Autumn warming delays downregulation of photosynthesis and does not increase risk of freezing damage in interior and coastal Douglas-fir seedlings

In the next several decades, warming in the northern hemisphere will result in asynchronous phasing between the temperature and photoperiod signals that evergreen conifers rely upon for cold hardening during autumn. Our study investigated intraspecific variation in photosynthetic and photoprotective mechanisms in Douglas-fir (Pseudotsuga menziesii) originating from contrasting climates during simulated summer and autumn conditions, as well as how autumn warming affects downregulation of photosynthesis and development of cold hardening. Following growth under long days and summer temperature (LD/ST; 16 h photoperiod; 22 °C/13 °C day/night), Douglas-fir seedlings from two interior and two coastal provenances were acclimated to simulated autumn conditions with short days and either low temperature (SD/LT; 8 h photoperiod; 4 °C/-4 °C day/night) or high temperature (SD/HT; 8 h photoperiod; 19 °C/11 °C day/night). Exposure to low temperature induced increase in size and de-epoxidation of the xanthophyll cycle pigment pool, development of sustained nonphotochemical quenching, and downregulation of photosynthetic activity. SD/HT seedlings exhibited no downregulation of photosynthesis, corresponding with no change in xanthophyll cycle pigment de-epoxidation and no development of sustained nonphotochemical quenching. However, freezing tolerance development for all provenances was not impaired under SD/HT relative to SD/LT. Interior Douglas-fir provenances developed greater freezing tolerance relative to coastal provenances under both temperature treatments. Our findings suggest that short photoperiod alone is insufficient to induce downregulation of photosynthesis in autumn for Douglas-fir. However, this prolonged period of photosynthetic activity does not appear to bear a trade-off of impaired freezing tolerance.