Understanding key adaptation traits is crucial to developing new cultivars with broad adaptations. The main objective of this research is to understand the genetic basis of winter hardiness (WH) and fall dormancy (FD) in alfalfa and the association between the two traits. QTL analysis was conducted in a pseudo-testcross F1 population developed from two cultivars contrasting in FD (3010 with FD = 2 and CW 1010 with FD = 10). The mapping population was evaluated in three replications at two locations (Watkinsville and Blairsville, GA). FD levels showed low to moderate correlations with WH (0.22–0.57). Assessing dormancy in winter is more reliable than in the fall in southern regions with warm winters. The mapping population was genotyped using Genotyping-by-sequencing (GBS). Single dose allele SNPs (SDA) were used for constructing linkage maps. The parental map (CW 1010) consisted of 32 linkage groups spanning 2127.5 cM with 1377 markers and an average marker density of 1.5 cM/SNP. The maternal map (3010) had 32 linkage groups spanning 2788.4 cM with 1837 SDA SNPs with an average marker density of 1.5 cM/SNP. Forty-five significant (P < 0.05) QTLs for FD and 35 QTLs for WH were detected on both male and female linkage maps. More than 75% (22/28) of the dormancy QTL detected from the 3010 parent did not share genomic regions with WH QTLs and more than 70% (12/17) dormancy QTLs detected from CW 1010 parent were localized in different genomic regions than WH QTLs. These results suggest that the two traits have independent inheritance and therefore can be improved separately in breeding programs.
The plant survival of three ecotypes of seashore paspalum (Paspalum vaginatum Swartz) submitted to freezing temperatures was evaluated by two methods: electrolyte leakage and freeze shock‐recovery. Bermudagrass [Cynodon dactylon (L.) Pets. cv. Midiron] was used as a check. Acclimated and nonacclimated plants were evaluated from 1994 to 1995 in Georgia by both methods to compare their performance. Results indicated that seashore paspalum was slower to acclimate than Midiron bermuda under the treatment conditions used in this experiment. However, a significant acclimation effect was found in HI‐1 paspalum, indicating the presence of variability for this trait within the species. The LT50 determinations demonstrated the superior cold hardiness of Midiron compared with seashore paspalum. Among the paspalum ecotypes, HI‐1 ranked superior to Adalayd and PI 299042, respectively, when plants were acclimated (AC). When nonacclimated (NA), Adalayd ranked superior to the other two ecotypes. These genotypic rankings were consistent for both methods of evaluation, indicating their effectiveness for efficiently screening cold hardiness responses among multiple ecotypes. Electrolyte leakage curves indicated that the lethal electrolyte leakage levels for AC and NA paspalum ecotypes were similar. Additional studies are required to evaluate the mechanism present in HI‐l, which showed significant acclimation effects on plant survival.
The purpose of the present study was to apply the Richards function to fit electrolyte and phenolic leakage data for several taxa of woody plants subjected to freezing stress and to determine how the curve inflection point relates to the lethal temperature range. The lowest survival temperature of Fraxinus americana, Lagerstroemia cv. Natchez, Magnolia grandiflora, Rhododendron cv. Red Ruffle and Zelkova serrata was determined based on visual evaluation of oxidative browning facilitated by a modified regrowth test and differential thermal analysis. Lethal injury occurred in all cases within a range of 3°C below lowest survival temperature. Using the Richards function inflection point as an estimate of lethal temperature led to an overestimation of freezing tolerance in most taxa. This overestimation was greater for stems than leaves, and was greater in winter than in summer. The lethal temperature range generally coincided with the initial increase in leakage caused by freezing. The lethal temperature range also was determined by using a point of interception of the lower asymptote of a curve with a line tangential to the inflection point. In most taxa tested estimated lethal temperature based on the point of interception provided an improvement over the estimate based on the point of inflection.
We determined whether increase in cold hardiness of Rhododendron cv. Catawbiense Boursault induced by water stress was correlated with changes in tissue water relations. Water content of the growing medium was either maintained near field capacity for the duration of the study or plants were subjected to drought episodes at different times between 15 July and 19 February. Watering during a drought episode was delayed until soil water content decreased below 0.4 m3 m−3 then watering was resumed at a level to maintain soil water content between 0.3 and 0.4 m3 m−3. Cold hardiness was evaluated in the laboratory with freeze tolerance tests on detached leaves. Water relations parameters were determined using pressure‐volume analysis. Exposure to drought episodes increased cold hardiness during the cold acclimation stage in late summer and fall but not during the winter. When water‐stressed plants were re‐watered to field capacity, the previous gain in cold hardiness gradually disappeared. Water relations parameters correlating with seasonal changes of cold hardiness included dry matter content (r =−0.67). apoplastic water content (r =−0.60), and water potential at the turgor loss point (r = 0.40). Changes of cold hardiness in water‐stressed plants in reference to well‐watered plants were correlated with changes of all water relations parameters, except for osmotic potential at full turgor (r = 0.13). It is proposed that water stress reduced the hydration of cell walls, thereby increasing their rigidity. Increased rigidity of cell walls could result in a development of greater negative turgor pressures at subfreezing temperatures and therefore increased resistance to freeze dehydration.
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