Search Results
Abstract
Deep supercooling was found in the stem tissues of all the Pyrus species studied. There was more than 1 low temperature exotherm resulting from the freezing of supercooled water in stem tissue, and these exotherms were associated with the tissue injury. The supercooled water in the stems of P. nivalis Jacq., P. cordata (Desv.) Schneider and P. elaeagrifolia Pall, was found in both xylem and bark tissues. The supercooling characteristics of vegetative and flower buds are also described. The hardiest and least hardy species found were P. caucasica Fed. and P. pashia D. Don., respectively.
Abstract
Deep supercooling of stem tissue water was found in all the native species of rose (Rosa spp.) studied. Freezing of this supercooled water was associated with injury to the stems, indicating that maximum cold hardiness of these species is limited to about −40°C. Therefore, these species have some potential for use in breeding to develop cold-hardy cultivated roses, but their hardiness would be limited to −40°C by the supercooling characteristic.
Abstract
Freezing injury of highbush blueberry flower buds (Vaccinium corymbosum L. cv. Rancocas and an experimental hybrid) were investigated after natural freezing and by differential thermal analysis (DTA). Terminal buds were less hardy than median and basal buds on the same twig. Apical, less mature florets were also less hardy than median or basal, more mature florets within each bud. DTA analysis of intact flower buds showed that a rate independent free water exotherm was followed by numerous rate dependent floret exotherms. Floret lethality was associated only with the rate dependent low temperature exotherms. Buds were held over water-glycerol solutions in desiccators so that they came into equilibrium with a relative humidity of 100%, 98% or 96% at 4°C. Artificially hydrated buds were hardy to –10° and artificially dehydrated buds were 15°C hardier. A mean survival temperature of the florets (MSTF) from DTA analysis, derived at slow cooling rates related closely to lowest survival temperature (LST66) from artificial freezing hardiness tests. In mid-winter, under environmental conditions of low temperature and low relative humidity, flower buds of a hardy hybrid lost all DTA exotherms and were hardy to at least –40°. Tissue dehydration and accompanying loss of floret low-temperature exotherms was associated with extreme cold tolerance of blueberry flower buds with the hardy experimental hybrid.
Abstract
Field and laboratory tests of cold hardiness were conducted on 8 cultivars of bermudagrass (Cynodon spp.) and 2 cultivars of Paspalum vaginatum. The cultivars of bermudagrass were more cold hardy than those of Paspalum. ‘Brookings’ bermudagrass was the hardiest turfgrass.
Abstract
The technique of differential thermal analysis was used to examine midwinter hardiness of bud and stem tissues of Vitis riparia. Low temperature freezing points (exotherms) were found to occur in both stem and deacclimated bud tissues. Exotherms were not exhibited by fully acclimated buds. The temperature at which the stem tissue exotherm occurred was independent of cooling rate and duration of a thawing pretreatment of 6 to 48 hours. The initiation temperature of the bud exotherms varied with cooling rate and duration of a thawing pretreatment of 6 to 48 hours; an increase in either caused the bud exotherm to occur at a higher temperature than that of the control. It is postulated that deep supercooling of the intracellular water in stem and bud tissues is the characteristic which limits the northern distribution of V. riparia.
Abstract
Stems of 49 woody species native to North America were collected from the field in Minnesota in January and subjected to controlled freezing tests. Characterization of the freezing process of xylem tissues by differential thermal analysis revealed that some species had no exotherms at low temperatures, some had small exotherms between –41° and –47°C, and others had large exotherms in the same temperature range. Generally very hardy species with ranges extending into northern Canada and Alaska had no exotherms. Species with small exotherms were native to the northern United States and southern Canada, and large exotherms were generally characteristic of the least hardy species studied. The low temperature exotherms occurred at the same temperature that results in xylem death of most of the plants studied. Plants with low temperature exotherms tend to have ring-porous xylem and the temperature of the low temperature exotherms was correlated with the minimum temperatures of the boundaries of the northern range of the species tested.
Abstract
Viability of pollen grains of isogenic sibling bean (Phaseolus vulgaris L.) selections of known tolerance of sensitivity to high temperatures (HT), as previously determined by pod retention and seed yield, was compared to that of a common parent bean selection and a cowpea [Vigna unguiculata (L.) Walp.] cultivar. Exposure of newly opened flowers to temperatures of 35° or 41°C reduced the viability of pollen grains in all bean selections. Pollen of all sibling selections was less affected by HT than pollen of their common parent suggesting transgressive segregation of factors for HT tolerance. At 41°, most pollen grains were destroyed in the parent bean selection and the 2 HT-sensitive siblings, whereas 44% to 55% of the pollen grains appeared to be viable in the 2 HT-tolerant siblings. Pollen viability of the HT-tolerant cowpea cultivar was not reduced by temperatures to 41°. Pollen staining indicated an interrelationship between pollen viability and tolerance to HT stress among the bean selections. The technique described has the potential for rapid selection of HT-tolerant genotypes in hybrid populations.
Abstract
Leaves of cold-acclimated lemon [Citrus limon (L.) Burm. f.], grapefruit (C. paradisi Macf.), orange [C. sinensis (L.) Osbeck], and mandarin (C. unshiu Marc.) trees ranged in cold hardiness from −4 to −11°C. No significant differences in water content (g H2O/g dry weight) or melting point depression were observed. Plots of liquid water content during freezing (g H2O/g dry weight) vs. temperature were similar for the 4 citrus species. The tissues apparently deviated from ideal freezing behavior because less ice was formed. The reduced ice formation could not be accounted for by osmotic effects. Negative pressure potential developed during freezing is hypothesized to play a role in tissue water potential in frozen systems. It was concluded that hardier Citrus leaves survive freezing of a larger fraction of their tissue water.
Abstract
The cold hardiness of 6 cool season grass genera was compared in mid-winter under controlled freezing conditions. Creeping bentgrasses (Agrostis palustris Huds.) tolerated the lowest temperatures whereas perennial ryegrasses (Lolium perenne L.) was the least hardy. Kentucky bluegrass (Poa pratensis L.) readily loses cold hardiness when exposed to warm conditions and did not reharden when exposed to cool temperatures. Appreciable hardiness in Kentucky bluegrass was induced by simulating drought conditions.
Abstract
Two bean cultivars and one breeding selection with different pod-retention characteristics were grown at mean soil moisture tension (MSMT) of 0.05 and 0.1 MPa in 2 separate plantings. In the 5 May planting, flower buds developed during the 1st 3½ weeks of flowering, were dated and counted, and those developing mature pods were identified. Sixty-five percent to 90% of all pods that reached full maturity were from floral buds that reached anthesis during the 1st 2 weeks of flowering. The percentage of pods reaching maturity varied among cultivars. About 40% of the floral buds that developed on the determinate bean selection were retained to full pod maturity. Only 20% to 25% of the floral buds developed on each of the indeterminate cultivars were retained to full pod maturity. An increase in the MSMT from 0.05 to 0.1 MPa in the 23 June planting reduced the number of pods and seeds/plant and total seed weight/plant by 20% to 40%, but the number of seeds/pod and weight/seed was not influenced by MSMT or by number of pods produced on either of the dry bean cultivars or the breeding selection.