Woody cover in African savannas: the role of resources, fire and herbivory

Aim To determine the functional relationships between, and the relative importance of, different driver variables (mean annual precipitation, soil properties, fire and herbivory) in regulating woody plant cover across broad environmental gradients in African savannas. Location Savanna grasslands of East, West and Southern Africa. Methods The dependence of woody cover on mean annual precipitation (MAP), soil properties (texture, nitrogen mineralization potential and total phosphorus), fire regimes, and herbivory (grazer, browser + mixed feeder, and elephant biomass) was determined for 161 savanna sites across Africa using stochastic gradient boosting, a refinement of the regression tree analysis technique. Results All variables were significant predictors of woody cover, collectively explaining 71% of the variance in our data set. However, their relative importance as regulators of woody cover varied. MAP was the most important predictor, followed by fire return periods, soil characteristics and herbivory regimes. Woody cover showed a strong positive dependence on MAP between 200 and 700 mm, but no dependence on MAP above this threshold when the effects of other predictors were accounted for. Fires served to reduce woody cover below rainfall‐determined levels. Woody cover showed a complex, non‐linear relationship with total soil phosphorus, and was negatively correlated with clay content. There was a strong negative dependence of woody cover on soil nitrogen (N) availability, suggesting that increased N‐deposition may cause shifts in savannas towards more grassy states. Elephants, mixed feeders and browsers had negative effects on woody cover. Grazers, on the other hand, depressed woody cover at low biomass, but favoured woody vegetation when their biomass exceeded a certain threshold. Main conclusions Our results indicate complex and contrasting relationships between woody cover, rainfall, soil properties and disturbance regimes in savannas, and suggest that future environmental changes such as altered precipitation regimes, N‐enrichment and elevated levels of CO₂ are likely to have opposing, and potentially interacting, influences on the tree–grass balance in savannas.     

Differential morphometric and micro-morpho-anatomical responses toward types of culture vessels used in micropropagation of Hemidesmus indicus (L.) R. Br.

Plant Cell, Tissue and Organ Culture (PCTOC) - Plantlets of Hemidesmus indicus (L.) R. Br. (Indian sarasaparilla) were developed in vitro in two different types of culture vessels and closure...     

Corticosteroid regulation of gonadotropin secretion and induction of ovulation in the rat

In the human polycystic ovarian syndrome, glucocorticoids have been demonstrated to have beneficial effects in inducing ovulation in a number of cases. These beneficial effects were assumed to be due to suppression of adrenal overproduction of androgens. However, the possibility exists that glucocorticoids may directly regulate gonadotropin secretion and thereby improve menstrual rhythm and ovulatory activity. Herein, we report that the corticoid, deoxycorticosterone, and the synthetic glucocorticoid, triamcinolone acetonide, like progesterone (P4), are able to induce luteinizing hormone and follicle-stimulating hormone surges and facilitate ovulation in the pregnant mare serum gonadotropin-primed rat. This effect is not shared by cortisol. Prolactin release was also stimulated by deoxycorticosterone, cortisol, and progesterone, but not by triamcinolone acetonide. Similar to progesterone, triamcinolone acetonide and deoxycorticosterone administration caused a loss of fluid retention in the uterus. This effect of triamcinolone acetonide and deoxycorticosterone may be related to progesterone action as opposed to anti-inflammatory action since cortisol had no effect on uterine fluid retention. These findings raise the possibility that the beneficial effects seen with glucocorticoids in inducing ovulation in polycystic ovarian syndrome may be due in part to their direct effects upon the release of gonadotropins.     

Structure-activity relationships of some unique estrogens related to estradiol are predicted by fit into DNA

The estrogenic activity of 11 beta-acetoxy estradiol, 11 beta-hydroxy estradiol, 11 alpha-hydroxy estradiol and 9 beta-estradiol was compared to estradiol using the restoration of uterine weight and prevention of LH rise in immature ovariectomized rats as endpoints of the assay. There was a good correlation between results using the two methods and estrogenic activity was found to be in the following order: 11 beta-acetoxy estradiol greater than estradiol greater than 9 beta-estradiol greater than 11 beta-hydroxy estradiol greater than 11 alpha-hydroxy estradiol. The biological activities of these compounds could be explained on the basis of stereochemical complementarity to the structure of DNA.     

Morphometric analysis of the autophagic and crinophagic lysosomal systems in mammotropes throughout the estrous cycle of the rat

Summary The crinophagic and autophagic lysosomal systems were studied in mammotropes (prolactin secreting cells) of the adenohypophysis throughout the estrous cycle of the rat. By means of morphometric analysis, it was found that the volume of secondary autophagic lysosomes was usually greater than that of the crinophagic type. Although the volumes of both secondary autophagic and crinophagic lysosomes were minimal throughout proestrus and diestrus 2, the autophagic lysosomal volume per mammotrope was elevated during the estrous period. The volume of secondary crinophagic lysosomes per mammotrope increased during late estrus and remained elevated throughout early diestrus 1. Furthermore, there was an inverse relationship between the volume of mature secretory granules per cell and of the crinophagic system. These data suggest a role for lysosomes in the regulation of synthesis and secretion of prolactin by the adenohypophysis of the rat.Supported by grant HD 11571 from the National Institutes of Health