Exploring how temperature affects dormancy induction and cold acclimation in hybrid poplar

Dormancy, cold hardiness and height growth were examined in four poplar clones exposed to four temperature conditions (13.5ºC/8.5ºC, 18.5ºC/3.5ºC, 18.5ºC/13.5ºC and 23.5ºC/8.5ºC day/night temperatures) under short photoperiod. The selected clones were ‘WP-69’ (‘Okanese’)- early acclimation, ‘Walker’ and ‘Katepwa’ - intermediate acclimation, and ‘Prairie Sky’- late acclimation. Changes in physical water properties and mobility within the vascular tissue region, vascular transition region into the axillary bud and the upper axillary bud were assessed during endodormancy development using Magnetic Resonance Microimaging (MRMI). In summary:a) There were distinct differences between poplar clones during dormancy induction in response to temperature. For example, ‘Katepwa’, ‘Walker’ and ‘WP-69’ clones became endodormant but ‘Prairie Sky’ did not enter endodormancy. Endodormancy development and cold acclimation in ‘WP-69’ were less affected by temperature than ‘Katepwa’ and ‘Walker’ suggesting that genotypic variation exists in response to temperature change.b) Growth cessation, not endodormancy, was a prerequisite for cold acclimation since cold hardiness increased in ‘Prairie Sky’ in the absence of endodormancy. However, increases in endodormancy coincided with increase in cold hardiness in other clones.c) Low night temperatures (18.5ºC/3.5ºC) delayed endodormancy development and cold acclimation in all clones compared to the warm night temperature treatment (18.5ºC /13.5ºC). Night temperature was negatively correlated with time to growth cessation, and cold hardiness and positively correlated with dormancy development. Changes in night temperature may affect time to growth cessation, subsequently altering timing of cold acclimation and endodormancy development since growth cessation appeared to be a prerequisite for both processes. d) ADC (Apparent Diffusion Coefficient), an indicator of water mobility within living tissues, was negatively correlated with endodormancy induction. Specifically, the transition region of vascular tissue between the stem and the lower axillary bud showed the highest correlation with endodormancy development. By contrast, decreases in T1 relaxation times, an indicator of biophysical water properties, were inconsistent with changes in endodormancy levels in axillary buds. Thus, ADC appears to correspond more closely with endodormancy development than changes in T1 relaxation times. It is apparent that temperature impacts dormancy development in hybrid poplar. Underlying changes in water appear to correspond with changes in endodormancy. Under future warming scenarios, genotypes such as WP 69 (‘Okanese’) that are less sensitive to temperature and maintain a consistent, endodormancy induction pattern, may be better fit to changing climates