The influence of shoot and root size on nitrogen uptake in wheat is affected by nitrate affinity in the roots during early growth.

Shoot and root system size influences N uptake in wheat (Triticum aestivum L.). Previously, we showed that four wheat genotypes with different biomass had similar N uptake at tillering. In the present study, we determined whether the similarity in N uptake in these genotypes was associated with genotypic differences in the affinity of the root system for NO3- uptake. Kinetic parameters of NO3- uptake were measured in hydroponic seedlings of vigorous and nonvigorous early growth wheat genotypes by exposing them to solutions with differing concentrations of K15NO3 for 15min. In the low concentration range, the high-affinity transport system of the nonvigorous cultivar Janz showed a higher maximum influx rate than the three vigorous lines and a higher affinity than two of the three vigorous lines. At high NO3- concentrations, where the low-affinity transport system was functional, the responsiveness of NO3- uptake to external concentrations was greater in Janz than in the vigorous lines. Both the high- and low-affinity transport systems were inducible. The genotypic variation in the kinetic parameters of NO3- uptake was large enough to offset differences in morphological traits and should be considered in efforts to improve N uptake. In a field trial, the growth and N uptake performance of the four wheat genotypes was investigated over the winter-spring growing season (June-November of 2010). The field trial showed that although early N uptake was disproportionately large relative to biomass accumulation, the differences in uptake at tillering can be changed by subsequent patterns of uptake.

Wheat genotypes with high early vigour accumulate more nitrogen and have higher photosynthetic nitrogen use efficiency during early growth.

Genotypic differences in early growth and nitrogen (N) uptake among 24 wheat (Triticum aestivum L.) genotypes were assessed in a field trial. At late tillering, large genetic variation was observed for shoot biomass (23-56gm-2 ground area) and N uptake (1.1-1.8gm-2 ground area). A strong correlation between aboveground biomass and N uptake was observed. Variation around this relationship was also found, with some genotypes having similar N uptake but large differences in aboveground biomass. A controlled environment experiment was conducted to investigate the underlying mechanisms for this variation in aboveground biomass using three vigorous genotypes (38-19, 92-11 and CV97) and a non-vigorous commercial cultivar (Janz). Vigorous genotypes had lower specific leaf N in the youngest fully expanded leaf than Janz. However, there was no difference in chlorophyll content, maximum Rubisco activity or the rate of electron transport per unit area. This suggests that Janz invested more N in non-photosynthetic components than the vigorous lines, which could explain the higher photosynthetic N use efficiency of the vigorous genotypes. The results suggest that the utilisation of wheat genotypes with high early vigour could improve the efficiency of N use for biomass production in addition to improving N uptake during early growth.

Large root systems: are they useful in adapting wheat to dry environments?

There is little consensus on whether having a large root system is the best strategy in adapting wheat (Triticum aestivum L.) to water-limited environments. We explore the reasons for the lack of consensus and aim to answer the question of whether a large root system is useful in adapting wheat to dry environments. We used unpublished data from glasshouse and field experiments examining the relationship between root system size and their functional implication for water capture. Individual root traits for water uptake do not describe a root system as being large or small. However, the recent invigoration of the root system in wheat by indirect selection for increased leaf vigour has enlarged the root system through increases in root biomass and length and root length density. This large root system contributes to increasing the capture of water and nitrogen early in the season, and facilitates the capture of additional water for grain filling. The usefulness of a vigorous root system in increasing wheat yields under water-limited conditions maybe greater in environments where crops rely largely on seasonal rainfall, such as the Mediterranean-type environments. In environments where crops are reliant on stored soil water, a vigorous root system increases the risk of depleting soil water before completion of grain filling.