Tolerance of citrus plants to the combination of high temperatures and drought is associated to the increase in transpiration modulated by a reduction in abscisic acid levels.
BACKGROUND
In natural environments, several adverse environmental conditions occur simultaneously constituting a unique stress factor. In this work, physiological parameters and the hormonal regulation of Carrizo citrange and Cleopatra mandarin, two citrus genotypes, in response to the combined action of high temperatures and water deprivation were studied. The objective was to characterize particular responses to the stress combination.


RESULTS
Experiments indicated that Carrizo citrange is more tolerant to the stress combination than Cleopatra mandarin. Furthermore, an experimental design spanning 24 h stress duration, heat stress applied alone induced higher stomatal conductance and transpiration in both genotypes whereas combined water deprivation partially counteracted this response. Comparing both genotypes, Carrizo citrange showed higher phostosystem-II efficiency and lower oxidative damage than Cleopatra mandarin. Hormonal profiling in leaves revealed that salicylic acid (SA) accumulated in response to individual stresses but to a higher extent in samples subjected to the combination of heat and drought (showing an additive response). SA accumulation correlated with the up-regulation of pathogenesis-related gene 2 (CsPR2), as a downstream response. On the contrary, abscisic acid (ABA) accumulation was higher in water-stressed plants followed by that observed in plants under stress combination. ABA signaling in these plants was confirmed by the expression of responsive to ABA-related gene 18 (CsRAB18). Modulation of ABA levels was likely carried out by the induction of 9-neoxanthin cis-epoxicarotenoid dioxygenase (CsNCED) and ABA 8'-hydroxylase (CsCYP707A) while conversion to ABA-glycosyl ester (ABAGE) was a less prominent process despite the strong induction of ABA O-glycosyl transferase (CsAOG).


CONCLUSIONS
Cleopatra mandarin is more susceptible to the combination of high temperatures and water deprivation than Carrizo citrange. This is likely a result of a higher transpiration rate in Carrizo that could allow a more efficient cooling of leaf surface ensuring optimal CO2 intake. Hence, SA induction in Cleopatra was not sufficient to protect PSII from photoinhibition, resulting in higher malondialdehyde (MDA) build-up. Inhibition of ABA accumulation during heat stress and combined stresses was achieved primarily through the up-regulation of CsCYP707A leading to phaseic acid (PA) and dehydrophaseic acid (DPA) production. To sum up, data indicate that specific physiological responses to the combination of heat and drought exist in citrus. In addition, these responses are differently modulated depending on the particular stress tolerance of citrus genotypes.
Department Ciències Agràries i del Medi Natural, Universitat Jaume I, E-12071, Castelló de la Plana, Spain.
Sara I
SI
Zandalinas
Departament de Nutrición Vegetal, Centro de Edafología Aplicada del Segura, Consejo Superior de Investigaciones Científicas, 30100, Murcia, Spain.
Rosa M
RM
Rivero
Departament de Nutrición Vegetal, Centro de Edafología Aplicada del Segura, Consejo Superior de Investigaciones Científicas, 30100, Murcia, Spain.
Vicente
V
Martínez
Department Ciències Agràries i del Medi Natural, Universitat Jaume I, E-12071, Castelló de la Plana, Spain.
Aurelio
A
Gómez-Cadenas
Department Ciències Agràries i del Medi Natural, Universitat Jaume I, E-12071, Castelló de la Plana, Spain. vicente.arbona@camn.uji.es.
Vicent
V
Arbona
D000040:Abscisic Acid / Q000378:metabolism*; D000222:Adaptation, Physiological / Q000235:genetics / Q000502:physiology*; D002245:Carbon Dioxide / Q000378:metabolism; D002957:Citrus / Q000145:classification / Q000235:genetics / Q000502:physiology*; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism; D049308:Dioxygenases / Q000235:genetics / Q000378:metabolism; D055864:Droughts; D018506:Gene Expression Regulation, Plant; D005838:Genotype; D004693:Glucan Endo-1,3-beta-D-Glucosidase / Q000235:genetics / Q000378:metabolism; D006358:Hot Temperature; D008315:Malondialdehyde / Q000378:metabolism; D010641:Phenotype; D045332:Photosystem II Protein Complex / Q000235:genetics / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D054046:Plant Stomata / Q000235:genetics / Q000502:physiology; D018526:Plant Transpiration / Q000235:genetics / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D020156:Salicylic Acid / Q000378:metabolism; D013045:Species Specificity; D020691:rab GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism
D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D002245:Carbon Dioxide; D008315:Malondialdehyde; D000040:Abscisic Acid; D003577:Cytochrome P-450 Enzyme System; D049308:Dioxygenases; C107203:abscisic acid 8'-hydroxylase; D004693:Glucan Endo-1,3-beta-D-Glucosidase; D020691:rab GTP-Binding Proteins; D020156:Salicylic Acid
Carrizo citrange; Cleopatra mandarin; Combined stress conditions; Heat; Hormone regulation; Salicylic acid