Nitrate Uptake during Recovery from Nitrogen Deficiency

Two-week-old nitrogen-deficient wheat plants attained a high rate of nitrate uptake on the first day of exposure to nutrient solutions supplemented with KNO₃. Ammonium uptake from similar solutions supplemented with NH₄NO₃ was also high during the first day of exposure, but nitrate uptake from this solution was lower than from the KNO₃ treatment. During the next two to three days there was a progressive decrease in uptake of both nitrogen ions. A steady increase in uptake then occurred as the plants fully recovered from the nitrogen-deficient state. The transient low nitrate uptake after three or four days of exposure to KNO₃ was not due to an excessive accumulation of nitrate in the tissue, nor to a failure in nitrate reduction as indicated by the rate of nitrate accumulation relative to the uptake rate. Nitrogen supplied as ¹⁵N-nitrite during the low uptake period was effectively incorporated into organic forms and effectively translocated to the shoots. Failure of the root tissue to increase in soluble carbohydrates during illumination was characteristic of the low uptake period. This contrasted with an increase in root soluble carbohydrates in the light during rapid uptake associated with full recovery from the nitrogen-deficient state. It is concluded that carbohydrate translocation to the root system was insufficient during the intermediate recovery period for optimal nitrate uptake, although it was sufficient for effective reduction and translocation of nitrate and reduced nitrogen. Ammonium uptake from NH₄NO₃ was restricted during darkness by the third day whereas there was little difference between light and dark periods in nitrate uptake from KNO₃ until about the sixth day of recovery. The extent to which ammonium restricted nitrate uptake increased progressively for two or three days following which a lessening influence seemed evident, and the effects were not directly associated with the rate of ammonium uptake.     

Phylogenetic Distribution and Significance of the Hypothalamic Releasing Hormones

SYNOPSIS. The mammalian hypothalamic releasing factors regulatingthyroid, gonadal and adrenal function as well as growth hormonesecretion have been isolated, characterized and their nucleotidesequences determined. In general, their hypophysiotropic effectsare replicated in lower vertebrates though thyrotropin releasinghormone (TRH) does not appear to stimulate thyroid functionin amphibia and fish. The releasing factors, or peptides structurallyrelated to these substances, are found throughout the CNS ofall vertebrates where they likely function as neurotransmittersor neuromodulators. High concentrations of TRH and other neuralpeptides includingsauvagine, which is related to corticotropinreleasing factor (CRF) and has CRF-like activity, are foundin amphibian skin, a neural crest derived tissue. mRNA extractedfrom the skin of Xenopus laevis was cloned and led to the identityof the DNA sequence of pre-pro TRH. Molecular variants of somatostatinhave been recognized from studies on the pancreatic islets ofthe anglerfish and catfish. Within mammalian species there isheterogeneity of growth hormone releasing factor (GRF), the44 (and 40) amino acid peptides isolated from a human(h) pancreatictumor. In the teleost brain-pituitary, 2 distinct hGRF-likeneuronal systems are present. Additionally, various molecularforms of hGRF exist in the fish brain showing structural changesfrom the human variety.     

4. Vavilov's Law of Homologous Series—is it relevant to potatoes?

Nikolai Ivanovich Vaviloc publsihed his Law of Homologous Series in 1920, but did not include any ideas oon homologous variation in potatoes. The relevance of this fundamental concept of genetic diversity to potatoes is discussed in relation to disease and post resistance in wild potatoes from mexico and the Andes of South America.     

Are dams hotspots for Didymosphenia geminata blooms?

