Dry matter partitioning and root growth in an annual wheat, a perennial wheatgrass and four perennial wheat derivatives

The development of perennial wheat could provide further options in the mixed farming systems of southern Australia, including greater flexibility in grain and/or forage production, while also providing improved ecosystem services. In recent studies, newly-developed perennial wheat derivatives, between bread wheat (Triticum aestivum L. [6n]) and perennial grass (e.g. Thinopyrum intermedium [6n]), have been reported to survive, regrow and produce grain in fields at Cowra NSW, for up to four years.
Following this initial proof of concept, this thesis examines the growth of perennial wheat derivatives in greater detail, under controlled conditions in large soil columns in a screen-house, and with some further field validation at Cowra. In each case, an annual wheat and a perennial grass were used as controls, for comparison with selected perennial wheat derivatives. First, the research examined the patterns of survival, regrowth, dry matter production and partitioning over three years, to determine attributes associated with sustained survival, regrowth and performance. Second, the research addressed how these patterns may change in response to limitations in the source, sink or both, to determine differences in sink priority, and hence, circumstances that produce any yield trade-off between annual and perennial growth habits. Third, the research examined whether perennial genotypes allocate more dry matter to roots, and if this can result in greater extraction of soil water from depth under prolonged water deficit. This was done to determine whether improved resource capture or greater initial dry matter at the beginning of a regrowth season can significantly extend the growing season of perennial wheat.
The first study with perennial wheat derivatives over successive generations proved their ability to survive and produce and increase root biomass in older plants. Grain yield was lower in perennials than in annual wheat, with the yields being similar in the first-and second-year of growth in perennial wheat derivatives, but increasing in the third year of growth. Some three-year-old perennial wheat derivatives produced2–3 fold more grain per plant than annual wheat controls. The above-ground biomass in three-year-old plants was 2–7 fold greater in perennial wheat derivatives than in annual wheat. In better performing perennial wheat derivatives and perennial grass root growth was found to be 2 fold greater in the first year and this increased to be 3–12 fold greater in older perennial wheat derivatives and perennial grass. Perennial wheat derivatives and perennial grass produced more tillers, biomass, and root growth with increasing plant age, all of which were likely contributors to the consistent regrowth and survival of these plants in successive years.
In the second study, resource allocation priorities with source or sink manipulation were explored in perennial wheat derivatives and perennial grass, and the outcomes were compared to those for annual wheat in two experiments. Under sink manipulation (the reduction of spikelets from one side of the spike), greater amounts of the available resources were translocated to the remaining seed, resulting in a larger seed size. This may indicate the potential for yield improvement as a result of breeding developments in perennial wheat and grass genotypes. In comparison, a smaller seed size and more biomass in the source-reduced study may have been correlated with greater priority being allocated to survival rather than grain production in perennial wheat derivatives and perennial grass, relative to annual wheat.
In the third study, the association of root growth and soil water extraction for the provision of drought resistance in perennial wheat derivatives and perennial grass in extended drying down conditions were investigated. After water was withheld, promising perennial wheat genotypes with more extensive and deeper root systems showed greater water extraction from deeper within the soil and, in turn, longer plant survival. This is the first comprehensive study with a third-year-old stand of perennial wheat derivatives and perennial grass that provides evidence of drought resistance directly due to the extensive root system and water extraction from deeper soil depths, thus confirming the hydrological benefits of perennial wheat during drought.
Perennial wheat derivatives showed variable responses in regrowth in different planting seasons and subsequent years, and produced more biomass and root growth. Increased root growth enabled plants to access soil water from greater soil depths, and was found in controlled conditions and in the field. The most promising derivatives were able to approach or exceed the perennial grass performance in similar conditions. These results added support to previous reports and provide a novel contribution to the growing literature on perennial wheat. In particular, I found source-sink balance in relation to the effects of plant age on resource acquisition and partitioning trends, indicating that perennial wheat have some physiological traits to withstand extended drying.