Optimising root growth to improve uptake and utilization of water and nitrogen in wheat and barley

Durum wheat (Triticum turgidum L. var durum) and spring barley (Hordeum vulgare L.) are the most widely grown crop species in the semi-arid to arid areas of the Mediterranean region. However, their average on-farm yields are relatively low, 1.95 and 2.60 t ha-1, respectively (FAO, 2007). Water is generally recognized as the most limiting factor for barley and durum wheat production in the Mediterranean, though it has been found, at least for some regions, that N fertilizer applications have been limiting (Passioura, 2002). Water in the Mediterranean is relatively scarce and predictions for 2025 show that water limitations for agricultural production in that region will intensify (IWMI, 2000). Nitrogen fertilizer represents a significant cost of production for the grower and may also have negative environmental impacts through nitrate leaching, use of fossil fuels for manufacture and application, and N2O emissions associated with denitrification. Reducing excessive N fertilizer inputs and increasing water productivity, whilst maintaining acceptable yields, will be aided by increases in uptake efficiency. To be in a position to manage irrigation and N inputs more effectively, an improved quantitative understanding of relationships between root traits and capture of water and nitrogen is required. The major phase of root growth in wheat and barley is during tillering and stem extension, and total length of the root system increases until about anthesis (Gregory et al., 1978b; Barraclough & Leigh, 1984). A theoretical model (van Noordwijk, 1983) indicated that the rooting trait best related with water and N capture is the root length density (root length cm/ soil volume cm3; RLD). Field data sets of barley grown on stored water in Syria indicated a RLD of about 1 cm cm-3 is required for extraction of ca. 90% of the available water, and it was defined as the critical root length density - CRLD (Gregory & Brown, 1989). In field-grown durum wheat and barley, RLD usually exceeds CRLD in the upper soil profile, while below 60 to 80 cm it is typically lower than 1 cm cm-3. The relationship between RLD in cereal root systems and below-ground resource capture was recently described in a quantitative model (King et al., 2003), linking the size (RLD) and cumulative distribution of the root system with depth (β) to the proportional capture of available water and nitrogen (φ) during grain filling (King et al., 2003). β describes the shape of the cumulative distribution with depth, according to: p = 1 - βd; where p is the fraction of the root system accumulated from the soil surface to a given depth (d). φ is calculated as: φ = 1 – e-k.RLD, where k is the resource capture coefficient (cm2). The overall aim of the present study was to: (i) identify root traits in barley and durum wheat for improved water and N capture under different intensities of water and/or N stresses, and (ii) quantify responses of root growth, root: shoot partitioning and water and N capture to simulated Mediterranean environments differing in water and N stresses, using controlled-environment experimental conditions. The main hypotheses tested were: 1. Mediterranean barley and durum wheat have a similar root system morphology, and comparable cumulative distribution of RLD with depth (βRLD). 2. Water and N deficits increase R:S, however total root weight and length will be reduced. 3. The proportion of roots deeper in the profile will increase with water and N deficits (higher β). 4. k can be defined from the relationship between RLD and φ, and hence a CRLD can be calculated; k should not differ between genotypes and the CRLD will be ca. 1 cm cm-3. 5. The k value for root volume density (root volume / soil volume; RVD) can be calculated according to King et al. (2003), and critical root volumes (CRVD) for a 90% water extraction can be calculated. However, RLD will explain a higher proportion of φ for water capture than RVD. 6. Aboveground dry weight (AGDW) and yield (Y) decrease with N and water deficits and there is an interaction between water availability and N fertilizer, such that responses to N are relatively greater under high than low water availability. 7. Water-use efficiency (AGDW / water use; WUE) increases with water stress and N availability, while grain Δ13C decreases, and responses are similar for spring barley and durum wheat. 8. Nitrogen-use efficiency (grain DM / N available; NUE), N-uptake efficiency (above groung N / N available; NupE) and N-utilization efficiency (grain DM / aboveground N; NutE) will decrease with increasing water deficits and increasing N supply and responses are similar for spring barley and durum wheat. In each of 2006, 2007 and 2008 one glasshouse soil column (15 cm diameter x 150 cm length) experiment was conducted at the University of Nottingham, School of Biosciences, Sutton Bonington Campus, UK (52.5o N, 1.3o W). The responses of Jordanian spring barley cv. Rum (2006-2008) and durum wheat cvs Hourani (2006-2007) and Karim (2007) to two levels of irrigation (drought and fully irrigated) and up to three levels of N fertilizer (nil, 50 and 100 kg ha-1 N, equivalents) were examined. In 2006 and 2007 for each genotype there were six treatment combinations (2 irrigation treatments x 3 fertilizer N levels), in 2008 for