1. The diatom Didymosphenia geminata has emerged in recent years as a globally invasive species. Although considered native to North America, reports of nuisance blooms have increased over the last decade.
2. Previously, we determined that D. geminata was ubiquitous in two major headwaters of the South Saskatchewan River Basin (SSRB), Alberta, Canada, but found it only bloomed at certain sites, including those immediately downstream from dam outfalls. To evaluate the role of dams in the abundance and blooming of D. geminata , we compared sites just below dams to unregulated upstream reference sites in six dammed rivers of the SSRB.
3. There was a high degree of seasonal variability in D. geminata abundance among sites, but statistical analyses showed a significant propensity for the diatom to have higher cell densities and an increased frequency of blooms at dam sites.
4. Important predictor variables of D. geminata abundance included dam presence, water clarity and total phosphorus concentration. When data from dam sites were analysed, a multiple regression model using mean discharge and pH as independent predictors explained 73% of the variation in D. geminata cell density.
5. Analysis of 3 years of data from one study river (Red Deer River) revealed consistently higher D. geminata cell densities at the dam site compared to the upstream reference. This analysis also showed that average cell density fluctuated by orders of magnitude from year-to-year.
6. Due to the ecological and aesthetic concerns regarding the global spread and blooming of D. geminata , we recommend that dams be viewed as key candidates for mitigating blooms in flow regulated systems.     

The ecology and social behaviour of the Neotropical butterfly Heliconius xanthocles Bates in Colombia

The mimicry, population ecology¹, adult behaviour and life cycle of Heliconius xanthocles were studied at two sites in Colombian tropical forest. The results were compared with the known biology of other Heliconius species. H. xanthocles is probably unpalatable and is a Mullerian comimic of some other species. In the Amazon basin H. xanthocles belongs to the 'dennis ray' mimicry ring of the Heliconiini, which includes Eueides tales, H. bumeyi, H. elevatus, H. melpomene and H. erato. In the Rio Negro valley, where the latter species either do not exist or have different mimetic allegiances, the 'clennis-ray' pattern of H. xanthocles breaks down. The adults are pollen-feeders, like other Heliconius. but they also visit other food sources such as bird droppings. During a mark-recapture programme in the Rio Negro, adults were found to be almost entirely restricted to small areas near their foodplant Passiflorapraeacuta. Females were rarer than males, but this was probably due to a behavioural difference between the sexes. The males had a Fisher-Ford survival rate of 0.95 per day and a life expectancy of 19 days. Population size estimates of males were between 11 and 17 individuals. These estimates are lower than, but comparable to those for other Heliconius. Males as well as females visit the larval toodplant, and an adult male was observed hovering over a small group of larvae. The females lav batches of 12–41 eggs. The largest batch (41 eggs) was laid cooperatively by two females. The larvae are gregarious feeders on shoot-tips of Passiflora praeacuta. and all lile stages are described for the first time. Predation on the larvae by a bug is described. The pupa has a distinctive morphology unlike that of other Heliconius. The adult population ecology, mimicry and larval behaviour are discussed.     

Effects of Mecoprop (an Auxin Analogue) on Ethylene Evolution and Epinasty in Two Biotypes of Stellaria media

Petiolar epinasty and the production of ethylene (ethene) werestudied in chickweed biotypes, Stellaria media, treated withthe herbicide and auxin analogue (RS)-2-(4-chloro-o-tolyloxy)propionicacid, potassium salt, common name mecoprop. This compound causedsevere epinasty and stimulated the production of ethylene fromshoot explants. However, when intact plants were treated withethylene, the leaves became only slightly epinastic. The ethyleneprecursor, 1-aminocyclopropane-I-carboxylic acid (ACC), at concentrationswhich stimulated the release of ethylene, was equally ineffectivein causing epinasty. Furthermore, 2, 5-norbornadiene, a specific,competitive inhibitor of ethylene action, only partly alleviatedmecoprop-induced epinasty. The responses observed in chickweedwere compared with those produced in tomato plants. ACC inducedepinasty in tomato within 2 h and these symptoms were completelyinhibited by norbornadiene. However, as in chickweed, the inhibitorgave only partial reversal of mecoprop-induced epinasty, implyingthat the epinastic response caused by the herbicide was notattributable to ethylene alone. We therefore suggest that mecoprop-inducedepinasty is a result of the combined ethylene-stimulating andgrowth-promoting properties of the herbicide. Mecoprop-stimulated ethylene evolution was initially significantlygreater in a herbicide-resistant, compared with a more susceptiblebiotype of chickweed. The significance of this finding is discussedin relation to the mechanism of mecoprop resistance in chickweed. Epinasty, ethylene, (RS)-2-(4-chloro-o-tolyloxy)propionic acid, mecoprop, herbicide resistance, chickweed, Stellaria media L., tomato, Lycopersicon esculentum L.