Version classiqueVersion mobile
OpenEdition Books

The Mediterranean region under climate change

 | 
Jean-Paul Moatti
, 
Stéphane Thiébault

Part 3. Adaptation, resilience, conservation of resources and prevention of risk

Sub-chapter 3.2.6. Adaptation of Mediterranean fruit tree cultivation to climate change

Evelyne Costes, Bouchaib Khadari, Hayat Zaher, Abdelmajid Moukhli, Raphaël Morillon, Jean-Michel Legave et Jean-Luc Regnard

Texte intégral

Context

1Ongoing changes are currently affecting and will continue to affect most climate components (IPCC, 2014). Increases in temperature and soil salinity and decreases in water availability are the main changes that threaten fruit tree cultivation in the Mediterranean area, where more frequent extreme temperatures and drought episodes are expected. Considering the long periods required for fruit tree selection, orchard installation and cultivation, global change consequences do not appear to have been sufficiently taken into account in the fruit tree industry, given the high vulnerability of the sector (Dingkhun et al., 2013). Since open field cropping systems largely prevail, fruit trees cannot escape the adverse conditions and must face, most of the time, the combined effects of elevated temperature and air drought during the leafy season whereas mild winter may affect the dormancy period of temperate species.

2A range of fruit tree species are currently cultivated in the Mediterranean area with varied uses: fresh fruits (Apple, Peach, Apricot, Citrus, Fig); dry fruits or nuts (Apricot, Fig, Dates, Almond, Pistachios) and oil (Olive). Despite their different adaptation capacities, likely linked to their geographic origin, all these species will have to face the consequences of climate change: production losses, financial losses for the growers and a possible shift of growing areas. In the following, we exemplify some of the impacts due to climate changes and known physiological responses, and explore perspectives for possible adaptations and opportunities that could emerge from breeding or new species and cultivation methods.

Abiotic constraints and physiological tree responses

3Temperature is the main climatic factor affecting fruit species – impacting their physiology and yield either during the vegetation cycle or during the winter dormancy period (from autumn to spring). Higher temperature increases the leaf-to-air vapor pressure deficit and this in turn increases tree water consumption. Even in irrigated orchards, heat waves associated with dry air accelerate water loss by transpiration, which can exceed the intrinsic tree hydraulic conductivity in xylem and leaves, particularly when transpiration is poorly regulated by stomatal closure. This may lead to cavitation of xylem vessel (i.e. the rupture of the sieve stream), associated with low stem hydric potential. In these circumstances and upon the loss of turgor, some organs (leaves, twigs, fruits) or even the whole tree can die. On the contrary, transpiration can be limited by leaf stomatal closure (avoidance strategy) but this limits leaf photosynthesis and can lead to heat stress, since leaf temperature is no more regulated by the efflux of latent heat resulting from transpiration. The elevation of leaf temperature (>40 °C) can also affect the photosynthetic machinery. All these stresses limit biomass accumulation, i.e. vegetative and fruit growth. They directly hamper tree yield and may indirectly impact the development of future flower buds, through depletion of carbon reserves at autumn and during winter. A subsequent reduced tolerance to pests can also be observed.

4Since fruit trees mostly have a C3 carbon cycle, the most favorable temperature for photosynthesis ranges from 25 °C to 35 °C. In apple, an optimum carbon balance (net photosynthesis minus maintenance respiration costs) is reached at a lower temperature than the optimum for photosynthesis, peaking at ca. 25 °C. In response to extreme temperatures (>40 °C), leaves display some plasticity through medium-term adaptation, e.g. synthesis of heat shock proteins. As the orchard annual yield results from a combination of instantaneous carbon budget with the duration of carbon assimilation from bloom to fruit harvest, the use of temperature-sensitive models can be helpful to predict the effect of general temperature rise.

5Changes in temperature regime also impact the winter dormancy period. In temperate fruit trees, warmer air temperatures from autumn to winter may delay the dormancy break (chilling requirement), also delaying flowering time, whilst warmer spring temperatures may hasten bud break (heat requirement) and accelerate flowering. Subsequent impacts include floral abnormalities, flowering asynchrony of cross-pollinating cultivars and extended periods of the blooming phase, which may lead to poor fruit set (Atkinson et al., 2013). Impacts on flowering phenology and their consequences on production (spring frost, poor pollination, extended fruit maturity, etc.) are particularly expected in the Mediterranean area. They reveal mostly vulnerabilities but also opportunities, according to regional and species differences (vulnerability to spring frost in altitude vs. poor frost risk at lower elevation).

