specific areas with Mediterranean climate (Sorkheh et al. 2011) . This species is one of the oldest tree nut crops, and today represents the largest production of any commercial tree nut product. The response to water deficit of almond trees is a well-documented process (Esparza et al. 2001, 2010; Klein et al. 2001; Gomes-Laranjo et al.; 2006, Rouhi et al. 2007; Egea et al. 2010). The results obtained on the agronomic response of almond trees to different deficit irrigation strategies demonstrate the prevalence of direct and strong links between the intensity of the water restriction and the response of several parameters related to tree growth, yield and water status (Rouhi et al. 2007; Egea et al. 2010). Besides predawn or midday LWP, midday stem water potential (SWP) and midday leaf sto-matal conductance, a series of indicators of plant water status were applied on drought-stressed almond trees. The results obtained by Nortes et al. (2005) indicate that both maximum daily trunk shrinkage and trunk growth rate in almond are sensitive to drought stress and that the second is the most useful parameter for quantifying water deficit intensity and duration.

As regard to fruit production, in almond trees (cv. "Nonpareil") an average loss in yield of 7.7 kg tree) 1 occurs in response to each 1 MPa decrease in stem water potential (SWP) below -1.2 MPa, if a severe irrigation deprivation is carried out during the harvest period of the previous year (Esparza et al. 2001). This yield loss is likely due to the decrease in the number of fruiting positions per tree, even though the authors did not observe effects of irrigation deficit on the percentage of spurs that flowered or set fruit during subsequent years. In another study, Esparza et al. 2001 confirmed that a severe drought stress during the harvest period in almond causes a reduction in non-structural carbohydrates content but not in N content per tree, so limiting vegetative growth in the following year and impacting subsequent fruit-bearing capacity rather than directly affecting flowering, fruit set or fruit growth. Differences in N-allocation patterns between fruiting and non-fruiting shoots were recently observed by Nortes et al. (2009) in drought-stressed almond plant (50% ET. during the entire growing season), if compared to fully watered plants (100% ET.). They found that in the 50% ET. treatment, a high N status is maintained in the leaves of fruit-bearing shoots, to the detriment of N resources allocated to vegetative shoots.

The studies on RDI applied in Mediterranean almond orchards and aimed to improve fruit yield and quality are numerous (Romero et al. 2004a, b, c; Girona et al. 2005b; Egea et al. 2010). It was found that an RDI of 20% ETc applied during the pre-harvest period (kernel-filling stage) does not cause reduction in kernel yield and size in almond (cv. "Cartagenera") and improves water-use efficiency, but only if predawn LWP is maintained above a threshold value of -2.0 MPa (Romero et al. 2004c). In the same cultivar, Romero et al. (2004a, b) indicated that a severe RDI (20% ETc) during the kernel-filling stage, and a recovery at 75% ETc during the post-harvest phase allows to save 220-273 mm yr 1 irrigation water without negatively affecting plant growth and fruiting.

Related almond species, interspecific crosses and spontaneous interspecific hybrids demonstrate a greater resistance to abiotic and biotic stresses and so represent valuable germplasm sources for rootstock breeding, especially under non-irrigated conditions (Browicz and Zohary 1996) . The wide adaptation of the related wild almond species indicate their potential as sources for resistance to drought stress as well as modified tree and nut traits. On this basis, the selection for drought resistance in almond rootstock material and for the increase of quality of cultivated almond production under stress conditions is particularly important. Among the different wild almond varieties, Rouhi et al. (2007) studied the intrinsic water use efficiency, defined as the ratio of assimilation rate over stomatal conductance, in cultivated and wild almond species, finding that P. dulcis is the species most tolerant to drought, P. scoparia tries to avoid drought, and P. lycioides has an intermediate behavior and this latter can have potential for use as rootstock for commercial almond production. As wild almond species can be very important for rootstock selection, in a recent paper, Sorkheh et al. (2011) examined the changes of antioxidant enzyme activities and the level of some antioxidant compounds involved in the ascorbate-glutathione cycle in drought-stressed plants of eight wild almond species from different geographical points of Iran. The authors found that after 70 days without irrigation, mean pre-dawn LWP in all the species fell from 0.32 to -2.30 MPa and marked decreases in CO2 uptake and transpiration occurred. The activities of the antioxidant enzymes involved in the ascorbate-glutathione cycle increased in relation to the severity of drought stress in all the wild species studied. Furthermore, the levels in total ascorbate and glutathione and H2O2 were directly related to the increase of drought stress. The up-regulation of the activities of some antioxidant compounds during drought stress is an immediate and efficacious response to scavenge the excess of activated oxygen species (AOS), such as superoxide anion (O2'~), hydrogen peroxide (H2O2), hydroxyl radical (HO) and singlet oxygen (1O2), and it was also observed in other fruit tree species, such as apricot (Scebba et al. 20013. olive (Sofo et al. 2004a) and plum rootstocks (Sofo et al. 2005).

