The antioxidative protection in leaves of four winter wheat (Triticum aestivum L.) varieties with different field drought resistance was studied under severe recoverable soil drought at seedling stage by withholding irrigation for 7 days (57-59% leaf water deficit) followed by rewatering. A 3-fold raise in electrolyte leakage and a sharp increase in proline accumulation corresponded to drought severity. Hydrogen peroxide content and catalase (CAT) activity were maintained low under stress. Peroxidase (GPX) activity increased, whereas superoxide dismutase (SOD) activity only slightly changed. The content of ascorbate and low-molecular thiols diminished under severe drought and was restored in recovery. Malondialdehyde level was not changed significantly in drought-treated plants but raised after re-watering. In recovery CAT activity became significantly higher whereas GPX activity diminished. Three isoforms of SOD, one of catalase and three of GPX were revealed. Proline accumulation had a predominant role in drought response. As for varieties, drought sensitivity or tolerance was not necessarily correlated with differences in the antioxidative response at early vegetative stage.

Bulgarian common wheat cultivars released in the period 1925-2003 were studied using the gibberellic acid (GA) test and microsatellite analysis of the Xgwm261 locus on chromosome 2DS to identify the semi-dwarfing (Rht) genes. The old cultivars, isolated through selection from landraces, carried rare alleles (211-and 215-bp) at Xgwm261 locus, and those developed by hybridisation to foreign cultivars, carried the 165-and 174-bp alleles. Forty-two (55.3%) of 76 modern cultivars were GA-responsive. The 192-bp allele, diagnostic for Rht8, was observed in 64 (84.2%) modern cultivars, of which 37 carry Rht8 alone, and 27 possess a combination of Rht8 and a GA-insensitive allele viz. Rht-B1d (17); Rht-D1b (6) and Rht-B1b (4). The 174-bp allele is present in seven cultivars, only one of which is photoperiod-sensitive, and the rest are day-length insensitive. The 203-bp allele was found in six modern cultivars. Cultivars carrying the Rht8 allele are the most widespread and some of them have been cultivated for a long period. Cultivars with the 'Saitama 27' allele (Rht-B1d) are the most productive and are second in distribution in the country. The recently observed trend for increasing the proportion of cultivars with GA-insensitive Rht genes is probably due to their combination with the 192-bp allele of Xgwm261 locus tightly linked to the Ppd-D1, to the break of the link between the 174-bp allele and ppd-D1, and to the introduction of other genes influencing flowering time.

Field drought studies were performed in order to assess oxidative stress, proteolytic activity and yield loss under natural stress conditions. Flag leaves of two drought-tolerant (Yantar and Zlatitsa) and two drought-sensitive (Miziya and Dobrudjanka) winter wheat varieties were analyzed. Stress intensity was assessed by relative electrolyte leakage and proline accumulation. Senescence progression was followed by loss of chlorophyll and protein. Lipid peroxidation, H 2 O 2 content, activities of superoxide dismutase (SOD), catalase (CAT), and non-specific peroxidase (GPX) isoforms, as well as proteolytic activities were analyzed from heading throughout grain filling. Weakening of membrane integrity and oxidative damage to lipids were more pronounced in the sensitive varieties under field drought. The activities of Fe-and Cu/Zn SOD isoforms decreased in the controls, but remained high in drought-treated plants. The activities of

Recurrent drought periods of varying duration often cause extensive crop damage and affect wheat production in Southern Europe. This study compares biochemical and ultrastructural responses of four wheat (Triticum aestivum L.) cultivars to long-term field drought, and their contribution to final grain yield. Gel electrophoresis and immunoblotting analyses combined with transmission electron microscopy and grain yield evaluation were employed to assess drought susceptibility of the wheat cultivars. Two of them behaved as droughttolerant, the other two presented as drought sensitive. Enhanced degradation of Rubisco large subunit (RLS), Rubisco small subunit (RSS) and Rubisco activase (RA) accompanied by an increased protease activity and reduced levels of heat shock proteins (HSP70) and dehydrins (DHNs) were associated with drought sensitivity. Drought tolerance coincided with relatively stable or increased HSP70 and DHN contents, and unchanged/higher levels of RLS, RSS and RA. Sensitive cultivars were more vulnerable to ultrastructural damages, showing obvious degradation of chloroplast membrane systems and depletion of leaf starch reserves. These drought responses affected yield potential, as tolerant cultivars gave higher yield under intense drought. Thus, our results provide additional insights into the complexity of plant drought responses, identifying multiple interacting traits that may serve as indirect selection criteria for wheat drought tolerance.

The study of inheritance and distribution of alleles linked to important agronomic characters is of a great value for planning breeding programs directed towards development of high-yielding hexaploid wheat varieties. The wheat microsatellite marker Xgwm 261 is of particular interest to international breeders because of its association with a commercially significant reduced height phenotype. The allele distribution at the tightly linked to the dwarfing gene Rht8 microsatellite Xgwm 261 locus was analyzed in the modern hexaploid wheat varieties and advanced breeding lines from the gene bank of Dobrudja Agricultural Institute (DAI), G. Toshevo, Bulgaria and CRAW, Gembloux, Belgium. A prevalence of 192 bp allele at Xgwm 261 was found in Bulgarian and other South-EasternEuropean wheat varieties collected at DAI, G. Toshevo, Bulgaria thus indicating the adaptive significance of the corresponding allele of Rht8 gene in this geographical region. The majority of wheat varieties from CRAW gene bank, Belgium carried 174 bp and 164 bp allele variants and only few of them were with 192 and 196 bp. The study demonstrated that a limited number of varieties and advanced breeding lines from the gene bank of Dobrudja Agricultural Bulgaria carried 164bp, 174bp, 194 bp, 196 bp and Xgwm 261 variants over 200bp.

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