Interactive effect of water and nitrogen regimes on plant growth, root traits and water status of old and modern durum wheat genotypes

The selection of the ideal root drought adaptive traits should take into account the production and maintenance of root tissues alongside the capacity to capture soil resources. Ten old and modern Spanish durum wheat (Triticum turgidum L. var durum) genotypes were grown in lysimeters under two contrasting water and nitrogen regimes to study the effect of such growth conditions on: (1) the aerial biomass, (2) the growth and structure of the roots and (3) the relationships of the root structure with aerial biomass, photosynthetic and transpirative characteristics and water use efficiency. Both high water and nitrogen regimes significantly increased aerial biomass. Root dry biomass and root length increased and decreased in response to improved water supply and nitrogen regimes, respectively. No significant correlations were detected between aerial biomass and any root trait under well-watered conditions. Under water stress aerial biomass was negatively correlated with root dry biomass, root length and root weight density and positively correlated with the specific root length, particularly for the subset of old genotypes. The high nitrogen regime significantly enriched the carbon isotope composition of the flag leaf (delta C-13(FL)) and hindered the effect of the high water regime on decreasing delta C-13(FL) enrichment. Thus, positive correlations of aerial biomass with delta C-13(FL) were detected regardless of the water regime. The study revealed: (1) the importance of root traits for higher aerial biomass under the low water regime; (2) that the interaction between nitrogen and the water regime may affect the predictive nature of the delta C-13 in drought breeding programs; and (3) the selection of the ideal root system structure should take into account the metabolic costs of the production and maintenance of root tissues alongside the capacity of capturing resources.