Water stress at the end of the pomegranate fruit ripening stage produces earlier harvest and improves fruit quality
Introduction
Pomegranate (Punica granatum L.) plants are equipped with xeromorphic characteristics such as a high leaf relative apoplastic water content and the ability to develop complementary stress avoidance and stress tolerance mechanisms to confront drought (Rodríguez et al., 2012). That means that it is able to thrive in arid and semi-arid areas, even under desert conditions (Aseri et al., 2008). Nevertheless, to reach optimal growth, yield and fruit quality for commercial production, the crop requires regular irrigation throughout the dry season Prasad et al., 2003, Shaliendra and Narendra, 2005, Sulochanamma et al., 2005, Holland et al., 2009).
The commercial production of pomegranate in the Mediterranean Basin is characterized by high quality fruits (Stover and Mercure, 2007, Holland et al., 2009) with high bioactive compounds content (Gil et al., 2000, Poyrazoğlu et al., 2002, Mena et al., 2011) and a correspondingly high antioxidant capacity and beneficial health effects (Lansky and Newman, 2007).
All Mediterranean agrosystems must cope with water scarcity, and any policy involving greater use of the water (Pereira et al., 2002). In this sense, pomegranate farming must be directed towards the use of deficit irrigation strategies, allowing significant water savings, and the profitable production of high quality fruits. Sustained deficit irrigation (SDI) is an irrigation strategy in which the amount of water applied at any moment of the season is lower than that needed to satisfy the full crop water requirements (English and Raja, 1996). Regulated deficit irrigation (RDI) is another irrigation strategy designed to save water while having a minimum impact on yield and fruit quality (Goldhamer, 1989, Naor, 2006). This requires precise knowledge of the crop response to drought stress during different phenological periods when adverse effects on productivity are minimal (non-critical periods) or maximal (critical periods).
Reports on the effect of irrigation management on pomegranate fruit yield and quality are very scarce. The first results indicated that it is possible to control the desired ripening time in pomegranates by applying different irrigation regimes (Sonawane and Desai, 1989). Also, Prasad et al. (2003), Shaliendra and Narendra (2005), and Sulochanamma et al. (2005) showed that irrigation has a positive effect on pomegranate vegetative growth, yield, and fruit weight. Recently, Galindo et al. (2014a) indicated that SDI applied throughout the pomegranate season reduces total yield per tree, the number of fruits per tree and the size of the fruits; however, such a strategy can advance the availability of fruits from late flowerings, which despite their smaller size are of high interest for the pomegranate industry due to their very high content of bioactive compounds. In contrast, Mellisho et al. (2012) concluded that SDI, under moderate water stress, showed some changes in colour and chemical characteristics, which reflected earlier ripening. However, Mena et al. (2013) indicated that pomegranate juice from trees submitted to SDI that produces severe water stress levels was of lower quality and less healthy than that from fully irrigated trees. On the other hand, Peña-Estévez et al. (2015) concluded that pomegranates from SDI trees had good sensory qualities, a higher content of most bioactive compounds, and suffered less chilling injury during cold storage and shelf-life than fully irrigated fruits. Recently, Laribi et al. (2013) showed that pomegranates from SDI trees, submitted to mild water stress during flowering and fruit set and more severe water stress during the linear phase of fruit growth and ripening, had a redder peel and higher level of total soluble solids in the juice.
To the best of our knowledge, there has been no scientific study evaluating the response of pomegranate to RDI, applying full irrigation in all the critical periods and deficit irrigation during the non-critical periods. However, Intrigliolo et al. (2013) and Laribi et al. (2013) studied pomegranate response to severe irrigation water restrictions applied during the phenological periods of (i) flowering, fruit set and early fruit growth, (ii) linear fruit growth, and (iii) the last part of fruit growth and ripening. These authors concluded that the phenological period comprising flowering and fruit set could be regarded as non-critical from the yield point of view. Moreover, Laribi et al. (2013) concluded that irrigation water restriction during pomegranate fruit growth and ripening enhances peel red colour intensity and total soluble solids in the juice, while irrigation water restriction during linear fruit growth period increased the concentration of many bioactive compounds in the juice, such as anthocyanins, that could be related to health and taste.
For this reason, the aim of this research was to (i) clarify whether the pomegranate fruit ripening phenological stage is a critical or non-critical period from the yield point of view, (ii) whether pomegranate yield response to water restriction during ripening depends on the exact point at which water stress takes place, and (iii) evaluate whether water restrictions during the ripening stage have secondary effects on fruit characteristics and composition.
Section snippets
Plant material, experimental conditions and treatments
The experiment was carried out in 2013 in a pomegranate (Punica granatum L.) orchard located near the city of Alhama de Murcia (Spain) (37°47′N, 1° 25′W). The trees were own-rooted 15 years old Mollar de Elche cultivar and the tree spacing was 3 m × 5 m. The soil of the orchard is a moderately saline silt loam (Hyposalic Calciorthid), with moderate lime content, very low organic matter content, low cationic exchange capacity, high available phosphorus levels and low available potassium. The
Climate and plant water status
The experimental conditions were semi-arid, characterized by a VPDm ranging from 0.33 to 1.87 kPa, average daily maximum and minimum air temperatures of 28.0 and 14.8 °C, respectively, and accumulated ETo of 162 mm. Total rainfall was 88.4 mm: DOY 270 (3.5 mm), DOY 271 (84 mm) and DOY 272 (0.9 mm)
Table 1 describes the cumulative effect of the irrigation treatments on the pomegranate trees using SIgleaf, SIΨstem, SIΨleaf and SIΨfruit values, whose values in the different irrigation treatments tended to
Discussion
A detailed analysis of the effects of the irrigation water withholding treatments on plant and fruit water status were described in a previous manuscript from our research network (Galindo et al., 2014b). Bearing in mind the results from that article and those shown in Table 1, it is clear that, in spite of the rainfall events (occurring on DOY 271), the cumulative water stress tended to increase with the number of days irrigation was withheld, the treatments in which irrigation was withheld
Conclusion
The present results indicated that the SI calculated from gleaf, Ψleaf, Ψstem and Ψfruit data vary as regards their ability to describe the cumulative water deficit reached by plants. SIΨfruit was the most feasible indicator for detecting differences between the treatments at moderate water stress levels while SIgleaf was the only indicator able to detect differences between the treatments at higher water stress levels. Moreover, pomegranate fruit ripening is a critical period from the yield
Acknowledgments
This research was supported by Ministerio de Economía y Competitividad (MINECO) (CICYT/FEDERAGL2013-45922-C2-1-R and AGL2013-45922-C2-2-R) grants to the authors. Also, this work is a result of the PR internship (19925/IV/15) funded by the Fundación Séneca − Agencia de Ciencia y Tecnología de la Región de Murcia (Seneca Foundation − Agency for Science and Technology in the Region of Murcia) under the Jiménez de la Espada Program for Mobility, Cooperation and Internationalization. AG and ZNC were
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These authors contributed equally to this work.