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.
Highlights
Mesembryanthemum crystallinum can switch its photosynthetic system from C3 photosynthesis to crassulacean acid metabolism (CAM) under drought or salt conditions
We used seedlings at the early developmental stage (Figure 1) before they shift into CAM photosynthesis
M. crystallinum leaves grew at a similar rate as the control group in the first 5 days, and slowed down the growth after 7 days of salt-treatment with a simulated equation: A = 29.7391 × (1.0830D+21 − 1) if D ≤ 0
Summary
Mesembryanthemum crystallinum can switch its photosynthetic system from C3 photosynthesis to crassulacean acid metabolism (CAM) under drought or salt conditions. CAM is a specialized mode of photosynthesis that has nocturnal fixation of atmospheric CO2 into organic acids (e.g., malic acid) by phosphoenolpyruvate carboxylase (PEPC), whereby the CO2-storing organic acids are remobilized and decarboxylated to provide CO2 for the Calvin cycle during the day (Winter et al, 2015). M. crystallinum is known as a succulent plant with more succulence in leaves at adult and flowering stages than at juvenile stage (Adams et al, 1998). Because the CO2-storing organic acids are mainly stored in mesophyll cells, some degree of succulence is required for CAM to be efficient (Males, 2017). The succulence ensures independence from limited or unpredictable water supply after the juvenile growth phase.
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