Box 1
Impact of climate change on the phenology and fruit production in olive tree
Olive flowering time is considered as a reliable bio-indicator of Mediterranean climatic variations. Several studies have revealed a flowering advance in the olive tree as a consequence of increased temperatures. The flowering duration (the delay between the beginning of flowering and fruit set) of some varieties of the Menara olive collection (Morocco) was reduced between 1975 and 2015-2016. Moreover, olive trees located in the northern part of the Mediterranean exhibit the greatest heat requirement for floral bud development while those located in the warmest winter areas (southern Mediterranean areas) have a more rapid floral development. In the latter case, lower heat requirements probably result from an adaptation to warmer regions.
Observations have been conducted on the worldwide collection (WOGB Marrakech, 663 Mediterranean olive varieties with 4 tree replicates) since 2014 to extend the comprehension of chilling requirements and climate impact on flowering time in the olive tree. Flowering intensity per tree is also subject to visual estimation and qualitative assessment. Based on the temperature data (especially the number of hours of temperature ≤ 9 °C during December-January) for consecutive years, the flowering intensity between and within varieties is currently being analyzed. Preliminary observations suggest that insufficient chilling may directly impact flowering intensity. Moreover, a survey of four olive farms with a high intensive system near Marrakech in 2016 showed some production reduction with respect to expected values, which may result from a warmer winter temperature and reduced flowering intensity compared to 2015. In the arid Mediterranean area, considering the current climate change, the selection of more early flowering olive varieties (with low chilling requirement) thus appears as a priority to ensure olive productivity and regularity under warmer and drought conditions.

Effects of high temperature on fruit yield and quality

6As many processes occur at cell and tissue levels, most reproductive development events are impacted by high temperature. Protective adaptations are triggered against thermic stress but with detrimental effects on the primary metabolism, limiting the fruit yield. Beyond the increase in respiration costs, these effects notably concern the duration of the bloom-to-fruit maturity period, with hastened fruit maturity. In stone fruits, a shorter bloom-to-harvest period negatively impacts the potential fruit size, since carbon acquisition is shortened and not compensated by a better carbon influx. Fruit texture and shape can also be affected: after a shortened cell multiplication phase and increased individual cell growth, apple fruits, for example, are less dense and crisp. At harvest maturity, the malic respiration rate is stimulated by warm temperatures, with detrimental effects on the fruit acidity to sugar ratio. Moreover, the scarcity of cold night temperatures delays the onset of red-skin apple pigmentation by anthocyanins. Finally, diverse effects on fruit post-harvest have been noted, for instance in apples, where sunburn is enhanced by warm temperatures and high irradiance in the field. Skin blemish is generally observed, lowering the commercial value at harvest.

Possible climate change adaptations for Mediterranean fruit trees

7Natural adaptation of species to afford and/or withstand non-optimal climate conditions relies on a series of morphological and/or physiological processes caused by avoidance and tolerance strategies. Heat avoidance can result to a certain extent from leaf pubescence, which increases leaf reflectance, and/or from leaf lamina inclination, but the plasticity of these traits in response to heat is poorly documented in fruit crops. Adaptation may also involve the co-evolution of each species with its biocoenosis and cohort of predators, pollinators, etc. (see the Fig tree case below).

8Regarding phenology, the negative consequences for fruit tree production could be reduced or even nullified by adapting the heat requirements of varieties to temperature changes at regional scale. However, in temperate species characterized by winter dormancy, the varietal adaptation of chill requirement and its interaction with heat requirement is complex and will require deeper knowledge of the signals and mechanisms linked to bud dormancy and growth (Cooke et al., 2012). Bud rehydration which plays a key role in the transition from endodormancy to ecodormancy, may also trigger winter dormancy break. The modeling of fruit tree flowering time in diverse climatic regions has shown that warming impacts and likely adaptations greatly differ between species and cultivars. According to the RCP4.5 et RCP8.5 scenarios of the Intergovernmental Panel on Climate Change (IPCC), some species show limited risk at all sites across South Australia up to the year 2090 whilst others show greater risk both historically and in the future. As a complement to model-based predictions, breeding strategy, based on genetic determinism of bud phenology, is likely to create new opportunities for fruit trees facing temperature increase.

Box 2
Pollination in the Fig trees and possible changes in varieties
The Fig tree is characterized by substantial diversity of local varieties, mainly present in limited areas such as north Morocco. Fig varieties differ by their fruits reaching maturity without pollination (“Common type”) mainly present in north Mediterranean areas or requiring pollination for fruiting, more suitable for drying (“Smyrna type”) and present in south Mediterranean areas. This is probably linked to the associated pollinator, Blastophaga psenes L. which, despite its presence, does not have a sufficient population dynamic to ensure regular pollination in the north, whereas the inverse is observed in the southern Mediterranean areas. Under warming conditions, the population dynamics of the pollinator may be disrupted, leading to a dramatic loss of fig production of “Smyrna” varieties. It is expected that plant material will evolve in favor of “common type” varieties and hence in product uses from dried to fresh fruits.

