Blizzard of TiO 2 price rises melts a bit as customers dig their heels in

Abstract

Ray parenchyma cells of poplar wood (Populus × canadensis Moench ‹robusta›) were studied for changes in ultrastructure and in the level of various sugars in fall, winter and spring in relation to freezing resistance as measured by LT50 (temperature at which 50% of the cells are injured). Before acclimation (mid-October), frost hardiness is low (LT50 −3 to −5 °C), sugar levels have not yet increased while the ray cell protoplast has already become dense in structure. This is due mainly to the elimination of large vacuoles, their replacement by small storage protein vacuoles, and the vast accumulation of storage material in general, i.e. in amyloplasts, protein bodies, and oleosomes. Frost acclimation started during leaf fall (end of October) concomitant with a sudden increase of total sugar content, consisting mainly of sucrose and its galactosides. An increase in these sugars is paralleled at the ultrastructural level by a prominent vesicular and cisternal ER-system originating in the cytoplasm. Full acclimation (LT50 lower than −50 °C) is obtained within 3 weeks (mid-November) and coincides with maximum content of sucrose and its galactosides (reaching 300 µg mg−1 DW in ray cells) and with a prominent development of vesicular and cisternal ER. The evidence for the accumulation in this elaborated ER-system of these sugars 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 made. Transient rises of winter temperature above freezing were paralleled by a noticeable decrease in frost resistance (LT50 −40 to −30 °C), declining sugar levels, and by gradual diminution of the particular ER-vesicles in cytoplasm. In March, at temperatures of 5 to 10 °C, sugar content rapidly lowers concomitandy with complete vanishment of vesicles from the cells. A decrease in frost resistance, in contrast, begins later, e.g. in mid-April, suggesting the involvement in frost hardiness of non-sugar compounds at this stage.