The Resistance to Salt of Brassica sps. and Improved Resistance by Direct Selection and Mutagenesis
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Salinity is one of the most serious environmental stresses influencing agriculture drastically decreasing the production of crop plants throughout the world. Due to salinity thousands of hectares of agricultural land are going out of production annually. To cope with the salinity problem, there is keen interest in the development of crop plants displaying resistance to the effects of salinity. A series of experiments was carried out under controlled environmental conditions to determine salt resistance in Rapid Cycling Brassica species (RCB's) at germination early seedling and late growth stage. The effect of increasing salinity concentration was determined in vivo using a compost based growing system and also in-vitro using adapted tissue culture techniques. At germination and during the early seedling stage, low salt concentrations (50-100 mM) stimulated germination and had no significant effect on growth in B. rapa and B. rapa appeared to have greater salt resistance than B. napus. There was no association between salt resistance at the early seedling stage and that at the adult stage. At later growth stages, salinity affected both the relative fresh and dry weights and tissue ion concentration with K: Na balance affected in favour of Na. The relative salt resistance in the six Brassica species was associated with the reduction in the total fresh weight of shoots of salt-treatment plants expressed as the percentage of control but was not associated with K., Na concentration or K/Na ratio in shoots. B. napus and B. carinata showed the greatest salt resistance, B. juncea, B. rapa and B. nigra were intermediate whilst B. oleracea was salt sensitive. Conventional selection for salt resistance was not successful in this study because B. oleracea, B. napus and B. carina/a were not able to complete either their vegetative or reproductive phases and died before completion of the first selection cycle. Whilst, ten percent of plants of B. rapa, B. nigra and B. juncea managed to complete the first selection cycle they failed to complete the second selection cycle. Although, callus induction and maintenance were successful for all 6 RCB's, regeneration of shoots from callus was poor. Also, callus-based selection for salt resistance was unsatisfactory and had variable results and it was concluded that this was not a promising avenue for improving salt resistance in RCB's A cauliflower curd meristem technique was adapted for in-vitro mutagenesis and selection. Mutagenesis was carried out using two mutagens N-nitroso-N-ethylurea or nitroso-methylurea at 1 mM and 2.5 mM. 300 green shoots were recovered from more than 1 ,000,000 explants mutagenised in liquid medium supplemented with 3 mM hydroxyproline as a selection agent. Of eighty in-vitro shoots which where measured for proline content, twelve showed higher proline level than controls. Leaf strip assays of the twelve selected in-vitro shoots and in-vivo weaned plants exposed to a 3 mM and 10 mM hydroxyproline assay showed greater resistance than controls. A few selections also had cross-resistance to salt at 550 mM NaCl and to frost at -7 °C. These results successfully indicated the existence of great opportunities for the production of stress resistance cauliflower plants via mutagenesis and hydroxyproline selection.
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