Abstract

Initially, experiments were conducted to prove that a new screening technique, namely the length of seedling roots in filter paper moistened with solutions with high concentrations of boron, for tolerance to high concentrations of boron could be used for distinguishing between tolerant and sensitive genotypes. Seedling were compared in the filter paper technique with those grown in boron enriched soil to investigate the response of wheat genotypes known to differ in tolerance to high concentration of boron. Under high boron concentrations in filter papers, the more tolerant genotypes had significantly longer rooted than those of the more sensitive genotypes. There was no significant correlation between the root lengths in the control treatment and the other three boron treatments (50, 100, 150 mgBl^-1). Thus, the differences in root lengths in the high boron treatments could not be attributed to inherent differences in root growth but to the genetic variation in response to high boron concentrations among varieties. Root lengths in the three boron treatments in filter papers were highly significantly correlated with the three characters routinely determined for plants grown in soil containing high levels of boron, namely the concentration of boron in the shoots, plant dry weight and leaf symptoms, indicating that root length could be used as a selection criterion in genetic studies or breeding programs for boron tolerance.

Genetic control of tolerance to boron was investigated between a moderately tolerant variety Halberd, a tolerant line G61450 and the moderately sensitive varieties Schomburgk and Condor. Two genes, Bo1 and Bo4 controlled tolerance to boron in Halberd and G61450, respectively. The genetic control of response to boron was the same for Condor, Schomburgk and a homozygous sensitive line 442S-1 extracted from the cross between G61450 and Halberd.

The chromosomal location of genes controlling tolerance to boron was studied by the use of F₂ monosomic and backcross reciprocal monosomic analysis. The results were consistent for both methods showing that chromosomes 7B and 4A were responsible for tolerance to boron in Halberd and G61450, respectively. Results of the backcross reciprocal monosomic analysis indicate that chromosome 4A was also the location of genes controlling tolerance to boron in the tolerant exotic lines India 126 and Benventuto Inca.

The genetic relationship, with respect to tolerance to boron, between an Australian moderately tolerant variety BT-Schomburgk and a number of tolerant exotic lines were investigated by testing the F₃ derived F₄ families. Transgressive segregations were observed for the cross between BT-Schomburgk and Klein Granador, and to Turkey 1473, indicating at least two different genes controlling response to boron between BT-Schomburgk and these two exotic lines. Monogenic segregations were observed from the cross between BT-Schomburgk and AUS 4903. The results of the cross between BT-Schomburgk and India 126 were more complicated than those of the other crosses and indicated that more than one gene conferred tolerance to boron in this cross.

This thesis demonstrates that it is possible to breed even more tolerant varieties than Halberd or BT-Shomburgk by transferring boron tolerant genes from tolerant lines including G61450, Turkey 1473, AUS 4903, and Klein Granador into less tolerant but otherwise well adapted varieties.

Yodsaporn