9In terms of the trade-off between carbon gain and transpired water, considering various climatic scenarios and the cost and/or availability of water resources for irrigation, increasing attention needs to be paid to the water use efficiency (WUE) of fruit trees, i.e., the ratio of biomass produced by water transpired over a crop cycle. Although increased WUE is generally desirable, the benefits of high WUE appear in very dry environments only. Moreover, thermoregulation resulting from transpired water plays a part in alleviating heat stress. Until recently, fruit breeding programs did not consider tree water economy as an important trait. Providing fruit growers with reliable information on variety behavior, in response to heat or drought stress, will be of increasing value (e.g. Virlet et al., 2014). Moreover, the choice of drought-resistant rootstocks is justified in semi-arid environments, but the selection of scion cultivars able to withstand sub-optimal conditions (heat waves, air drought, lower irrigation, soil salinity, etc.) without prejudice to yield will become more relevant.

Box 3
Screening for new rootstock selection: the example of Citrus
Citrus are grown on the various coasts of the Mediterranean basin where they represent almost 17% of world production. Spain, Morocco and Egypt are the main producers. Oranges, small citrus such as clementines, limes, lemons and grapefruits are the main varieties. Citrus production requires irrigation during summer: nearly 80 liters of water are consumed by a tree to produce an orange. In the southern Mediterranean area, deep boreholes for agriculture led to lowering of groundwater levels and increase of salt contents in irrigation water. Citrus are extremely sensitive to salt stress, especially grapefruit and orange.
Citrus are grown grafted on rootstocks that allow the tree to withstand many diseases (e.g. Tristeza or Phytophthora), but also confer tolerance to drought, salt stress or calcareous soils. The adaptation of citrus to salinity requires rigorous management of cultural practices: tree age, rootstock, grafted variety, irrigation systems, soil type and climate being the main factors that may modulate the stress impact. In addition to improved irrigation practices to limit the extraction of salts from the soil, rootstocks are required to limit the absorption of Na+ and Cl- in roots and limit their impact on the canopy, since these ions may be transferred passively into the transpiration stream through the xylem.
As with grapes, Cl- is responsible for chloroses leading to growth stop and reduction in fruit production. Leaf Cl- content is thus a good criterion to evaluate the tolerance/sensitivity of seedlings to salt stress. Poncirus (trifoliate orange) cultivars are of potential interest since they are tolerant to Tristeza, but they do not limit Cl-absorption in roots and transfer these ions to the aerial part. Rootstock cultivars such as Cleopatra or sour orange that maintain toxic ions in roots are the most suitable for adaption to salt stress.
However, such cultivars are susceptible to Tristeza. The current strategies are therefore (i) to select hybrids combining tolerance to Tristeza and salt stress; (ii) to create tetraploid genotypes that may confer tolerance to salinity. Some spontaneous tetraploids may be present among seedlings, and studies at the research institutes CIRAD in France and IVIA in Spain have shown that such rootstocks are more tolerant to salt stress and water deficit than the respective diploids. Other studies suggest that tetraploids have better capabilities of toxic ion compartmentalization and detoxification systems compared to diploids. Even though the impact of salt stress in orchards where tetraploid rootstocks are used remains to be fully explored, selecting varieties with appropriate physiological and molecular mechanisms is likely to limit the impact of toxic ions in the leaves. The use of tetraploids hybrids of Poncirus and Cleopatra mandarin as rootstocks in combination with varieties selected for fruit quality and less salt sensitivity thus seems promising.

Innovative cropping systems for warm and dry conditions

10Presently, the agronomical performance of intensive orchard depends on the satisfaction of water requirements, particularly during periods of active fruit growth. In this context, the water economy mainly relies on irrigation scheduling, and possibly regulated deficit irrigation (Steduto et al., 2012). In pip fruits, netting over orchards has been developed, either for protection against hail or pests (e.g. codling moth in apple) or to influence the microclimate: trees receive 10 to 15% less solar radiation and have a lower evaporative demand, since extreme temperatures and high vapor pressure deficit are avoided. This is globally beneficial if the fruit quality is maintained. Cultural practices based on the use of reflective sprays have also been developed for the protection of fruits, limbs and trunks, and evaporative cooling through overhead sprinkling to mitigate heat periods. In addition, experimentations on the potential benefit of associated crops or on traditional agro-systems experienced between the tropics and in southern Mediterranean areas, where species are often mixed or in multi-layers, could represent new solutions. In extensive cropping conditions, such as traditional rainfed fig or olive groves, soil water deficit cannot be avoided during the dry season, but it can be overcome by the capacity of root systems to explore deep soil layers and/or low planting density which increases access to water for each tree. The relative benefit of intensive / traditional / associated crops / multi-layer cropping systems will certainly have to be further evaluated in the light of climate change and regional priorities.

