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Universidade de Trás-os-Montes e Alto Douro (2006)

Ecophysiological responses of olive (Olea europaea L.) to restricted water availability : limitations, damages and drought resistance mechanisms

Bacelar, Eunice Luís Vieira Areal

Titre : Ecophysiological responses of olive (Olea europaea L.) to restricted water availability : limitations, damages and drought resistance mechanisms.

Auteur : Bacelar, Eunice Luís Vieira Areal

Université de soutenance : Universidade de Trás-os-Montes e Alto Douro

Grade : Tese de Doutoramento em Engenharia Biológica 2006

Résumé
Olive (Olea europaea L.) is an evergreen tree traditionally cultivated in the Mediterranean basin for oil and table fruit consumption. During the summer months, olive, like other Mediterranean xerophytes, is usually subjected to high solar irradiances, high air temperatures, high vapour pressure deficits and limited water availability in the soil. The reduction of moisture availability anticipated in the climate change scenarios would inevitably add to the problem of water scarcity throughout the Mediterranean region. So, how olive trees are affected by restricted water availability, is of great concern. A survey of the scientific literature revealed that olive has important attributes that enable survival and production in drought-prone environments (chapter 1), but important areas are still untouched and few differences have been established between O. europaea cultivars. Therefore, the aim of this thesis was to identify the drought resistance mechanisms of olive tree cultivars and the limitations and damages imposed by water shortage. It was also our goal to investigate how irrigation assists the olive tree to withstand Mediterranean field conditions. The experimental work was divided in five chapters, which include the information published or submitted to journals belonging to the Journal Citation Reports. Three experiments have been conducted. In the first experiment, we investigated the morpho-anatomical, physiological and biochemical adaptations of five field-grown olive cultivars with different geographical origins (Arbequina, Blanqueta, Cobrançosa, Manzanilla and Negrinha) to drought conditions (chapters 2 and 3). Chapter 2 presents data on the leaf-level morphological and structural adaptations to reduce water loss. Leaf measurements included leaf tissue thickness, stomatal density, leaf area, leaf mass per unit area, density of leaf tissue, relative water content, succulence, water saturation deficit, water content at saturation and cuticular transpiration rate. The results revealed that olive cultivars native to dry regions, such as Cobrançosa, Negrinha, and Manzanilla, have more capability to acclimate to drought conditions than cultivars originated in regions with a more temperate climate, like Arbequina and Blanqueta. Adaptation was effected by an increase in sclerophylly. Cobrançosa avoids water loss with high density of foliar tissue and the presence of the thick cuticle and trichome layers. Manzanilla and Negrinha enhanced their sclerophylly by building parenchymatous tissues and increasing protective structures like the upper cuticle (Negrinha) and both the upper and lower epidermis (Manzanilla). In chapter 3, gas exchange rates, chlorophyll a fluorescence, photosynthetic pigments, plant water relations, total soluble sugars, starch, soluble proteins and proline concentrations were investigated in the five olive cultivars. Stomatal control of water loss was identified as an early response of all olive cultivars to water deficit, leading to limitation of carbon uptake by the leaves. However, the degree of midday depression of photosynthesis was genotype dependent, with a maximum in Arbequina and a minimum in Negrinha. Non-stomatal factors also play an important role in limiting photosynthesis when olive cultivars (mainly Blanqueta) are submitted to prolonged drought under field conditions. In all cultivars, but mostly in Manzanilla, free proline was accumulated in the foliage. However, olive trees have other mechanisms of drought resistance in addition to the solely proline accumulation, such as stomatal closure and soluble protein accumulation. In a subsequent study, a glasshouse experiment was conducted. One-year-old plants of three Portuguese O. europaea cultivars (Cobrançosa, Madural and Verdeal Transmontana, the more representative cultivars in Trás-os-Montes) were submitted to contrasting water availability regimes during the dry season (chapters 4 and 5). Chapter 4 presents the effect of cultivar and watering regime in the vegetative growth, gas exchange, xylem hydraulic properties and water use efficiency of biomass production. Low water availability (LW) affected growth and biomass accumulation of the three cultivars. However, Cobrançosa plants were the less affected. Under