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The growth and yield of seven wheat and two barley cultivars or lines, previously found to show different degrees of boron tolerance under field conditions, were compared in a pot experiment at a range of soil boron treatments . Soil treatments ranged up to 150 mg/kg applied B . Extractable B in soils ranged up to 103 mg/kg . At the highest B treatment seedling emergence was delayed, but the percentage emergence was not reduced . The degree of boron toxicity symptom expression varied between the wheat cultivars and lines, with the two most tolerant, Halberd and (Wq*KP)*WmH)/6/12, displaying the least symptoms .The concentration of boron applied to the soil which produced a significant depression of growth and yield varied between cultivars . For example, the yield of (Wq*KP)*WmH)/6/12 was not affected at the 100 mg/kg applied boron treatment, while the grain yield for (Wl*MMC)/W1/10 was significantly reduced at the 25 mg/kg treatment.There was a linear increase in boron concentration in tillers at the boot-stage with increasing concentration of boron in the soil . The most boron tolerant genotypes had the lowest tissue boron concentrations in each of the treatments . Halberd and (Wq*KP)*WmH)/6/12 had approximately half the boron concentrations of the more sensitive genotypes at the 25 and 50 mg/kg treatments . Differential tolerance of boron within the tissue was also observed . Both Stirling and (WI*MMC)/W1/10 had significantly reduced total dry matter and grain yields at the 25 mg/kg treatment, while the concentrations of boron in boot stage tillers at this treatment were 118 and 100mg/kg, respectively . On the other hand, Halberd and (Wq*KP)*WmH)/6/12 had tissue boron concentrations of 144 and 131 mg/kg, respectively, at the 50 mg/kg treatment but yield was unaffected .The relative responses in the pot experiment, for wheat, were in close agreement with field results . Halberd and (Wq*KP)*WmH)/6/12 had the highest grain yields, with the lowest concentrations of boron in the grain when grown under high boron conditions in the field . In pots these two genotypes proved to be the most tolerant of boron . For barley the advantage in grain yield in the field, expressed by WI-2584 compared with Stirling, was not repeated in pots . WI-2584 was, however, more tolerant than Stirling on the basis of total dry matter production .The results show that useful variation in boron tolerance exists among wheat, and that breeding should be able to provide cultivars tolerant to high levels of boron .

Abstract. Nax1 and Nax2 are two genetic loci that control the removal of Na + from the xylem and thereby help to exclude Na + from leaves of plants in saline soil. They originate in the wheat ancestral relative Triticum monococcum L. and are not present in modern durum or bread wheat. The Nax1 and Nax2 loci carry TmHKT1;4-A2 and TmHKT1;5-A, respectively, which are the candidate genes for these functions. This paper describes the development of near-isogenic breeding lines suitable for assessing the impact of the Nax loci and their performance in controlled environment and fields of varying salinity. In young plants grown in 150 mM NaCl, Nax1 reduced the leaf Na + concentration by 3-fold, Nax2 by 2-fold and both Nax1 and Nax2 together by 4-fold. In 250 mM NaCl, Nax1 promoted leaf longevity and greater photosynthesis and stomatal conductance. In the uppermost leaf, the Na + -excluding effect of the Nax loci was much stronger. In the field, Na + in the flag leaf was reduced 100-fold by Nax1 and 4-fold by Nax2; however, Nax1 lines yielded 5-10% less than recurrent parent (cv. Tamaroi) in saline soil. In contrast, Nax2 lines had no yield penalty and at high salinity they yielded close to 25% more than Tamaroi, indicating this material is suitable for breeding commercial durum wheat with improved yield on saline soils.