As seen for olive (Sofo et al. 2004b), the role of proline during drought stress is particularly important for the osmotic homeostasis of the plants. The results of a forthcoming paper on wild almond (Sorkheh et al. 2011) highlight that the cell membrane damage is a direct consequence of oxidative stress by H2 O2 and that the application of exogenous proline can alleviate these detrimental effects. Thus, it is possible to recommend exogenous proline treatment of wild species of almond in order to increase their anti-oxidant defenses when subjected to drought.

2.3 Plum and Cherry

Many plum genotypes are used as rootstock for almost all other Prunus species and, among them, Myrobalan plum (Prunus cerasifera L.) clones often show positive agronomic features for resistance to pathogens and abiotic stresses (Lecouls et al. 2004; Intrigliolo and Castel 2006) . In the Mediterranean regions, drought is the main limiting factor for plum growth (Rato et al. 2008). Sofo et al. ( 2005) studied the effects of water deficit on photosynthetic performance and on the components of the ascorbate-glutathione cycle in four interspecific plum hybrids, used as root-stocks, hypothesizing that an excess of reducing power, with the consequent increase in H2O2 and other AOS concentration, causes the up-regulation of some antioxidant enzymes during a drought period. Their results showed that the activities of antioxidant enzymes and the levels of the molecules involved in the ascorbate-glutathione cycle (antioxidant enzymes, total ascorbate and glutathione and H2O2) increased in all the hybrids examined in parallel to the severity of drought stress. After 70 days of water shortage, mean predawn LWP of all the hybrids fell from -0.34 to -3.30 MPa and marked decreases in net photosynthesis and transpiration occurred. All these physiological and biochemical responses could limit cellular damage caused by AOS during periods of water deficit. On the basis of these results, it appears that the ability of Prunus hybrids to regulate the enzymatic antioxidant system during drought stress can be an important attribute linked to drought tolerance. As regard to yield susceptibility to reduced irrigation in plum (P salicina ; cv. "Black Gold"), Intrigliolo and Castel (2006) pointed out that an RDI applied from pit hardening to harvest reduces fruit weight by 10-21%, indicating that phase III of fruit growth is a phenological period highly sensitive to water deficits in this species.

As in plum, water relations and photosynthesis of sweet cherry (P. avium L.) grown in Mediterranean environments are mainly influenced by the rootstock genotype, and the regulation of fruit quality is mainly dependent on the cultivar genotype (Gongalbes et al. 2005; Godini et al. 2008). An interesting and wide comparison among cherry rootstocks subjected to non-irrigated conditions in Southern Italy highlighted the satisfactory performance of "SL 64" and the promising performance by dwarfing "Weiroot® 158" and semi-dwarfing "MaxMa 14" under water-limited growing conditions. An optimal water and nutrient management is of primary importance for cherry trees grown under semiarid conditions, and drip fertigation is a valid irrigation system for this species (Neilsen et al. 2005) ; Furthermore, water scarcity could determine an higher percentage of double fruits as well as a phytohormonal disequilibrium, with consequent losses of commercial product (Engin and Unal 2008).

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