Elsevier

Plant Science

Volume 169, Issue 2, August 2005, Pages 403-412
Plant Science

Influence of water deficit and rewatering on the components of the ascorbate–glutathione cycle in four interspecific Prunus hybrids

https://doi.org/10.1016/j.plantsci.2005.04.004Get rights and content

Abstract

The activities of ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate reductase (MDHAR, EC 1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1) and glutathione reductase (GR, EC 1.6.4.2), as well as the levels of ascorbate pool, glutathione pool and H2O2 were studied in plants of four interspecific Prunus hybrids subjected to water deficit and shade conditions. After 70 days of water shortage, plants were subjected to a rewatering treatment. During water recovery, leaves fully exposed to sunlight and leaves in shade conditions of about 30% of environmental irradiance were sampled. After 70 days without irrigation, mean pre-dawn leaf water potential of all the hybrids fell from −0.34 to −3.30 MPa and marked decreases in net photosynthesis and transpiration occurred. The activities of APX, MDHAR, DHAR and GR increased in relation to the severity of drought stress in all the clones studied. Generally, APX, MDHAR, DHAR and GR were down-regulated during the rewatering phase and their activities decreased faster in shaded leaves than in non-shaded leaves. The levels of total ascorbate, total glutathione and H2O2 were directly related to the increase of drought stress and subsequently decreased during rewatering. This response could limit cellular damage caused by active oxygen species during periods of water deficit. The ability of Prunus hybrids to regulate the enzymatic antioxidant system during different water and irradiance conditions might be an important attribute linked to drought tolerance.

Introduction

Acclimation of plants to drought is often associated with increased levels of activated oxygen species (AOS), such as superoxide anion (O2radical dot), hydrogen peroxide (H2O2), hydroxyl radical (HOradical dot) and singlet oxygen (1O2), which are toxic for the cells [1], [2]. AOS are by-products of aerobic metabolism and their production is enhanced during drought conditions through the disruption of electron transport system and oxidizing metabolic activities occurring in chloroplasts, mitochondria and microbodies [3], [4]. Excessive levels of AOS damage cellular structures and macromolecules, causing photoinhibition of photosynthetic apparatus [1] but the production and accumulation of AOS activate multiple defence responses, thus having also a positive role [4], [5]. In particular, the presence of H2O2 in the apoplast is toxic for pathogens, is involved in gene transcription and systemic acquired resistance, and slows down the spread of invading organisms by cell death round the infection and a rapid local cross-linking of the cell wall [6], [7].

Under non-stressful conditions, AOS are efficiently eliminated by non-enzymatic and enzymatic antioxidants, whereas during drought conditions the production of AOS exceeds the capacity of the antioxidative systems to remove them, causing oxidative stress [1], [8]. The antioxidant non-enzymatic system includes ascorbate and glutathione, two constituents of the antioxidative ascorbate–glutathione cycle which detoxify H2O2 in the chloroplasts [3] and are located both within the cell and in the apoplast [6], [9]. Ascorbate (AsA) is a major primary antioxidant synthesized on the inner membrane of the mitochondria which reacts chemically with 1O2, O2radical dot, HOradical dot and thiyl radical [3], [8], and acts as the natural substrate of many plant peroxidases [10]. Moreover, AsA is involved in other functions such as plant growth, gene regulation, modulation of some enzymes and redox regulation of membrane-bound antioxidant compounds [6], [7], [8]. Glutathione (GSH) is a tripeptide synthesized in the cytosol and the choloroplast which scavenges 1O2 and H2O2 and is oxidized to glutathione disulfide (GSSG) when acts as an antioxidant and redox regulator [1], [8], [11]. GSH is the substrate of glutathione S-transferases (GSTs), which have a protective role in the detoxification of xenobiotics and dehydroascorbate reductase (DHAR) [9]. Finally, GSH is a precursor of phytochelatins, which regulate cellular heavy metals levels, and is involved in gene expression [8].

The antioxidant enzymatic system includes the enzymes of the ascorbate–glutathione cycle, that operates both in the chloroplasts and in the cytosol: ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate reductase (MDHAR, EC 1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1) and glutathione reductase (GR, EC 1.6.4.2).

The genus Prunus comprises more than 400 species adapted to temperate areas and cultivated in Europe [12]. In particular, stone fruit crops, such as peach (Prunus persica L.), plum (Prunus cerasifera L. and Prunus domestica L.), almond (Prunus dulcis L.), apricot (Prunus armeniaca L.) and cherry tree (Prunus avium L.), are typical and economically important cultures mainly localized in Mediterranean regions, where the spring–summer period is often characterized by high temperatures, high irradiance levels and lack of precipitation. Productive stone fruit trees are usually grafted plants with a lower part, the rootstock and an upper grafted part, which is the genotype of the commercial variety. Rootstocks have different genetic background compared to the commercial varieties and can be used to confer various traits, such as drought stress resistance. Many plum genotypes are used as rootstock for almost all other Prunus species and, among them, Myrobalan plum (P. cerasifera L.) clones show positive agronomic features and are resistant to root-knot nematodes [12]. The response to water deficit of these species is a well documented process [13], [14], [15], [16], [17] but only few studies highlighted the importance of antioxidant enzymes in genus Prunus and in other fruit trees [18], [19], and very little is known about the linkages between drought and the components of the ascorbate–glutathione cycle in these species.

The aim of this work was to study the changes of antioxidant enzyme activities (APX, MDHAR, DHAR and GR) and the level of some compounds (ascorbate and glutathione pools and H2O2) involved in the ascorbate–glutathione cycle in plants of four Prunus interspecific hybrids grown under water shortage followed by a rewatering phase, and to determine the differences of antioxidant and physiological responses among hybrids during stress conditions. Finally, on the basis of previous findings [19], we also hypothesize different patterns of enzyme activities in leaves under different levels of irradiance during the rewatering phase.

Section snippets

Study site, plant material and experimental design

The study site was located at the ‘Università degli Studi della Basilicata’ in Potenza (Southern Italy – Basilicata Region – 40°39′N, 15°47′E). The experimental period started on July 10 and ended on October 24, 2002.

Trials were conducted on virus free plant material obtained from the breeding programs of INRA Bordeaux and SIA Zaragoza (EU funded project FAIR-6-CT-98-4139). The material, presenting different levels of resistance against nematodes of Meloidogyne spp., included four interspecific

Environmental conditions and physiological parameters

The highest value of air temperature was 29.2 °C after 13 days from the beginning of the drought period. The mean values of all the daily values of air temperature and RH were 21.6 °C and 68.2%, respectively. VPD range was between 2.2 and 0.3 KPa, with a mean value of 0.9 KPa (Fig. 1). The mean values of soil humidity measured at different levels of depth decreased progressively during the drought phase and then increased during rewatering, showing a range between 9.3 and 27.4% (Fig. 2).

The water

Discussion

During periods of water deficit, species of the genus Prunus, such as almond, peach and apricot tree, show significant decrease in gas exchange [14], [16], [30]. Our results confirm that the decrease of soil humidity (Fig. 2), together with high values of VPD (Fig. 1), caused a reduction of LWP and gas exchange in Prunus plants (Fig. 3). Recent studies demonstrated that the decrease in net CO2 assimilation in response to environmental stresses reduces the capacity of the photosynthetic electron

Acknowledgement

This research was financed by the project FAIR (CT 98-4139) ‘An alternative to methyl bromide using resistant Prunus rootstocks to root-knot nematodes’.

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