Root growth of perennial grasses in acid, high-strength soils and the application of molecular-based assays to quantitative root studies

[Truncated abstract] Perennial species play an important role in sustainable pasture production in south-eastern Australia. However, poor persistence of perennial grasses can lead to a decline in the productivity and environmental benefits of the pasture systems. Poor persistence is frequently associated with acid soils in which aluminium (Al) toxicity constrains root growth, resource acquisition and plant productivity. Perennial grasses demonstrate varying levels of resistance to acid soils, ranging from resistant species such as cocksfoot (Dactylis glomerata L.) and weeping grass (Microlaena stipoides (Labill.) R.Br.), to very sensitive species such as tall wheatgrass (Thinopyrum ponticum (Podp.) Z. –W. Liu & R. –C. Wang.). Phalaris (Phalaris aquatica L.) is considered to be moderately sensitive to acid soils but cultivars have been bred for improved resistance to Al-toxicity. Under field conditions, these cultivars have not always exhibited better persistence when compared with the Al-sensitive genotypes they were intended to replace and this has cast doubt on the value of breeding for Al-resistance. The first objective of this thesis was to understand how the growth and morphology of roots of perennial grasses are affected by acid, Al-toxic soils and how this is influenced by genotypic differences in acid-soil resistance (Chapter 2). Al restricted root elongation (primarily lateral roots), root hair development and rhizosheath development in both acid-soil sensitive (phalaris cv Sirosa and tall wheatgrass) and acid-soil resistant genotypes (cocksfoot and phalaris cv Advanced AT), with the exception of weeping grass, which was highly acid-soil resistant. Root growth of acid-soil sensitive genotypes was affected to a greater extent, and at lower levels of Al, than acid-soil resistant genotypes. It was hypothesised that the value of acid-soil resistant genotypes for persistence of perennial grasses in the field may be modified by other soil conditions.