PhD Exit Seminar – Christine Chang (Ensminger lab)

PhD Exit Seminar

Wednesday September 7^th, 10:10 am – Room DV 3129, University of Toronto at Mississauga

Christine Chang (Ensminger lab)

“Impacts of Short Photoperiod, Elevated Temperature, and Elevated CO₂ on Cold Hardening in Eastern White Pine“

Abstract

Cold hardening in evergreen conifers is induced during autumn by decreasing temperature and photoperiod, and may be delayed or impaired by climate warming. This work aimed to characterize the control of photoperiod versus temperature over the downregulation of photosynthesis, changes in carbohydrate metabolism and development of freezing tolerance that occur during cold hardening in Eastern white pine (Pinus strobus L.) seedlings. Furthermore, this work assessed the impact of elevated temperature with ambient or elevated CO₂ on the timing and extent of cold hardening under controlled and field conditions.

Under controlled conditions, short photoperiod rapidly induced leaf starch degradation and accumulation of sucrose, while low temperature rapidly induced adjustments of leaf pigments, chlorophyll fluorescence and accumulation of glucose. Prolonged exposure to short photoperiod and low temperature resulted in downregulation of photosynthesis, accumulation of cryoprotective carbohydrates, and development of freezing tolerance. A novel 16-kD dehydrin protein was induced by short photoperiod but maximally expressed under the combination of short photoperiod and low temperature; expression of this dehydrin strongly correlated with freezing tolerance.

Elevated temperature suppressed the downregulation of photosynthesis, accumulation of carbohydrates and dehydrins; freezing tolerance was impaired, but provided sufficient protection against average historical winter temperatures at the seedlings’ native origin. The combination of elevated temperature and elevated CO₂ enhanced photosynthesis and, under long photoperiod, enhanced accumulation of starch. Elevated CO₂ did not further impair dehydrin expression or development of freezing tolerance. Lastly, we assessed the impact of local projected temperatures for the year 2050 on cold hardening in field-grown seedlings. In a complex natural environment with large seasonal and diurnal temperature changes, elevated temperature did not perceptibly delay downregulation of photosynthesis or impair freezing tolerance.

The presented findings distinguish the control of short photoperiod and low temperature signals over the downregulation of photosynthesis, changes in carbohydrate metabolism and development of freezing tolerance during cold hardening in P. strobus. The results suggest that rising temperature and CO₂ levels may allow P. strobus seedlings to benefit from increased carbon uptake when photosynthesis is active, and that impaired freezing tolerance is not likely to pose a significant threat to their winter survival.