Date Published: June 17, 2016
Publisher: Public Library of Science
Author(s): Soumaya Dbara, Matthew Haworth, Giovani Emiliani, Mehdi Ben Mimoun, Aurelio Gómez-Cadenas, Mauro Centritto, Panagiotis Kalaitzis.
The productivity of olive trees in arid and semi-arid environments is closely linked to irrigation. It is necessary to improve the efficiency of irrigation techniques to optimise the amount of olive fruit produced in relation to the volume of water used. Partial root-zone drying (PRD) is a water saving irrigation technique that theoretically allows the production of a root-to-shoot signal that modifies the physiology of the above-ground parts of the plant; specifically reducing stomatal conductance (gs) and improving water use efficiency (WUE). Partial root-zone drying has been successfully applied under field conditions to woody and non-woody crops; yet the few previous trials with olive trees have produced contrasting results. Thirty year-old olive trees (Olea europaea ‘var. Chetoui’) in a Tunisian grove were exposed to four treatments from May to October for three-years: ‘control’ plants received 100% of the potential evapotranspirative demand (ETc) applied to the whole root-zone; ‘PRD100’ were supplied with an identical volume of water to the control plants alternated between halves of the root-zone every ten-days; ‘PRD50’ were given 50% of ETc to half of the root-system, and; ‘rain-fed’ plants received no supplementary irrigation. Allowing part of the root-zone to dry resulted in reduced vegetative growth and lower yield: PRD100 decreased yield by ~47% during productive years. During the less productive years of the alternate bearing cycle, irrigation had no effect on yield; this suggests that withholding of water during ‘off-years’ may enhance the effectiveness of irrigation over a two-year cycle. The amount and quality of oil within the olive fruit was unaffected by the irrigation treatment. Photosynthesis declined in the PRD50 and rain-fed trees due to greater diffusive limitations and reduced biochemical uptake of CO2. Stomatal conductance and the foliar concentration of abscisic acid (ABA) were not altered by PRD100 irrigation, which may indicate the absence of a hormonal root-to-shoot signal. Rain-fed and PRD50 treatments induced increased stem water potential and increased foliar concentrations of ABA, proline and soluble sugars. The stomata of the olive trees were relatively insensitive to super-ambient increases in [CO2] and higher [ABA]. These characteristics of ‘hydro-passive’ stomatal behaviour indicate that the ‘Chetoui’ variety of olive tree used in this study lacks the physiological responses required for the successful exploitation of PRD techniques to increase yield and water productivity. Alternative irrigation techniques such as partial deficit irrigation may be more suitable for ‘Chetoui’ olive production.
The production of olives, and products derived from olives, is a major agro-industry in Mediterranean areas with the global market worth over €11 billion per annum . The sustainability of this industry faces a number of converging pressures associated with climate change, population growth and unsuitable agricultural practices [2, 3]. The productivity of olive trees (Olea europaea L.) is largely constrained by the availability of water during the summer months when the fruit develops . The majority of European olive groves are currently rain-fed without supplementary irrigation . Global climate models predict that Mediterranean summers will likely become hotter, with an increased frequency and duration of drought events that will coincide with episodes of raised temperatures relative to the norm . Olive trees possess a number of physiological adaptations to cope with drought [7–9]. Nevertheless, longer and more severe droughts may have significant implications for the production of olives [10, 11]. Supplementary irrigation increasing soil water content to field capacity dramatically increases the yield of olives per tree, but also promotes vegetative growth reducing the efficiency of irrigation when measured relative to crop production . The effectiveness of irrigation is gauged by ‘water productivity’: the amount of yield produced per unit of water applied in irrigation . Furthermore, in the future the availability of irrigation water will likely be constrained by increased population levels, industrialisation and urbanisation, combined with the possible effects of climate change on the temporal and spatial distribution of water . It is therefore necessary to optimise the impact of irrigation on yield through development of irrigation technologies based on physiological studies of plant responses to water deficit [15, 16].
The majority of olive groves are rain-fed, particularly those in hilly areas where water for irrigation is either expensive or impractical. Irrigation with relatively low volumes of water (70–200 mm3 ha-1 per week) can increase yields to 80% of those of plants supplied with sufficient water to replace ETc [4, 72]. In 2010, within the EU ~40% of Spanish, 26% of Italian and ~36% of Greek olive groves were irrigated; with irrigated trees responsible for 52% of olive fruit production . However, the availability of fresh-water for irrigation will likely be constrained by population growth, urbanisation and industrialisation, combined with the potential effects of climate change on precipitation patterns . This necessitates the optimisation of water-use in irrigation techniques, often termed ‘more crop per drop’ . Partial root-zone drying has been successfully applied to numerous crops (see summary in introduction) and to olives grown in split-pot experiments (eg. ). However, the results of this and previous studies (eg. [44, 48]) suggest that PRD may not be as effective in certain varieties of olive trees under field conditions, or it may not be possible to achieve sufficiently rigorous control of the distribution of water under field conditions.
Partial root-zone drying has been utilised to improve the water productivity of numerous crops. The successful application of PRD to olives would permit the optimisation of yield relative to water-use in a crop grown in drought prone areas. However, while the results of laboratory based split-root studies of olive trees have been promising; the efficacy of PRD irrigation in the field has been equivocal. In this study, during productive ‘on-years’, yield was significantly reduced by 47% in the PRD100 treatment relative to the control, despite receiving the same volume of water. Yield was 68 and 95% lower in the PRD50 and rain-fed treatments. The yield of fruit relative to the amount of water used was significantly lower under PRD in comparison to application of water to the whole root-zone. Supplementary irrigation did not enhance olive fruit yield during the less productive ‘off-year’, suggesting that co-ordination of the supply of water with the alternate bearing cycle may enhance water-productivity on a two-year basis. The quality and quantity of oil produced by equal amounts of olive fruit from each irrigation treatment was identical. Lower A was observed in the PRD50 and rain-fed treatments due to higher diffusive (Table 2) and biochemical (Fig 3) constraints to CO2-uptake. A similar pattern was not observed in the PRD100 treatment, possibly indicating that a root-to-shoot signal inducing stomatal closure was not present. Stomatal conductance was identical in the control and PRD100 treatments, as were Ψs and foliar [ABA]. Stomatal closure occurred in the PRD50 and rain-fed olive trees, with a relatively small reduction in gs of 19–29%, which corresponded to lower Ψs and higher concentrations of the osmotic regulators ABA, proline and soluble sugars (Fig 4). The lack of clear active physiological stomatal behaviour to [CO2] (Fig 2c) and [ABA] (Fig 4c) may indicate that the dominant component of stomatal control in the Chetoui variety of olive trees is hydro-passive. The physiological mechanisms required to produce a root-to-shoot signal of soil drying and then induce stomatal closure to enhance the WUE of photosynthesis, may be absent in the Chetoui variety of olive tree; thus constraining the effectiveness of PRD in optimising the water productivity of irrigation. Nonetheless, the required physiological mechanisms for successful application of the PRD technique may be present in other olive varieties. The apparent absence of physiological mechanisms required for PRD in Chetoui olive may negate the effectiveness of PRD in Chetoui olive groves. Periodic deficit irrigation of the entire root-zone may be a more successful approach in optimising crop yield and water productivity in olive trees than applying water to part of the root-system.