Physiological Changes in Mesembryanthemum crystallinum During the C3 to CAM Transition Induced by Salt Stress

Abstract

Salt stress impedes plant growth and development, and leads to yield loss. Recently, a halophyte species Mesembryanthemum crystallinum has become a model to study plant photosynthetic responses to salt stress. It has an adaptive mechanism of shifting from C₃ photosynthesis to crassulacean acid metabolism (CAM) photosynthesis under stresses, which greatly enhances water usage efficiency and stress tolerance. In this study, we focused on investigating the morphological and physiological changes [e.g., leaf area, stomatal movement behavior, gas exchange, leaf succulence, and relative water content (RWC)] of M. crystallinum during the C₃ to CAM photosynthetic transition under salt stress. Our results showed that in M. crystallinum seedlings, CAM photosynthesis was initiated after 6 days of salt treatment, the transition takes place within a 3-day period, and plants became mostly CAM in 2 weeks. This result defined the transition period of a facultative CAM plant, laid a foundation for future studies on identifying the molecular switches responsible for the transition from C₃ to CAM, and contributed to the ultimate goal of engineering CAM characteristics into C₃ crops.

Keywords: C3 to CAM transition; Mesembryanthemum crystallinum; photosynthesis and photorespiration; physiology; salt stress.

FIGURE 1

Morphology of M. crystallinum seedlings growing under control and salt treatment for 14 days. Each image is representative of the seedlings at 12 pm in the four biological replicates.

FIGURE 2

Whole leaf area measurement of M. crystallinum seedlings during their growth for 42 days. The salt treatment began at day 0. (A) Leaf area changes in control group (blue line) and salt-treated group (orange line) of M. crystallinum. (B) Simulation of M. crystallinum growth of the control group. The line represents the simulation and the dots represent exact leaf area of each biological replicate. (C) Simulation of M. crystallinum growth of the salt treated group. The line represents the simulation and the dots represent exact leaf area of each biological replicate.

FIGURE 3

Leaf CO₂ assimilation and stomatal aperture changes in the control and salt-treated M. crystallinum seedlings. (A) Leaf CO₂ gas exchange and (B) stomatal movement behavior. The blue line represents the control group and the orange line represents the salt-treated group. The gray bars represent the night time and the white bars represent the day time. The error bar represents standard error. The data were obtained from three biological replicates.

FIGURE 4

Changes in leaf succulence and relative water content (RWC) in the control and salt-treated M. crystallinum seedlings. (A) Leaf succulence. The blue dots represent the control group and the orange dots represent the salt-treated group. The error bar represents standard error. (B) RWC. The blue dots represent the control group and the orange dots represent the salt-treated group. The error bar represents standard error. An asterisk indicates a Student’s t-test (p < 0.05) and two asterisks indicate a Student’s t-test (p < 0.01). The data were obtained from four biological replicates.

FIGURE 5

Changes in MDA contents in the control and salt-treated M. crystallinum seedlings. The blue dots represent the control group and the orange dots represent the salt-treated group. The error bar represents standard error. An asterisk indicates a Student’s t-test (p < 0.05) and two asterisks indicate a Student’s t-test (p < 0.01). The data were obtained from four biological replicates.