Small understorey trees have greater capacity than canopy trees to adjust hydraulic traits following prolonged experimental drought in a tropical forest.

Future climate change predictions for tropical forests highlight increased frequency and intensity of extreme drought events. However, it remains unclear whether large and small trees have differential strategies to tolerate drought due to the different niches they occupy. The future of tropical forests is ultimately dependent on the capacity of small trees (<10 cm in diameter) to adjust their hydraulic system to tolerate drought. To address this question, we evaluated whether the drought tolerance of neotropical small trees can adjust to experimental water stress and was different from tall trees. We measured multiple drought resistance-related hydraulic traits across nine common neotropical genera at the world's longest-running tropical forest throughfall-exclusion experiment and compared their responses with surviving large canopy trees. Small understorey trees in both the control and the throughfall-exclusion treatment had lower minimum stomatal conductance and maximum hydraulic leaf-specific conductivity relative to large trees of the same genera, as well as a greater hydraulic safety margin (HSM), percentage loss of conductivity and embolism resistance, demonstrating that they occupy a distinct hydraulic niche. Surprisingly, in response to the drought treatment, small trees increased specific hydraulic conductivity by 56.3% and leaf:sapwood area ratio by 45.6%. The greater HSM of small understorey trees relative to large canopy trees likely enabled them to adjust other aspects of their hydraulic systems to increase hydraulic conductivity and take advantage of increases in light availability in the understorey resulting from the drought-induced mortality of canopy trees. Our results demonstrate that differences in hydraulic strategies between small understorey and large canopy trees drive hydraulic niche segregation. Small understorey trees can adjust their hydraulic systems in response to changes in water and light availability, indicating that natural regeneration of tropical forests following long-term drought may be possible.

Psoralen phototoxicity: correlation with serum and epidermal 8-methoxypsoralen and 5-methoxypsoralen in the guinea pig.

Serum and epidermal concentrations of 8-methoxypsoralen and 5-methoxypsoralen 2 hours after oral administration to guinea pigs were determined by high-performance liquid chromatography. A linear relation was found between the serum and epidermal concentrations of 8-methoxypsoralen. In addition, a relation was found between serum concentrations of 5-methoxypsoralen and 8-methoxypsoralen and the appearance of phototoxicity. The lower phototoxicity of orally administered 5-methoxypsoralen as compared to 8-methoxypsoralen in the guinea pig appears to be due to its reduced concentrations in the epidermis, the primary site of the phototoxic events.

Sensitization potentials of methyl, ethyl, and n-butly methacrylates and mutual cross-sensitivity in guinea pigs.

Guinea pigs could be strongly sensitized to methyl, ethyl, and n-butyl methacrylates in ethanol or olive oil by the topical route, or in saline by the intradermal route. For elicitation of skin reactons, topical challenge with the compounds in olive oil or intradermal challenge with saline as the solvent was neccessary. Topical challenge with the methacrylates in ethanol failed to elicit any allergic skin reactions because of their volatility. All sensitized animals responded strongly not only to the inducing methacrylate but also to the other methacrylates, showing that mutual cross-sensitivity had occurred. Since methyl methacrylate has been reported to be a potent sensitizer in humans, the guinea-pig model described here may be useful for screening products before marketing.

Dermal carcinogenicity study by mouse-skin painting with 2,4-toluendediamine alone or in representative hair dye formulations.

The chronic toxicity of 2,4-toluenediamine (2,4-TDA) alone or in conbination with a hair dye complex (2,5-toluenediamine, p-phenylenediamine, and resorcinol) was studied in Swiss-Webster mice of both sexes by a skin-painting technique. The predominant neoplasms seen in these mice were primary pulmonary adenomas and adenocarcinomas. Skin neoplasms were seen in most groups of mice, including untreated control mice. Statistical analysis of the incidences of skin neoplasms among the various groups of mice did not show any significant differences. The 2,4-TDA alone or mixed with the hair dye complex did not produce any abnormal proliferation and maturation of the squamous epithelium of the skin. The 2,4-TDA under our experimental conditions was found to be nontoxic and noncarcinogenic to the skin of mice.

Difference in topical and systemic reactivity of psoralens: determinations of epidermal and serum levels.

Topical application of 8- and 5-methoxypsoralen (8-MOP, 5-MOP), psoralen, and 4,5',8-trimethylpsoralen (TMP) to the epilated backs of albino guinea pigs followed by UV irradiation at 320-400 nm resulted in elicitation of similar phototoxicity. However, the systemic phototoxicity determined after oral administration of these psoralen derivatives differed significantly. Serum and epidermal levels of 8-MOP, 5-MOP, and TMP were determined 2 hours post administration. We found a linear relationship between serum and epidermal levels of 8-MOP, but the levels of the other psoralen derivatives were significantly lower. We believe that the reduced oral phototoxicity of 5-MOP and TMP relative to 8-MOP in the guinea pig is due to its reduced levels in the epidermis.