| Aluminum is the main factor limiting plant growth on acid soil. In fact, acidification could bring about many other changes of physical and chemical properties in soil, which in turn also affect plant growth and development. On acid soil, some nutrients, such as P, Ca and Mg are usually deficiency, while others, such as Mn and B could attain very high available concentration toxic to plants. Similarly, the bio-availability of some heavy metals e.g. Cd increases markedly, causing toxicity to the plants. Plant species and genotypes within species greatly differ in tolerance to acidic stress. Consequently, it is one of the best approaches to develop acid-soil tolerant genotypes in increasing crop productivity on acid soil. However, there is little information about the mechanisms of aluminum toxicity and the genetic difference in aluminum tolerance of plants, thus exposing a constraint on carrying out relevant breeding and cultivation programs. In the present research, approximately 500 barley genotypes were planted initially on the acid soil with pH 4.9. According to performance of seedling growth, identified as the tolerance to acid soil, 243 genotypes with the different growth performance were selected for use in further field and laboratory trials for identifying acid soil and Al toxicity tolerance, respectively. The consistency was compared to the results obtained from both field and laboratory experiments to test the possibility of identifying acid soil tolerance by means of hydroponic culture. Meanwhile, other hydroponic experiments were conducted to study the effect of aluminum alone and aluminum+ cadmium toxicity on barley growth and the physiological mechanism of genotypic difference in aluminum tolerance. The main results are as follows:1. Studies on the methodology for screening barley genotypes tolerant to acid soil or aluminumIn the present study, both field and laboratory (hydroponic) trials were carried out to identify acid soil and Al toxicity tolerance in barley genotypes. The two methods proved that most of the genotypes used in the experiments showed aluminum (acid soil) sensitive or intermediately sensitive. However, the rank in tolerance was not consistent between thetwo methods for a great deal of genotypes. The difference may be described to different growth medium, pH and Al concentration and form in the medium and other elements, suggesting it is important to control nutrient composition and other experimental conditions in order to obtain reasonable results in hydroponic culture. The other reason for the inconsistence in the two screening methods may be related to complex root properties in barley. There was a wide difference among barley genotypes in their tolerance to Al toxicity. It was noted that some genotypes showed as high as even higher Al tolerance than control Dayton, which is considered as an Al-tolerant genotype. The correlation analysis, based on grey system theory showed that emergence date, tillers per plant, tillering date and spikes per plant could be used as indicators in identifying barley genotypes tolerant to acid soil in light of their sensitivity to acid soil.2. Effect of acid soil on dry matter accumulation and Al and nutrient contents of barleys differing in acid soil toleranceThe experiment was conducted by using 10 barley genotypes with different acid soil tolerance at seedling stages, each 5 genotypes for acid tolerance and sensitivity, to determine the difference in biomass accumulation, Al and nutrient content in the roots and shoots among genotypes under the condition of acid soil stress. The results showed that acid soil stress reduced dry matter accumulation of all genotypes, the extent of reduction varied with genotypes, being much more affected for the acid soil sensitive genotypes, such as Shang 70-119 than the acid soil tolerant genotypes, in comparison with the control. The difference between two kinds of genotypes in biomass inhibition by acid soil was much larger at maturity than at milking stage. Acid soil stress significantly red...
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