LW conditions, total leaf area was sharply reduced due to a combination of leaf growth reduction and shedding of older leaves, minimising water losses by transpiration. Water stress also caused a marked decline on photosynthetic capacity and stomatal control was the major factor affecting photosynthesis. Under LW, water use efficiency of biomass production was improved in Cobrançosa, whereas it decreased in Madural and Verdeal Transmontana. In all cultivars, water stress induced an increase in xylem vessel frequency, providing a greater vascular potential and a greater security of xylem sap conduction under drought conditions. From the behaviour of Cobrançosa plants, with a lower allocation of assimilates to leaves, smaller leaves, a smaller reduction in net assimilation rate and photosynthetic rate, a more efficient water transport through the xylem, and an enhanced water use efficiency of biomass production under LW, we consider this cultivar the more appropriate for cultivation under restricted water availability. Chapter 5 presents the effect of cultivar and watering regime in leaf anatomy, sclerophylly, pressure–volume relationships, chemical composition and oxidative stress symptoms. Anatomically, Cobrançosa and Madural were more capable than Verdeal Transmontana to cope with LW availability, with a thicker upper epidermis, a thicker palisade parenchyma and higher stomatal density. Cobrançosa leaves also revealed the lowest specific leaf area and the highest density of the foliar tissue. Under LW conditions, Cobrançosa and Madural showed greater capability for osmotic adjustment and increased tissue rigidity. By contrast, Verdeal Transmontana did not exhibit osmotic adjustment, but was able to increase tissue elasticity and total soluble protein concentration. Leaves grown under LW conditions revealed signs of oxidative stress, with decreases in chlorophyll, carotenoid and total thiol concentrations and increased levels of lipid peroxidation. Nevertheless, LW plants developed some defence mechanisms against oxidative stress, like the increase in total phenol and total soluble protein concentrations. Comparatively, Cobrançosa revealed more protection against oxidative stress. In opposition, the increased levels of lipid peroxidation and the decreased total thiol concentration under LW conditions suggest that the mechanisms against oxidative stress were less effective in Madural. In chapter 6, the influence of different irrigation regimes were investigated on an 8- year-old olive (Olea europaea L., cv. Cobrançosa) commercial orchard located in northeast Portugal. Trees were subjected to a rainfed control (T0) and 3 treatments (T1, T2, T3) that received a seasonal water amount equivalent to 30, 60 and 100% of the estimated local evaporative demand by a drip irrigation system. All irrigation levels improved carbon assimilation of olive trees and reduced the midday and afternoon depression of photosynthesis and stomatal conductance. The experiment also revealed the occurrence of a dynamic photoinhibition in irrigated trees, mainly in trees irrigated with 60 and 100% of the estimated local evaporative demand, which seemed to be effective in protecting the photosynthetic apparatus from photodamage. Leaves grown under rainfed conditions revealed symptoms of oxidative stress, like the reduction in chlorophyll concentration and the increased levels of lipid peroxidation. We also found that the scavenging function of superoxide dismutase was impaired in rainfed plants. In contrast, the low thiobarbituric acid reactive substances concentration in irrigated trees indicates that irrigation decreases the oxidative damage by lipid peroxidation. Furthermore, we concluded that irrigation enhanced antioxidant protection at leaf level since leaves of irrigated trees had high levels of –SH compounds and the highest antioxidant potential. Meanwhile, the finding that guaiacol peroxidase activity increased in rainfed plants, associated with the appearance of oxidative damage, suggests that this enzyme has no major antioxidative function in olive. Additionally, this experiment demonstrated that olive trees irrigated with 30% of the estimated local evaporative demand had high intrinsic water use efficiency at midday and afternoon, saving consistent amounts of water and revealing a relative water content similar to trees irrigated with high amounts of water. Moreover, this level of irrigation seemed to be sufficient to reduce oxidative damage at leaf level. This is an important finding since a sagacious irrigation approach is essential for viable olive industry due to the limited water resources available in the Mediterranean region. Chapter 7 present the general conclusions from the research work that was undertakenand indicates some directions for future research.

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