Abstract
Photosynthetic organisms are able to sense energy imbalances brought about by the overexcitation of photosystem II (PSII) through the redox state of the photosynthetic electron transport chain, estimated as the chlorophyll fluorescence parameter 1-qL, also known as PSII excitation pressure. Plants employ a wide array of photoprotective processes that modulate photosynthesis to correct these energy imbalances. Low temperature and light are well established in their ability to modulate PSII excitation pressure. The acquisition of freezing tolerance requires growth and development a low temperature (cold acclimation) which predisposes the plant to photoinhibition. Thus, photosynthetic acclimation is essential for proper energy balancing during the cold acclimation process. Eutrema salsugineum (Thellungiella salsuginea) is an extremophile, a close relative of Arabidopsis thaliana, but possessing much higher constitutive levels of tolerance to abiotic stress. This comparative study aimed to characterize the photosynthetic properties of Arabidopsis (Columbia accession) and two accessions of Eutrema (Yukon and Shandong) isolated from contrasting geographical locations at cold acclimating and non-acclimating conditions. In addition, three different growth regimes were utilized that varied in temperature, photoperiod and irradiance which resulted in different levels of PSII excitation pressure. This study has shown that these accessions interact differentially to instantaneous (measuring) and long-term (acclimation) changes in PSII excitation pressure with regard to their photosynthetic behaviour. Eutrema accessions contained a higher amount of photosynthetic pigments, showed higher oxidation of P700 and possessed more resilient photoprotective mechanisms than that of Arabidopsis, perhaps through the prevention of PSI acceptor-limitation. Upon comparison of the two Eutrema accessions, Shandong demonstrated the greatest PSII operating efficiency (ΦPSII) and P700 oxidizing capacity, while Yukon showed greater growth plasticity to irradiance. Both of these Eutrema accessions are able to photosynthetically acclimate but do so by different mechanisms. The Shandong accessions demonstrate a stable response, favouring energy partitioning to photochemistry while the Yukon accession shows a more rapid response with partitioning to other (non-photochemical) strategies.
Highlights
Photosynthesis is a highly coordinated and environmentally sensitive metabolic process.Photoautrophs modulate the structure and function of their photosynthetic apparatus to changes in the environment to maintain cellular energy balance called photostasis
Such a subsidence in qO due to warm measuring temperature was estimated for the photosynthetic photon flux density (PPFD) levels above 310 μmol photons m−2 s−1. This was higher in plants of Eutrema (50% in Yukon and 52% in Shandong) than that in Arabidopsis (41%) (Table 3). These results show that the effects of low measuring temperature and cold acclimation on qO was more pronounced in Arabidopsis plants
We have performed comparative experiments aimed to characterize basic photosynthetic properties of the Yukon and Shandong accessions of Eutrema as well as Arabidopsis grown under different growth regimes
Summary
Photoautrophs modulate the structure and function of their photosynthetic apparatus to changes in the environment to maintain cellular energy balance called photostasis. It is well established that modulation of light and/or temperature cause similar energy imbalances and a change in PSII excitation pressure [2,3,4,5,6]. The level of freezing tolerance attained during cold acclimation is a coordinated response to environmental cues dependent on the genotype of the plant. The changes occurring in leaves during the transition to low temperature are thought to represent transient stress responses whereas leaves that develop at low temperature establish a new metabolic homeostasis that represents the true cold acclimated state [11,12]. It has been demonstrated that photosynthesis interacts with other processes during cold acclimation involving crosstalk between photosynthetic redox, cold acclimation and sugar-signalling pathways to regulate plant acclimation to low temperatures [3,6]
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