The lack of evolution of weed resistance to the herbicide glyphosate has been considered from several perspectives. Few plant species are inherently resistant to glyphosate. Furthermore, the long history of extensive use of the herbicide has resulted in no verified instances of weeds evolving resistance under field situations. Unique properties of glyphosate such as its mode of action, metabolism, chemical structure and lack of residual activity in soil may explain this observation. Selection for glyphosate resistance of crops using intense whole plant and cell/tissue culture techniques, including mutagenesis, has had only limited success and is unlikely to be duplicated under normal field conditions. Information obtained in the development of glyphosate-resistant crops suggests that target-site alterations that decrease the herbicidal activity of glyphosate also may lead to reduced survival of a weed. In addition, the complex manipulations that were required for the development of glyphosate-resistant crops are unlikely to be duplicated in nature to evolve glyphosate-resistant weeds.
Studies were conducted to determine species response to low—temperature environments. Apical development was found to progress at near—freezing temperatures throughout the winter except when the plants were encased in a solid soil—ice matrix. Claytonia lanceolata and Nemophila breviflora shoots developed above the soil surface, under snow cover. In both species, chlorophyll content was inversely proportional to depth of snow cover. Spectrophotometric analysis of light penetration through snow indicated about 10% penetration through shallow layers (ca. 3.5 cm), and 0.01% through layers of 69 cm. C. lanceolata, Orogenia linearifolia, and Erythronium grandiflorum plants developed complete floral structures during the winter months, these being protected by leaves or an apical sheath (Orogenia).
Effects of low temperatures on cell ultrastructures of three grass species were studied. Secale cereale L., Cynodon dactylon (L.) Pers., and Paspalum notatum Flugge seedlings were given treatments of −5, 0, 10, or 25 C for 3 days and then evaluated for cellular response. Electron micrographs showed chloroplasts to be the most sensitive organelle in all three species, with the temperate Secale plastids responding differently than plastids from the subtropical and tropical species. Rough endoplasmic reticulum increased, but dictyosome numbers decreased in all species as treatment temperatures were lowered to 0 C. Mitochondria expanded slightly at reduced temperatures, but with little difference detected between species. The study indicated that the various cellular components of these species responded differently to reduced temperatures, with the species differing mainly in degree of response.
The influence of reduced osmotic potentials on soybean (Glycine max L.) tissue cultures was evaluated. Reduced osmotic potentials were generated by addition of mannitol, sorbitol, glucose, or sucrose to a standard medium. Cells in control callus were comparatively large and about half were elongated or irregularly shaped. As osmotic potential decreased, all cells became smaller and spherical, regardless of which osmoticum was used. The addition of metabolically active or inert osmotica resulted in increased callus production. Maximum callus growth (fresh weight) occurred when the osmotic potential of the culture medium was lowered 8 to 12 bars. Decreasing osmotic potentials by 2 to 20 bars did not increase the frequency of adventitious bud differentiation above the low frequency observed on the standard medium.
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