Response of Clementina de Nules citrus trees to summer deficit irrigation. Yield components and fruit composition

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Abstract

The effects of mid-summer regulated deficit irrigation (RDI) treatments were investigated on Clementina de Nules citrus trees over three seasons. Water restrictions applied from July, once the June physiological fruit drop had finished, until mid September were compared with a Control treatment irrigated during all the season to match full crop evapotranspiration (ETc). Two degrees of water restrictions were imposed based on previous results also obtained in Clementina de Nules trees (Ginestar and Castel, 1996, González-Altozano and Castel, 1999). During the RDI period, deficit irrigation was applied based on given reductions over the ETc, but also taking into account threshold values of midday stem water potential (Ψs) of −1.3 to −1.5 MPa for RDI-1 and of −1.5 to −1.7 MPa for RDI-2. Results showed that water savings achieved in the RDI-2 treatment impaired yield by reducing fruit size. On the contrary, the RDI-1 strategy allowed for 20% water savings, with a reduction in tree growth but without any significant reduction in yield, fruit size nor in the economic return when irrigation was resumed to normal dose about three months before harvest. Water use efficiency (WUE) in the RDI trees was similar or even higher than in Control trees. RDI improved fruit quality increasing total soluble solids (TSS) and titratable acidity (TA). In conclusion, we suggest that the RDI-1 strategy here evaluated can be applied in commercial orchards not only in case of water scarcity, but also as a tool to control vegetative growth improving fruit composition and reducing costs associated with the crop management.

Research highlights

► We studied the application of two RDI levels during summer in commercial Clementina de Nules citrus orchards. ► We determined the effects on yield and fruit quality. ► Summer stem water potentials of −1.3 to −1.5 MPa did not decrease yield. ► Water restriction increased fruit total soluble solids and titratable acidity.

Introduction

Citrus is the most important irrigated crop in Eastern Spain, with more than 304,000 ha and an average annual production of about five million tons of which between 50% and 60% is exported as fresh fruit. More than a third of this comes from the mandarin group where ‘Clementina de Nules’ is one of the most common cultivars.

Since water is a scarce resource in Spain, with irrigated agriculture accounting for 80% of global water consumption (MMA, 2008), it is thus important to increase crop water use efficiency (crop marketable yield divided by crop water use). Regulated deficit irrigation (RDI) was developed in the 1980s as a strategy to reduce growth of vigorous fruit trees and to save water (Chalmers et al., 1981, Behboudian and Mills, 1997). Water restrictions are applied in phenological periods, when fruit growth is less sensitive to soil water deficit, while during the rest of the season full tree water requirements are applied.

Previous research investigated the effects of reducing water application in citrus trees during certain phenological periods (see revision by Ruiz-Sánchez et al., 2010). It has been demonstrated that even moderate water stress applied during phase I (i.e. flowering and fruit set) normally compromises yield by increasing June fruit drop (Doorenbos and Kassam, 1979, Ginestar and Castel, 1996, Romero et al., 2006, García-Tejero et al., 2010). On the other hand, water restrictions applied during the last phase of fruit growth and ripening might decrease yield by reducing the final fruit weight (González-Altozano and Castel, 1999, Pérez-Pérez et al., 2009, García-Tejero et al., 2010). Thus, in general, the period after June fruit drop is the less sensitive to water restrictions, providing that water applications return at full dosage sufficiently before harvest in order to allow for a compensation in fruit growth (Cohen and Goell, 1988).

González-Altozano and Castel (1999) in experimental orchards determined that moderate water restrictions (i.e. replacing only 50% of full crop evapotranspiration) during July and August successfully allowed reducing water application without any yield or fruit size reductions. They identified a threshold value of plant water stress determined by measurement of midday stem water potential (Ψs) of around −1.3 to −1.5 MPa.

Despite the promising results obtained by González-Altozano and Castel (1999) RDI in ‘Clementina de Nules’ trees is not a practice commonly applied in the area, because growers are often reluctant to deliberately apply RDI based on the lack of more local research under commercial situation.

The aim of the present work was to assess the extrapolation of the previous research by González-Altozano and Castel (1999) under commercial situations. In addition, the plant water stress threshold values previously identified were used to avoid the appearance of severe plant water stress. The overall goal is to provide growers with more reliable information on the use of RDI for optimizing water management. It is believed that the release of the present results will increase the possibility of transferring this tool under commercial situations.

Section snippets

Experimental plot

The experiment was performed during 2007, 2008 and 2009 in a commercial ‘Clementina de Nules’ orchard (Citrus clementina, Hort ex Tan) grafted on Carrizo citrange (Citrus sinensis, Osb. × Poncirus Trifoliata, Raf.). The orchard belongs to a farmers Cooperative located in Liria, Valencia, Spain (40 °N, 0 °W elevation 300 m), and was planted in 1999 at a spacing of 6 m × 4 m.

The soil was clay to clay loam texture, rich in calcium carbonate and with 21% by weight stones. Trees were drip irrigated with 8

Meteorological conditions and irrigation volume applied

Reference evapotranspiration (ETo) registered during the three growing seasons, ranged between 1090 and 1046 mm (Table 1). Year 2008 was the rainiest, with 553 mm of seasonal precipitation, while 2009 was the driest, with 371 mm. Thus, 2009 was the year with the highest irrigation volumes applied and, therefore, with the less water savings in the RDI treatments. These were, for the three years of study, 20%, 22% and 8% in the RDI-1 and 31%, 24% and 8% in the RDI-2, respectively.

Plant water status

The Ψs registered

Discussion

Overall, the present results confirm previous research by González-Altozano and Castel (1999) that showed that the summer is a suitable period for applying water restrictions in Clementina de Nules citrus trees. However, the final effect of RDI on yield and fruit size distribution depends on both the degree and the duration of plant water stress. For successful RDI application it is therefore important to frequently check plant water status. In fact, a moderate plant water stress applied

Acknowledgements

The authors thank D. Pérez, L Pérez, F. Sanz and A. Yeves for their help in both field and laboratory work. We also thank B. Tamargo and the rest of personnel from “Cooperativa de Lliria” for their help with the orchard management. This work was supported by the CSD2006-00067 grant from CONSOLIDER-INGENIO 2010.

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