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

Sahar Baccari,Olfa Elloumi,Lotfi Fki,Erola Fenollosa,Melanie Morales,Ferjani Ben Abdallah,Noureddine Drira,Anissa Chaari-Rkhis,Sergi Munné-Bosch

doi:10.3389/fpls.2020.614144

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.

Highlights

Olive tree (Olea europaea L.) is one of the most widespread crops in Mediterranean-type agroecosystems, with high economic and social importance
Leaf production rate (LPR) dropped severely in all olive trees exposed to drought stress, with the seedlings “Chemlali” (“Chemlali S”) and “Oleaster” (“Oleaster S”) being the most affected already at 30 days of drought (Figure 1B)
The effects of water stress on growth may be considered the first line of defense, as it has been previously reported on olive trees (Trabelsi et al, 2019)

Summary

Introduction

Olive tree (Olea europaea L.) is one of the most widespread crops in Mediterranean-type agroecosystems, with high economic and social importance. In semi-arid areas of the Mediterranean basin, the scarcity of precipitation combined with extreme climatic conditions such as high temperatures increase water stress in olive trees. During the drought episode of 1999–2002, the decrease of olive production reached 90%, which adversely affected farmers income (Gargouri et al, 2012). Considering this situation, much attention has recently been drawn to the valorization and conservation of plant genetic resources to achieve relatively high yields under severe drought. Several researchers have demonstrated that combinations of rootstocks and scions decrease the depressive effects of water stress and increase resistance to drought (Trifilo et al, 2007; Therios, 2009)

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