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Abstract
Interest in the thermal acclimation of photosynthesis has been stimulated by the increasing relevance of climate change. However, little is known about intra-specific variations in thermal acclimation and its potential for breeding. In this article, we examined the difference in thermal acclimation between alfalfa (Medicago sativa) cultivars originating from contrasting origins, and sought to analyze the mechanisms in play. A series of experiments was carried out at seven growth temperatures between 5 and 35 °C using four cultivars from temperate and Mediterranean origin. Leaf traits, the photosynthetic rate at 25 °C (A400 (25)), the photosynthetic rate at optimal temperature (A400 (opt)), the thermal optimum of photosynthesis (Topt), and the photosynthetic parameters from the Farqhuar model were determined. Irrespective of cultivar origin, a clear shift in the temperature responses of photosynthesis was observed as a function of growth temperature, affecting thermal optimum of photosynthesis, photosynthetic rate at optimal temperature and photosynthetic rate at 25 °C. For both cultivars, Topt values increased linearly in leaves grown between 5 and 35 °C. Relative homeostasis of A400 (25) and A400 (opt) was found between 10 °C and 30 °C growth temperatures, but sharp declines were recorded at 5 and 35 °C. This homeostasis was achieved in part through modifications to leaf nitrogen content, which increased at extreme temperatures. Significant changes were also recorded regarding nitrogen partitioning in the photosynthetic apparatus and in the temperature dependence of photosynthetic parameters. The cultivars differed only in terms of the temperature response of photosynthetic parameters, with Mediterranean genotypes displaying a greater sensitivity of the maximum rate of Rubisco carboxylation to elevated temperatures. It was concluded that intra-specific variations in the temperature acclimation of photosynthesis exist among alfalfa cultivars, but that Mediterranean genotypes presented no evidence of superior performance at high temperatures.
Highlights
Because plants cannot move, adaptation of their photosynthetic characteristics is essential to maximize performance at their growth temperature (Tgrowth) (Berry and Bjorkman 1980; Sage and Kubien 2007; Way and Yamori 2014; Yamori et al 2014)
The thermal acclimation of photosynthesis has been associated with modifications to several photosynthetic variables (Way and Yamori 2014) which include: (i) shifts in the thermal optimum (Topt) of photosynthesis toward a new The impacts of growth temperature (Tgrowth), (ii) a relative homeostasis of the maximum photosynthetic rate between Tgrowth (Cowling and Sage 1998; Gunderson et al 2010) and (iii) altered photosynthetic characteristics measured at 25 C, such as the ratio between the maximum electron transport rate (Jmax) and the maximum rate of Rubisco carboxylation (Vcmax) (Berry and Bjorkman 1980; Leuning 1997, 2002; Medlyn et al 2002)
A significant intra-specific variability was found in terms of the thermal acclimation of photosynthesis in alfalfa
Summary
Adaptation of their photosynthetic characteristics is essential to maximize performance at their growth temperature (Tgrowth) (Berry and Bjorkman 1980; Sage and Kubien 2007; Way and Yamori 2014; Yamori et al 2014). Growth effects alter leaf traits, such as the final leaf area (Larea), the specific leaf area (SLA) and the amount of nitrogen per unit area (Na) (Field and Mooney 1986; Garnier et al 1999; Hikosaka 2004; Reich et al 1998), which are tightly related to photosynthetic capacity (Evans 1989; Yamori et al 2005) These growth responses may in part be adaptive, and contribute to the thermal acclimation of photosynthesis (Onoda et al 2004; Sage and Kubien 2007)
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