Effects of freezing temperatures on early life stages of native trees of different elevational origin: implications for tree recruitment in seasonally dry mountain forests.

In mountain forests, tree regeneration is limited by increasingly frequent frosts with increasing elevation. We investigated the effects of exposure to freezing temperature on early life stages of two native trees of different elevational origin in a seasonally dry mountain forest. We hypothesized that the negative effects of freezing exposure on performance of early life stages increases as freezing temperature decreases, and that frost resistance increases in plants of high elevational origin. We collected seeds of two tree species (Kageneckia lanceolata and Lithraea molleoides) from populations located at different elevations and grew seedlings and saplings in a greenhouse. Dry seeds, imbibed seeds and 1-month-old seedlings were exposed to seven temperature treatments ranging from 4 °C to -20 °C, while 12-month-old saplings were exposed to four temperature treatments from -8 °C to -20 °C. After freezing exposure in a climate chamber, we monitored seed germination and seedling and sapling survival. Germination of K. lanceolata decreased with decreasing temperature only for imbibed seeds from mid- and high elevations, whereas germination of L. molleoides slightly increased with decreasing temperature only for imbibed seeds from high elevations. For both species, seedling survival decreased with decreasing temperature. For K. lanceolata, the negative effects of freezing temperatures were weaker as elevational origin of seeds increased, whereas L. molleoides showed the opposite pattern. For both species, saplings only survived at the mildest applied freezing temperature (-8 °C). We conclude that effects of climatic variation associated with elevation depend on the study species and life stage. The observed patterns could be caused by maternal effects, which are absent at the sapling stage. Moreover, temperatures below -8 °C can limit recruitment since partial mortality of seedlings and saplings occurred at such values.

Effect of ethanol on choline transport in rat jejunum.

The effect of ethanol on choline transport across the rat jejunum was studied by intraluminal perfusion in vivo and by influx measurement across the brush-border membrane in vitro. Acute ethanol administration (4 g/kg) through a gastric tube caused an increase in net choline absorption within 1 h. The increase was prevented by pretreatment with pyrazole, an inhibitor of ethanol metabolism. Chronic ethanol administration also caused an increase in choline absorption, the effect being unrelated to the nutritional changes that occur with ethanol ingestion. In contrast, direct instillation of 0.65 M ethanol through the perfusate caused no changes in choline absorption, and the perfusion of a 1.14 M solution even decreased absorption. The in vitro influx of choline across the mucosal membrane of the isolated rat jejunum was also enhanced by pretreatment with ethanol given by gavage 1 h prior to experimentation. Similarly, the ethanol-related increase in the influx rate was inhibited by pyrazole but was unaffected by acetaldehyde or acetate. Like ethanol, pretreatment of rats with methanol stimulated the choline influx rate. The results suggest that ethanol metabolism, rather than the direct effect of ethanol by itself, stimulates the absorption and influx of choline into the rat jejunum. The effect is not produced by the primary metabolites of ethanol, acetaldehyde, or acetate but is very likely related to stimulation by other products of ethanol metabolism.

Filiform polyposis of the colon presenting as massive hemorrhage: an uncommon complication of Crohn's disease.

A case of localized filiform polyposis of the colon is presented. This lesion was part of a more generalized inflammatory bowel disease with clinicopathological features indicative of Crohn's disease. An unusual feature of this case was profuse rectal bleeding, initially from a solitary sigmoid ulcer and later from the area of filiform polyposis. A step by step clinicopathological follow-up emphasizing the development of filiform polyps is provided.