Linking Leaf Water Potential, Photosynthesis and Chlorophyll Loss With Mechanisms of Photo- and Antioxidant Protection in Juvenile Olive Trees Subjected to Severe Drought

Abstract

The identification of drought-tolerant olive tree genotypes has become an urgent requirement to develop sustainable agriculture in dry lands. However, physiological markers linking drought tolerance with mechanistic effects operating at the cellular level are still lacking, in particular under severe stress, despite the urgent need to develop these tools in the current frame of global change. In this context, 1-year-old olive plants growing in the greenhouse and with a high intra-specific variability (using various genotypes obtained either from cuttings or seeds) were evaluated for drought tolerance under severe stress. Growth, plant water status, net photosynthesis rates, chlorophyll contents and the extent of photo- and antioxidant defenses (including the de-epoxidation state of the xanthophyll cycle, and the contents of carotenoids and vitamin E) were evaluated under well-watered conditions and severe stress (by withholding water for 60 days). Plants were able to continue photosynthesizing under severe stress, even at very low leaf water potential of -4 to -6 MPa. This ability was achieved, at least in part, by the activation of photo- and antioxidant mechanisms, including not only increased xanthophyll cycle de-epoxidation, but also enhanced α-tocopherol contents. "Zarrazi" (obtained from seeds) and "Chemlali" (obtained from cuttings) showed better performance under severe water stress compared to the other genotypes, which was associated to their ability to trigger a higher antioxidant protection. It is concluded that (i) drought tolerance among the various genotypes tested is associated with antioxidant protection in olive trees, (ii) the extent of xanthophyll cycle de-epoxidation is strongly inversely related to photosynthetic rates, and (iii) vitamin E accumulation is sharply induced upon severe chlorophyll degradation.

Keywords: drought tolerance; olive germplasm; photoprotection; xanthophyll cycle; α-tocopherol.

FIGURE 1

Stem and leaf growth in olive trees subjected to mild and severe drought stress. Effects of drought stress periods (30 and 60 days after start of treatments) on (A) stem growth and (B) leaf production rate in olive trees obtained from cuttings (cv. “Chemlali” and “Chetoui”) and obtained from seeds (“Chemlali S,” “Chetoui S,” “Zarrazi S” and “Oleaster S”). Values represent means ± S.E. of n ≥ 3 for irrigated plants and n ≥ 8 for water-stressed plants. A two-way analysis of variance (ANOVA) was used to examine if the effects of treatment (water regime) and genotype (cultivar) were statistically significant and to look at the significance of interaction effects. Different letters indicate significant differences between genotypes in each condition using Duncan post hoc tests (p ≤ 0.05).

FIGURE 2

Water status and leaf gas exchange in olive trees subjected to severe drought stress. (A) Leaf water potential (Ψ_L), (B) net photosynthesis rate (An), (C) stomatal conductance (gs), (D) transpiration (E), (E) water use efficiency (A/E) and (F) intercellular to ambient carbon dioxide concentration ratio (Ci/Ca) in olive trees obtained from cuttings (cv. “Chemlali” and “Chetoui”) and obtained from seeds (“Chemlali S,” “Chetoui S,” “Zarrazi S” and “Oleaster S”) after 60 days of drought stress. Values represent means ± S.E. of n = 3. A two-way analysis of variance (ANOVA) was used to examine if the effects of treatment (water regime) and genotype (cultivar) were statistically significant and to look at the significance of water regime × cultivar interaction effects. Means having the same letter are not significantly different according to Duncan post hoc test at p ≤ 0.05.

FIGURE 3

Relationship between leaf water status and photo- and antioxidant protection in olive trees. Relationship between leaf water potential and the de-epoxidation state of the xanthophyll cycle (A), the contents of the xanthophyll cycle pool (B), lutein (C), β-carotene (D), and α-tocopherol (E) per unit of chlorophyll of olive trees after 60 days of treatments. Correlation was determined according to Pearson test. p values and corresponding correlation coefficient (r) are shown.

FIGURE 4

Relationship between photosynthesis and photo- and antioxidant protection in olive trees. Relationship between net photosynthesis rates and the de-epoxidation state of the xanthophyll cycle (A), the contents of the xanthophyll cycle pool (B), lutein (C), β-carotene (D), and α-tocopherol (E) per unit of chlorophyll of olive trees after 60 days of treatments. Correlation was determined according to Pearson test. p values and corresponding correlation coefficient (r) are shown.

FIGURE 5

Relationship between chlorophyll loss and photo- and antioxidant protection in olive trees. Relationship between chlorophyll contents and the de-epoxidation state of the xanthophyll cycle (A), the contents of the xanthophyll cycle pool (B), lutein (C), β-carotene (D), and α-tocopherol (E) per unit of chlorophyll of olive trees after 60 days of treatments. Correlation was determined according to Pearson test. p values and corresponding correlation coefficient (r) are shown.

FIGURE 6

Tolerance to severe water stress of olive genotypes. The color gradation ranges from dark red to dark green to indicate progressively increased stress tolerance. Three levels of susceptibility and three levels of tolerance were given for each of the parameters in red and green, respectively, and the mean of tolerance was then calculated considering all parameters measured after 60 days of treatments. SGR, Stem growth rate; LPR, Leaf production rate; Ψ_L, Leaf water potential; A_n, Net photosynthesis rate; gs, stomatal conductance; E, transpiration; A/E, water use efficiency; Ci/Ca, intercellular to air carbon dioxide concentration ratio; Chl a + b, Total chlorophyll; Chl a/b, chlorophyll a/b ratio; VZA/Chl, xanthophyll cycle pool per unit of Chl; DPS, de-epoxidation state of the xanthophyll cycle; Lut/Chl, lutein per Chl ratio; β-car/Chl, β-carotene per Chl ratio; α-Toc/Chl, α-tocopherol per unit of Chl. Genotypes: Rooted cuttings, “Chemlali” and “Chetoui”; Seedlings, “Chemlali S,” “Chetoui S,” “Zarrazi S” and “Oleaster S.”