Abstract
During autumn, evergreen conifers utilize the decrease in daylength and temperature as environmental signals to trigger cold acclimation, a process that involves the downregulation of photosynthesis, upregulation of photoprotection, and development of cold hardiness. Global warming will delay the occurrence of autumn low temperatures while daylength remains unaffected. The impact of autumn warming on cold acclimation and the length of the carbon uptake period of species with ranges that encompass diverse climates, such as Douglas-fir (Pseudotsuga menziesii), remains unclear. Our study investigated intraspecific variation in the effects of autumn warming on photosynthetic activity, photosynthetic pigments, and freezing tolerance in two interior (var. glauca) and two coastal (var. menziesii) Douglas-fir provenances. Following growth under simulated summer conditions with long days (16 h photoperiod) and summer temperatures (22/13°C day/night), Douglas-fir seedlings were acclimated to simulated autumn conditions with short days (8 h photoperiod) and either low temperatures (cool autumn, CA; 4/−4°C day/night) or elevated temperatures (warm autumn, WA; 19/11°C day/night). Exposure to low temperatures in the CA treatment induced the downregulation of photosynthetic carbon assimilation and photosystem II efficiency, increased the size and de-epoxidation of the xanthophyll cycle pigment pool, and caused the development of sustained nonphotochemical quenching (NPQ). Seedlings in the WA treatment exhibited no downregulation of photosynthesis, no change in xanthophyll cycle pigment de-epoxidation, and no development of sustained NPQ. Albeit these changes, freezing tolerance was not impaired under WA conditions compared with CA conditions. Interior Douglas-fir seedlings developed greater freezing tolerance than coastal seedlings. Our findings suggest that autumn warming, i.e., short photoperiod alone, does not induce the downregulation of photosynthesis in Douglas-fir. Although autumn warming delays the downregulation of photosynthesis, the prolonged period of photosynthetic activity does not bear a trade-off of impaired freezing tolerance.
Highlights
Climate change is projected to increase global average surface temperatures by 2.5–5◦C by the end of the century (IPCC, 2014)
For Little Elk Creek (LIT) seedlings in particular, Rd significantly decreased by almost 50% under cool autumn (CA) (P < 0.001)
The transition from simulated summer control (SC) conditions to simulated CA conditions caused substantial decreases in photosynthetic carbon assimilation (Figure 2A) and photosystem II (PSII) efficiency (Figure 3). These changes are indicative of the downregulation of photosynthesis that is expected in evergreen conifers undergoing cold acclimation during autumn (Wong et al, 2019; Chang et al, 2020)
Summary
Climate change is projected to increase global average surface temperatures by 2.5–5◦C by the end of the century (IPCC, 2014). Even larger increases are projected for the middle to high latitudes of the Northern Hemisphere, which are dominated by temperate and boreal forests. In these forests, evergreen conifers undergo cold acclimation during autumn to protect their overwintering tissues (Chang et al, 2020). The impact of projected autumn warming on evergreen conifer species will be strongly influenced by the importance of photoperiod versus temperature for the induction of the physiological changes that constitute cold acclimation (Way and Montgomery, 2015)
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