barley there were two irrigation treatments at one level of N fertilizer (50 kg N ha-1, equivalent). The experiments used a factorial randomised block design in three (2006) or five replicates (2007 and 2008). In each experiment, detailed analysis at sequential samplings through the season was carried out, including anthesis and harvest, of root growth and morphology (by root digital image analysis), as well as for the aboveground growth and dry matter partitioning. Water and N uptake were measured and their use-efficiencies evaluated. In 2006, water uptake was gravimetrically measured by weekly weighing a sub-set of soil columns for each treatment. While in 2007 and 2008, water content was weekly measured at different soil-depth intervals using a Theta-T probe (ML2x Delta T Devices, Cambridge, UK) via access apertures in columns for a sub-set of columns. WUE was calculated as the AGDW /total water use, from the date of transplantation to harvest and also by the slope of the linear regression of cumulative AGDW on cumulative water uptake through time. This project attempted a comprehensive study of root (and shoot) responses of barley and durum wheat to water and/ or nitrogen stresses, to identify root characteristics for resource acquisition in Mediterranean type environments. However the conditions were atypical of Mediterranean ecosystems. High soil N available (at sowing + mineralization through the season) and/or leaching led to inconsistent and contradictory response to the ones usually found in the literature. Excessive temperatures known to be inhibitory to plant growth and development were felt in the glasshouse, with peaks exceeding 50 ºC. In the field, roots usually experience much lower temperatures below ground. However in these experiments they were subject to the same high temperatures as shoots, this would have had a major impact on the observed root distributions. Moreover, soil in the columns was found to have quite large bulk densities (1.61, 1.85 and 1.76 g cm-3 in 2006, 2007, and 2008, respectively), offering a quite high resistance to root growth and consequently shoot growth (Bowen, 1981). To avoid roots growing in the edges, only one plant per soil column was used. However when compared to field grown crops, it only represents an half to a fifth of the plant densities usually found in wheat and barley grown in the Mediterranean. Therefore the usual cropping inter-competition for soil resources was not accounted for. For this reasons the root densities presented in this work might not be representative of those found in the field grown crops, and hence its use has to be cautiously. Due to the large amount of time needed to extract the root system from the soil, and posterior fine cleaning before scanning, only the top (0 – 20 m); mid (60 - 80 cm) and bottom (>125 cm) of the root system where possible to be analysed. Consequently the total root weight, length and volume, are not real totals but the sum of the layers analysed. Other root morphology parameters, like mean root diameter, specific root length (SRL) and root volume root weight ratio (rV:rW) where calculated in function of those layers. The calculation of the root parameters distribution with depth, using β coefficients (βW - weight, βL – length and βV – volume), was also done taking in account those soil depth sections. This partial analysis can result in a different distribution with depth when compared with a full analysis. Moreover the root shoot ratio (R:S) was estimated using the βW, hence those values may not be the same as if all root system was analysed. Root growth of barley was generally representative of values reported in the literature in the present experiments, but root growth of durum wheat genotypes showed some signs of restriction in the soil columns, particularly in 2007, possibly in part due to the high soil bulk density (BD = 1.85 g cm-3). The root to shoot dry weight ratio (R:S) increased with drought, but relatively more for wheat than for barley, so that total root weight (TRW) was actually higher under water limitations for durum wheat than under full irrigation. After anthesis for all genotypes under the droughted treatment, there was a consistent increase in the allocation of root biomass deeper in the soil profile (higher βW). Total root weight (TRW), total root length (TRL) and total root volume (TRV) were well correlated; therefore RWD, RLD and RVD distribution with depth followed similar patterns. Hence, an increase of βL and βV was also found under drought. Beta values for root length were (averaged across 2006, 2007 and 2008): 0.97, 0.97 and 0.96 for barley cv. Rum, wheat cv. Hourani and wheat cv. Karim under irrigation; and 0.98, 0.98 and 0.97 under drought, respectively. N was shown to occasionally reduce RLD, possibly associated with extreme and uniform N concentration in the soil (due to a high mineralization) causing lateral root formation to cease (Linkohr et al., 2002). The sub-traits most affecting TRL differed between genotypes. For durum wheat changes in length were mainly associated with increases in R:S and TRW, whilst for barley cv. Rum specific root length (SRL; root length cm / root weight g) was more important in determining TRL. Therefore SRL could be a promising trait to target in breeding, since it may be possible to increase RLD without increasing the allocation of biomass from the