Degree Type

Dissertation

Date of Award

2008

Degree Name

Doctor of Philosophy

Department

Agronomy

First Advisor

Dr. Allen D. Knapp

Second Advisor

Dr. Charles E. Brummer

Abstract

Medicago truncatula is a close annual relative to alfalfa. Understanding cold acclimation (CA) is important for concurrently increasing autumn yield and winter survival in alfalfa. Such investigations are difficult because of its complex genetic makeup, and outcrossing with inbreeding depression. This study was conducted to understand CA in M. truncatula for determining if it could be used as a model to help dissect the genetic control of autumn yield and cold tolerance in alfalfa. Although M. truncatula has less agronomic value than alfalfa, its small genome and short generation time makes it useful as a model crop for legume genomics. Because M. truncatula is exposed to cool weather during the growing season in its native mediterranean region, it is plausible that it has evolved mechanisms to tolerate low temperatures.

Assessing growth variation among M. truncatula accessions under autumn conditions is an important first step for identifying potential as a model to understand cold tolerance in alfalfa and as a winter annual forage crop. Wide variation in root and shoot dry matter accumulation revealed the presence of exploitable yield variation among M. truncatula accessions. The majority of the accessions produced higher dry matter in roots and shoots under the simulated autumn condition, patterned after 20-years Ames's Iowa autumn temperature and photoperiod than in control. These results indicate that M. truncatula have considerable variation in cold tolerance abilities. Time to first flower, the only trait associated consistently with differences among geographical locations, was delayed by simulated autumn condition showing a plastic response with temperature treatments and proved a clear evidence for adaptation ability of the species to diverse locations.

Cold acclimation is characterized as a biphasic process in M. truncatula based on the two distinct levels of freezing tolerance (FT) observed with response to temperatures, similar to what has been reported for winter hardy alfalfa. Simulated autumn conditions reduced stem length in M. truncatula , the response associated with fall dormancy in alfalfa. Jemalong-6 showed higher cold tolerance than W6 5018 with lower reduction in dry weights, stem length and number of leaves per stem and lower injury measured in CA compared to nonacclimation regimes. Differences in cold tolerance in M. truncatula may partly be related to cultivars differences in their ability to cold acclimation and indicate its possible use as a model for understanding genetics of cold tolerance in other legume crops, including alfalfa.

Freezing injury occurs as a result of destabilized membranes from freeze-induced dehydration and thus, maintaining membrane stability is a key function of CA. Ion leakage test to assess injury in leaf tissues after freeze-thaw event was efficient for determining both, FT and CA ability differences in genotypes. The maximum FT was achieved with CA3 regime, the only regime that included subzero temperature, can be used to induce maximal CA. Quantifying CA capabilities among a large number of accessions is possible using this screening procedures in future. Time course of accumulation of sugar correlates well with development of freezing tolerance in many plants. Sugars, particularly, glucose, fructose, sucrose and total sugars increased in CA regimes, reached maximum in CA3 and correlated with FT suggesting that its contribution to membrane stability. Raffinose was only detected in cold acclimated tissues could be indicative of FT, but requires further studies to confirm if differences in its concentrations explain the genetic differences in FT among M. truncatula accessions.

Copyright Owner

Babita Thapa

Language

en

Date Available

2012-04-30

File Format

application/pdf

File Size

116 pages

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