Differences in root impedance have been demonstrated to have a major impact on penetration resistance.Fifteen blocks measuring 33 x 33m were marked out in an even grid with five blocks in each of three north-south columns representing the three treatment replicates.Blocks were separated from each other by 3m wide strips which were sown with grass seed after the first trial year was sown.Within each block, trial plots measured 1.55m wide by 2.1m long and were sown at 250 seed m-2 in early November 2007 or at 360 seed m-2 in early April 2008 for winter and spring varieties, respectively.Fifty-six lines of winter barley and 64 lines of spring barley, identified as a population useful for association genetics , were sown to each cultivation treatment.Prior to the year of sampling, the trial was sown in three successive seasons.The data presented in this paper were derived from samples taken from only the plough treatment, which represents the typical agricultural practice for the region,strawberry gutter system and minimum tillage, which is gaining popularity because of fuel costs and perceived benefits to soil conservation.Flag leaf samples were taken at growth stage 21 from both barley plants grown in pots and each genotype grown in the field and samples were kept on ice until they could be frozen.Samples were freeze dried before milling to a flour.One hundred mg of shoot material was digested in 5 mL of 18 M H2SO4 at 360 °C for 20 mins, after which an excess of 30% hydrogen peroxide was added until digests cleared.

Concentrations of P in diluted digests were determined by reaction with malachite green.All data are presented as the mean of three replicates and bars represent standard errors of the mean.Significant differences were established using ANOVA, and treatment means compared by LSD.Relationships between both shoot biomass and P concentration in the glasshouse experiment were related to the P added using regression.From these regression it was possible to determine the critical P concentration for barley to reach 75% growth in this soil.Mean data from the different cultivation treatments within each population of genotypes were compared using regression and r2 and 95% confidence intervals derived.The critical P concentration to achieve 75% growth was also plotted.All data were tested for normality prior to analysis.The shoot dry weight of spring barley cultivar Optic increased significantly to an asymptote with increasing P addition to the soil.This growth response was described by an exponential rise to a maximum of the form y = a + b.Using this equation it was possible to calculate the amount of P that needs to be added to this soil to achieve a critical proportion of maximal growth.For the purposes of this experiment 75% of maximal growth was used to define the critical growth of the barley plants, which was achieved by an addition of 175 mg P kg-1 soil.The shoot P-concentration of these plants also increased significantly with the addition of P to soil,however this did not reach an asymptote with the relationship being described by a linear equation.This allowed the calculation of the critical P concentration in the shoots to achieve 75% growth which, on this soil, was 2.56 µg P g-1 DW.These results demonstrate that soils typical of the arable north east of Scotland are responsive to the addition of P when barley is grown and that reasonably large additions of P are required to achieve growth approaching the physiological maximum for the barley.When grown in the field, winter barley genotypes had significantly greater P concentration in flag leaves than spring barley genotypes.This may be an indication of the greater root growth associated with winter germplasm which would allow greater access to P resources.

Within the spring and winter genotypes there was significant variation in shoot P concentration ranging from 1.7 to 3.0 and 1.5 to 2.5 µg P g-1 DM, respectively.Of particular interest was that some of the winter barley genotypes were able to achieve the critical shoot P concentration necessary to achieve 75% maximal growth in this soil under conventional cultivation.In contrast none of the spring genotypes were able to achieve this.Moreover, none of either winter or spring varieties passed the critical level in the minimum till cultivation treatment.There was also a significant impact of the cultivation treatment on the concentration of P in shoots, however this effect was dependent on which set of genotypes were considered.In the winter germplasm the minimum till cultivation treatment caused a decline in the shoot P concentration while in the spring germplasm there was a slight increase in the average shoot P concentration.The impact of the cultivation treatment on the ability of the various genotypes to acquire P is demonstrated even more strongly when the shoot P concentration of the various genotypes is correlated between the different cultivation treatments.The lack of correlation demonstrates that there is no significant relationship between the genotypes, either winter or spring germplasm, when grown in the different cultivation treatments.In fact, only 32% of the winter genotypes fall within the 95% confidence intervals of the relationship, while even fewer, only 19%, of the spring genotypes fall within the corresponding confidence interval.This suggests that the vast majority of barley genotypes have a differential response in P-nutrition to cultivation treatments, with approximately equal numbers being more suited to minimum tillage and vice versa.Interestingly, of the five winter genotypes which showed P concentrations greater than the critical for 75% maximal growth in conventional cultivation treatments, only one demonstrated a suitability for growth in minimum till cultivation.The availability and cycling of P for the conventionally cultivated and minimum till treatments are likely to be quite different due to differences in the biological and physiocochemical conditions of these treatments.For example microbial community structure and size, rooting depth, and water relations are all likely to be different.These differences will go some way to explaining why there is no relationship between genotypes of the association mapping populations between the two treatments.It is likely that the traits which allow some genotypes to successfully acquire P in the conventional plough cultivation treatment will be quite different to those of which benefit P-nutrition in minimum tillage treatment, therefore compromising the use of the genetic variation to predict useful genotypes for the different treatments.Results here are analogous with results on studies on genotypic variation in P-use efficiency in wheat which demonstrate that the ability of the wheat lines to acquire P was greatly dependent on soil type and only a small proportion of the variability in shoot growth and P content was attributable to genotypic differences.

