Abstract
Genetic modification of shoot and root morphology has potential to improve water and nutrient uptake of wheat crops in rainfed environments. Near-isogenic lines (NILs) varying for a tillering inhibition (tin) gene and representing multiple genetic backgrounds were phenotyped in contrasting, controlled environments for shoot and root growth. Leaf area, shoot and root biomass were similar until tillering, whereupon reduced tillering in tin-containing NILs produced reductions of up to 60% in total leaf area and biomass, and increases in total root length of up to 120% and root biomass to 145%. Together, the root-to-shoot ratio increased two-fold with the tin gene. The influence of tin on shoot and root growth was greatest in the cv. Banks genetic background, particularly in the biculm-selected NIL, and was typically strongest in cooler environments. A separate de-tillering study confirmed greater root-to-shoot ratios with regular tiller removal in non-tin-containing genotypes. In validating these observations in a rainfed field study, the tin allele had a negligible effect on seedling growth but was associated with significantly (P<0.05) reduced tiller number (–37%), leaf area index (–26%), and spike number (–35%) to reduce plant biomass (–19%) at anthesis. Root biomass, root-to-shoot ratio at early stem elongation, and root depth at maturity were all increased in tin-containing NILs. Soil water use was slowed in tin-containing NILs, resulting in greater water availability, greater stomatal conductance, cooler canopy temperatures, and maintenance of green leaf area during grain-filling. Together these effects contributed to increases in harvest index and grain yield. In both the controlled and field environments, the tin gene was commonly associated with increased root length and biomass, but the significant influence of genetic background and environment suggests careful assessment of tin-containing progeny in selection for genotypic increases in root growth.
Highlights
Wheat crops are commonly grown in rainfed environments germplasm and/or management practices must take account where variable rainfall and the occurrence of terminal water of the dominant stress development patterns in the target envideficits limit biomass and grain yield
Up to 10 tillers comprising mainstem, secondary, and occasionally tertiary tillers were observed on individual plants in both Banks and Kite non-tin near-isogenic line (NIL)
The influence of the tin gene on tiller reduction at final harvest was greatest for the Banks background and for the biculm B++ (1–2 tillers per plant) and to a lesser extent oligoculm B+ (2–3 tillers per plant) NILs
Summary
Wheat crops are commonly grown in rainfed environments germplasm and/or management practices must take account where variable rainfall and the occurrence of terminal water of the dominant stress development patterns in the target envideficits limit biomass and grain yield. Increasing water deficits around early stem elongation can limit tiller production to slow canopy development (Duggan et al, 2005a; Mitchell et al, 2013) while drought immediately prior to and during anthesis can reduce floret fertility to reduce grain number (Saini and Aspinall, 1981). Water limitation during grain-filling reduces assimilate supply to developing grains, reducing final yield and quality (van Herwaarden et al, 1998; Mitchell et al, 2013). The timing and severity of water deficit influence the final impact on grain yield, with some plasticity in yield possible through compensating processes such as changes in tiller or ear growth (Sadras and Rebetzke, 2013)
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