aboveground to the roots. Overall water use (WU) was higher for barley than wheat cultivars associated with its more extensive root system and higher aboveground growth. However, differences between the WU of plants subjected to drought of wheat cultivars and barley were not large. As expected WU decreased with drought and WUE increased. For barley WUE was relatively higher than for wheat cv. Hourani, and wheat cv. Karim. However, ∆13C in the grains, across years was similar between genotypes. Leaf SPAD values measured at anthesis were always higher for barley than wheat, possibly indicating higher specific leaf nitrogen (SLN) resulting in higher assimilation rate per unit leaf area (Cabrera-Bosquet et al., 2009). N-use efficiency was higher for barley than for durum wheat cultivars and decreased with drought and N application for all genotypes. Differences in NUE were mainly explained by NupE. Fitting the King et al. (2003) equation to the RLD and φ data for water, a kRLD was found for barley cv. Rum of 2.4 cm-2 under drought (2007-08), resulting in a CRLD of 0.97 cm cm-3. This was similar to the value previously found by Gregory & Brown (1989); however no value could be fitted under irrigation. For wheat cv. Karim relatively higher values of kRLD were found: 0.59 and 0.40 cm-2 under irrigation and drought, respectively, in 2007; corresponding to CRLD values of 0.41 and 0.64 cm cm-3, respectively. Overall results indicated that under drought CRLD values were higher than under irrigation. Fitting an adapted King et al. (2003) equation to RVD vs φ for water showed a more consistent relationship than was found for RLD. Similar values of kRVD were observed for barley (5.13 and 4.45, under irrigation and drought, respectively) and wheat cv. Hourani (5.03 and 4.00, under irrigation and drought, respectively), though wheat cv. Karim had relatively higher values (10.04 and 5.86, under irrigation and drought, respectively). Therefore, CRVD values for wheat cv. Karim were lower than for the other two genotypes. kRVD values under drought were lower than those found under irrigation, resulting in higher CRVD under drought. AGDW and grain yield (Y) was relatively higher for barley than for both wheat varieties. Furthermore wheat cv. Karim showed the lowest values of Y. Those yield differences were mainly associated with a higher fertile shoot number per plant for barley. Indeed Y was strongly positively correlated with the fertile shoots and grain number per plant (R2 = 0.76 and 0.97, respectively). Drought decreased AGDW, number of fertile shoots and therefore Y for all genotypes but more severely for barley than for the wheat genotypes. N fertilizer effects were only consistent in 2006 where the N50 treatment increased fertile shoot number, AGDW and Y per plant, as well as WU, but only under irrigation, consistent with the literature (Ebrahim, 2008). Barley proved to have higher WUE associated with a higher SLN, and produced a higher Y. Water use and NupE were also higher under drought for barely than for wheat genotypes due to its more extensive root system. Therefore, it seems that on the basis of the present results under Mediterranean conditions, barley cv. Rum should be preferred when high rain or irrigation is available. When water is limited durum wheat varieties will probably maintain Y relatively better than barley. Nevertheless, these findings should be interpreted cautiously since wheat growth in this work was possibly limited more by the CE growing conditions in the UK than barley. Furthermore, the root growth of wheat cv. Karim was apparently susceptible to mechanical impedance that usually increases in drying soil. Overall, root systems of barley and wheat and their distribution with depth were broadly similar. However, under drought durum wheat seemed more adapted, not only relatively increasing the biomass allocated to the roots (high R:S), but in fact absolutely increasing TRW when compared to the irrigated plants. Traits underlying variation in total root length were different between genotypes; durum wheat was mainly dependent on the amount of biomass allocated to roots, while barley seems able to produce more root length by increasing SRL due to changes in tissue density. Therefore breeding programs should consider SRL a potential target trait. The relation between RLD and φ was verified resulting in a k value for RLD for barley of 2.4 cm-2. A CRLD of approximately 1cm cm-3 for barley was confirmed. However, results suggested that it might be lower under irrigation than under drought and lower for wheat than for barley. RVD was slightly better related to φ than RLD. Therefore more studies relating proportional resource capture and RVD are needed to confirm these findings and establish the basis of that relationship. β was confirmed by the present results to be a good trait to summarize the overall effects of changes of root distribution with depth and with drought. A simple framework relating the biomass allocated to roots (R:S), the investment in length (SRL) and the cumulative distribution of roots with depth (β) to a potentially higher RLD at depth and water and N uptake is suggested. Finally, the implications of the current findings for establishing agronomic and breeding strategies to improve below-ground resource capture, utilization and yield production under water and/or N stresses are discussed