Conclusion

11The risks and adaptation strategies of Mediterranean fruit trees to climate change depend on the species, its intrinsic ability and plasticity linked to its original geographic area, and its ecotype, but also on fruit use (e.g. fresh/dry/oil) and storage possibility. Many adaptation options can help address climate change impact, but no single option is sufficient by itself. We therefore recommend combining several approaches, notably breeding for new material and innovative cropping systems. While the ongoing global warming threatens fruit tree industry all around the Mediterranean Sea, it also opens new challenges and opportunities for renewing plant material (scions and rootstocks) and cropping systems. Some fruit species from tropical dry areas could perhaps be considered for the diversification of cultivated fruit species around the Mediterranean Sea, thus creating new opportunities. Biologic and agronomic options will also have to be complemented by measures for accompanying populations, especially the poorest and most fragile, in order to balance inequalities and contribute to social stability, taking into account the resilience of farming structures and the proximity between growers and consumers.

Bibliographie

References

Atkinson, C. J., Brennan, R. M., Jones, H. G. (2013)
Declining chilling and its impact on temperate perennial crops. Environmental and Experimental Botany, 91, 48-62.

Cooke, J. E., Eriksson, M. E., Junttila, O. (2012)
The dynamic nature of bud dormancy in trees: environmental control and molecular mechanisms. Plant, Cell & Environment, 35(10), 1707-1728.

Dingkuhn, M., Gerardeaux, E., Gate, P., Legave, J.M. (2013)
Les productions végétales. In S’adapter au changement climatique: Agriculture, écosystèmes et territoires. Editions Quae, JF Soussana coord., 6: 91-106, 282p.

IPCC (2014)
Fifth Assessment Report. Climate Change 2014: Synthesis Report. Cambridge & New York: Cambridge University Press, 31p.

Virlet, N., Lebourgeois, V., Martinez, S., Costes, E., Labbé, S., Regnard, J. L. (2014)
Stress indicators based on airborne thermal imagery for field phenotyping a heterogeneous tree population for response to water constraints. Journal of experimental botany, eru309.

Steduto, P., Hsiao, T.C., Fereres, E., Raes, D. (2012)
Crop yield response to water. In FAO Irrigation and drainage paper 66, Roma, 501p.

Auteurs

INRA, France
Biologist-Modeler, INRA “Genetic Improvement and Adaptation of Mediterranean and Tropical Crop” Mixed Research Unit, CIRAD-INRA-Montpellier SupAgro “Fruit Tree Architecture and Functions (AFEF)” Team, Montpellier, France
costes@supagro.inra.fr

CBNMed, France
Geneticist, Conservatoire Botanique National Méditerranéen de Porquerolles (CBNMed) “Genetic Improvement and Adaptation of Mediterranean and Tropical Crop” Joint Research Unit, CIRAD-INRA-Montpellier SupAgro “Fruit Tree Architecture and Functions (AFEF)” Team Montpellier, France
khadari@supagro.inra.fr

INRA, Morocco
Breeder, INRA Morocco, Centre Régional de la Recherche Agronomique de Marrakech (CRRA de Marrakech), Morocco
hayatzaher@yahoo.fr

INRA, Morocco
Breeder, INRA Morocco, Centre Régional de la Recherche Agronomique de Marrakech (CRRA de Marrakech), Morocco
moukhli2000@yahoo.fr

CIRAD, France
Physiologist, CIRAD “Genetic Improvement and Adaptation of Mediterranean and Tropical Crop” Mixed Research Unit, CIRAD-INRA-Montpellier SupAgro «Breeding Plants with Vegetative Propagation (APMV)» Team Station de Roujol, Petit Bourg, Guadeloupe, France
raphael.morillon@cirad.fr

INRA, France
Agronomist – Modeler, “Genetic Improvement and Adaptation of Mediterranean and Tropical Crop” Mixed Research Unit, CIRAD-INRA-Montpellier SupAgro “Fruit Tree Architecture and Functions (AFEF)” Team, Montpellier, France
legave@supagro.inra.fr

Montpellier SupAgro, France
Eco-physiologist, Montpellier SupAgro, “Genetic Improvement and Adaptation of Mediterranean and Tropical Crop” Mixed Research Unit, CIRAD-INRA-Montpellier SupAgro “Fruit Tree Architecture and Functions (AFEF)” Team, Montpellier, France
regnard@supagro.inra.fr

© IRD Éditions, 2016

Conditions d’utilisation : http://www.openedition.org/6540