Enhanced Photosynthetic Capacity, Osmotic Adjustment and Antioxidant Defenses Contribute to Improve Tolerance to Moderate Water Deficit and Recovery of Triploid Citrus Genotypes

Abstract

Currently, drought stress is a major issue for crop productivity, and future climate models predict a rise in frequency and severity of drought episodes. Polyploidy has been related to improved tolerance of plants to environmental stresses. In Citrus breeding programs, the use of triploidy is an effective way to produce seedless fruits, one of the greatest consumer expectations. The current study used physiological and biochemical parameters to assess the differential responses to moderate water deficit of 3x genotypes compared to 2x genotypes belonging to the same hybridization. Both parents, the mandarin Fortune and Ellendale tangor, were also included in the experimental design, while the 2x common clementine tree was used as reference. Water deficit affects leaf water status, as well as physiological and detoxification processes. Triploid genotypes showed a better ability to maintain water status through increased proline content and photosynthetic capacity. Moreover, less oxidative damage was associated with stronger antioxidant defenses in triploid genotypes. We also found that triploidy improved the recovery capacity after a water deficit episode.

Keywords: Citrus; ROS accumulation; antioxidant system; drought stress; enzymatic antioxidant; osmolytes; polyploidy.

Figure 1

Plant water status parameters of 2x and 3x genotypes under three water treatments: 70–100% pot capacity (black point, control), 45–55% pot capacity (grey points, water deficit), and five days after re-watering (blue point, recovery). (A) Pre-dawn water potential (Ψ_PD), (B) relative water content (RWC), and (C) transpiration rate (E). All data are mean values (± S.E.) of 3 independent measurements for Ψ_PD (n = 3) and 15 independent measurements for RWC and E (n = 15). Data were analyzed with ANOVA and Fisher’s LSD test (p < 0.05). Uppercase letters compare water treatments for each genotype. Lowercase letters compare genotype within the same water treatment.

Figure 2

Leaf physiological parameters of 2x and 3x genotypes under three water treatments: 70–100% pot capacity (black point, control), 45–55% pot capacity (grey points, water deficit), and five days after re-watering (blue point, recovery). (A) Net photosynthesis (P_net), (B) stomatal conductance (g_s), (C) maximal quantum yield of PSII (F_v/F_m), and (D) non-photochemical quenching (NPQ) rate. All data are mean values (±S.E.) of 15 independent measurements (n = 15). Data were analyzed with ANOVA and Fisher’s LSD test (p < 0.05). Uppercase letters compare water treatments for each genotype. Lowercase letters compare genotype within the same water treatment.

Figure 3

Oxidative stress markers and antioxidant enzymes in 2x and 3x genotypes under three water treatments: 70–100% pot capacity (black point, control), 45–55% pot capacity (grey points, water deficit), and five days after re-watering (blue point, recovery). (A) Malondialdehyde (MDA); (B) H₂O₂ content; (C) specific activities of SOD, (D) CAT, and (E) APX. All data are mean values (±S.E.) of 15 independent measurements (n = 15). Data were analyzed with ANOVA and Fisher’s LSD test (p < 0.05). Uppercase letters compare water treatments for each genotype. Lowercase letters compare genotype within the same water treatment.

Figure 4

Changes in (A) proline, (B) tAsa content, (C) Asa/DHA ratio, and (D) specific activity of dehydroascorbate reductase (DHAR) in 2x and 3x genotypes under three water treatments: 70–100% pot capacity (black point, control), 45–55% pot capacity (grey points, water deficit), and five days after re-watering (blue point, recovery). All data are mean values (±S.E.) of 15 independent measurements (n = 15). Data were analyzed with ANOVA and Fisher’s LSD test (p < 0.05). Uppercase letters compare water treatments for each genotype. Lowercase letters compare genotype within the same water treatment.

Figure 5

Principal component analysis (PCA) performed on leaves of 2x and 3x genotypes subjected to water deficit. (A) Clustering of genotypes on the two first components and (B) contribution of each physiological and biochemical parameter to the two first components of PCA.

Figure 6

Principal component analysis (PCA) performed on leaves of 2x and 3x genotypes after the recovery period. (A) Clustering of genotypes on the two first components and (B) contribution of each physiological and biochemical parameter to the two first components of PCA.