It is therefore imperative that screening for P-use efficiency in cereals be performed on soils rather than in hydroponics or agarand ideally on a range of different soil types and under different agronomic treatments as the genetic control of this trait on one soil is likely to be different under a range of conditions.The development of SNP based high throughput genome wide assays which use hundreds of SNP’s to identify single markers from association mapping populations are a powerful tool to allow genotypic variation identified in studies like this to identify markers for P-use efficiency in barley.However, while there may be significant genotypic variation in these association mapping populations, the genetic component of this variation is not always robust between treatments.In conclusion, we have shown that genotypic variation in P-use efficiency is present in association mapping populations grown in the field.However, such variation is not related between soil cultivation treatments, where differences in root abiotic stresses will have a large impact on root growth and nutrient acquisition.It is therefore important that when screening for markers for multi-mechanistic traits, such as P-use efficiency, that screening occurs in a number of environments so that the robustness of the genotypic variation can be tested.In recent decades,grow strawberry in containers ubiquitous use of groundwater in California and other parts of the world have led to chronic groundwater overdraft and water quality issues.Worldwide recognition of groundwater depletion and its adverse effects on human and environmental well-being has increasingly led to actions, policy, and legislative change to manage water resources jointly and sustainably.For example, the Sustainable Groundwater Management Act formed in 2014 in California requires groundwater users to achieve long term groundwater sustainability by managing groundwater extraction and intentionally replenishing water in groundwater aquifers.One possible technique for groundwater replenishment is agricultural managed aquifer recharge in which farmland is flooded using excess surface water in order to recharge the underlying aquifer.As most agricultural fields have lower infiltration capacities compared with dedicated recharge basins, Ag-MAR is designed to capture high-volume excess surface water by flooding large areas of farmland at relatively low recharge rates of less than one meter per Ag-MAR event.Ideally, flooding for Ag-MAR is preferably done on fallow fields or during crop dormancy periods, when agricultural fields have the potential to serve as percolation basins for groundwater recharge.O’Geen et al.recommended potential areas in California for Ag-MAR using an index that combines five soil characteristics: deep percolation, root zone residence time, chemical properties, topography, and surface conditions.An ideal Ag-MAR site will comprise an effective deep percolation , adequate crop tolerance for flooding, low soil salinity, leveled soil surface, and lack of compaction and erosion.O’Geen et al.identified an area of 22,500 km2 of agricultural land , mostly in the Central Valley, as having excellent to moderately good potential for Ag-MAR.Root zone residence time is defined as the duration of saturated conditions in the soil root zone after water is applied.It is a key factor in Ag-MAR, as prolonged saturated conditions in the rhizosphere can damage perennial crops due to O2 deficiency— hypoxia—a well-known situation in agricultural soils under intensive irrigation.

Root functioning, nutrient and water uptake, vegetative growth, and crop yield are all affected by low O2 concentration.Moreover, in flooded fields, complete depletion of O2—anoxia— may occur, which affects crops severely and can lead to plant mortality.Therefore, quantifying the soil aeration status is of great importance for implementation of Ag-MAR on fields with perennial cropsD.A.Webb], grapes [ViThis vinifera L.], alfalfa [Medicago sativa L.]) with potentially no yield lost.Soil aeration is essential to support aerobic soil respiration which includes O2 consumption by plant roots and microbial population.The O2 level in unsaturated soils depends highly on the gas phase since the O2 concentration in atmospheric air is ∼250 mg L−1 while water in equilibrium with the atmosphere contains dissolved O2of only ∼8 mg L−1.The two transport mechanisms for meeting soil oxygen demand are diffusion of O2 in both the liquid and gas phases and convection of O2 in flowing water or air.However, low diffusion rates in water and the low solubility of O2 in water make the liquid phase contribution for O2 replenishment negligible.Therefore, gas transport is considered the main mechanism for supplying soil O2, and gas diffusion, driven by the O2 concentration gradient between the atmosphere and the soil, is considered the dominant transport process.The composition and magnitude of the gas phase in soils determines the soil aeration status, which controls O2 availability to soil respiration.Soil aeration status can be evaluated by the following quantifiers: volumetric air content , O2 concentration in the gas or liquid phase , O2 diffusion rate and soil redox potential.Under flooded conditions, as expected during Ag-MAR, O2 supply by gas transport is suppressed by soil saturation, as water occupies most of the air-filled soil pores.If ponding occurs, the ponded water layer at the soil surface will act as a barrier that effectively blocks soil gas exchange with the atmosphere, because the diffusivity of O2 in water is 10,000 times lower than in air.Under these conditions, hypoxia is expected to develop rapidly as the result of root respiration, microbial activity, displacement of air by water and impeded soil gas exchange.The depletion rate of O2 from the soil solution and entrapped air pockets will depend on temperature and respiration activity, so depletion will be slow at low temperatures and low organic-matter content.Upon water logging, the rate of decline in soil O2 from ∼21% to 0% can vary, ranging from one to several days.To minimize crop damage during Ag-MAR due to poor soil aeration, an adequate supply of O2 to the root zone must be provided.This can be achieved by natural- or forced-aeration of the root zone.