Interrelationships between ultrastructure, sugar levels, and frost hardiness of ray parenchyma cells during frost acclimation and deacclimation in poplar (Populus × canadensis Moench "robusta") wood.
The ray parenchyma cells of poplar wood (Populus × canadensis 'Robusta') were studied for changes in ultrastructure and in the level of various sugars in the autumn, winter and spring in relation to freezing resistance as measured by LT50 (temperature at which 50% of the cells are injured). Before acclimatization (mid-October), frost resistance was low (LT50 -3 to -5°C), sugar levels were not high, while the ray cell protoplast had become dense in structure. Frost acclimatization started during leaf fall (end of October) concomitant with a sudden increase in total sugar content, consisting mainly of sucrose and its galactosides. An increase in these sugars was paralleled at the ultrastructural level by a prominent vesicular and cisternal ER-system originating in the cytoplasm. Full acclimatization (LT50 lower than -50°C) was obtained within 3 weeks (mid-November) and coincided with the maximum content of sucrose and its galactosides (reaching 300 µg/mg DW in ray cells) and with a prominent development of vesicular and cisternal ER. Evidence for the accumulation of these sugars in this ER-system is discussed, and a proposal for its functional significance both as an intrinsic device for dehydrating the protoplast during frost hardening, and for delivery of membrane material to the plasma membrane, is presented. Transient increases of winter temperature above freezing were paralleled by a noticeable decrease in frost resistance (LT50 -40 to -30°C), declining sugar levels and the gradual diminution of the ER-vesicles in the cytoplasm. In March, at temperatures of 5-10°C, sugar content rapidly decreased concomitantly with the disappearance of vesicles from the cells. A decrease in frost resistance, in contrast, began later (mid-April), and it is suggested that non-sugar compounds were involved in frost hardiness at this stage.