Wheat can utilize either form alone , but mixed N nutrition typically produces the best grain yields and quality in hydroponically grown and field-grown plants .Ammonium requires less energy to assimilate into organic compounds , but can prove toxic if it accumulates to high concentrations within plant tissues . Nitrate is generally the predominant form available in aerated, temperate agricultural soils , and may accumulate within plant tissues to high concentrations without toxicity . In wheat, the N form supplied has been found to influence many physiological parameters profoundly including biomass , leaf area , tillering , seed mass , protein content , and mineral nutrient acquisition and distribution , although such differences can vary among cultivars . The presence of NH4 + , as either a sole N source or in mixed N nutrition, increased organic N concentration in shoots, roots, and grain and decreased partitioning of dry matter to the roots in wheat . Decreased cation uptake has been found in wheat under NH4 + nutrition , although results varied among cultivars . For example, NH4 + nutrition decreased whole plant and shoot accumulations of K, Cu, Ca, Mg, Fe, Mn, and Zn . Nutrient allocation to plant tissues also varied between N forms. NH4 + -fed plants distributed a smaller percentage of total P, K, Cu, and B to roots relative to NO3 + -fed plants . Also, a greater percentage of reduced N was allocated to the shoots in NH4 + -fed plants . Elevated atmospheric concentrations of CO2 alter growth and N dynamics of wheat and other C3 plants. Under elevated CO2, wheat has lower protein and N concentrations ,vertical grow racks and lower macro- and micro-nutrients concentrations . Grain phytate concentrations are also thought to increase or remain the same under elevated CO2,and in conjunction with decreased concentrations of micro-nutrients, bio-available Zn and Fe are expected to decrease even further under elevated CO2 , as these micro-nutrients form indigestible complexes with phytate.
By contrast, wheat yields , harvest index , whole plant biomass , shoot biomass , and root biomass typically increase under CO2 enrichment. In addition, elevated CO2 concentration can increase tillering , nitrogen use efficiency , and micro/macro-nutrient use efficiencies. The influence of elevated CO2 on many of these characteristics may vary among cultivars and research protocols . Wheat grown under CO2 enrichment behaves differently under NO− 3 and NH4 + nutrition. Exposure to elevated CO2 inhibits NO− 3 photoassimilation in wheat as well as in all other C3 and C3– C4 intermediate plants tested . At elevated CO2, NH4 + -fed plants showed greater increases in leaf area and smaller decreases in shoot protein concentration than NO− 3 -fed plants , which could have consequences for human nutrition. Vegetative plants receiving NH4 + had greater shoot, stem, and root biomass at elevated CO2 . Wheat receiving NO− 3 grew slower at elevated CO2 than at ambient CO2 . Shoot NO− 3 concentrations in NH4 + -fed plants were undetectable while those in NO− 3 -fed plants increased by 62% with CO2 enrichment . This increase was associated with an inhibition in NO− 3 and NO− 2 reductase activities under elevated CO2 . The interaction between atmospheric CO2 concentration and inorganic N form and how it influences plant growth and nutrient concentrations has not been examined in wheat or any other crop species grown to senescence. Here, we grew wheat hydroponically in controlled environment chambers and measured mineral nutrition, biomass, and nutrient allocation in response to three concentrations of atmospheric CO2 and two forms of N nutrition . We tested the following hypotheses: plant nutrient concentrations and allocation patterns will respond differently to CO2 enrichment under the two N forms, and NO− 3 -fed plants will show a smaller biomass and yield enhancement in response to CO2 enrichment than NH4 + -fed plants as a result of CO2 inhibition of shoot NO− 3 assimilation. Also, we observed both differences in the Zn concentration between plants grown on NH4 + and NO− 3 and a strong dependence of Zn absorption on Zn and phytate concentration, indicating that phytate and bio-available Zn are affected by N form and CO2. Therefore, we used the well supported Miller equation to estimate how N and CO2 might impact a hypothetical human population. Iron, another important micro-nutrient that forms complexes with phytate, was not analyzed because we observed no significant differences in iron concentrations between the N forms and because how best to estimate Fe absorption in humans is still uncertain . Wheat seeds were surface sterilized for one minute in 2.6% sodium hypochlorite solution and thoroughly rinsed with DDI water.
The seeds were then rolled up in germination paper saturated with 10 mM CaSO4. The germination paper was placed in a 400 mL beaker with approximately 75 mL of 10 mM CaSO4 solution, covered with a plastic bag and placed in an incubator for four days. Seedlings were transplanted into 20 L tubs filled with an aerated nutrient solution that contained 1 mM CaSO4, 1 mM K2HPO4, 1 mM KH2PO4, 2 mM MgSO4, and 0.2 g L−1 Fe-NaEDTA and micro-nutrients 2HPO4 as the N source, Epstein and Bloom, 2005. The nutrient solution was replaced weekly and an additional 0.2 mM of NO− 3 – or NH4 + − N was added midweek until harvest. The solution volume was maintained by daily addition of deionized water. Solution pH varied between 6.8 and 7.0 for both of the N forms, and the NH4 + and the NO− 3 solutions did not differ by more than 0.1 pH units. The plants were grown in controlled environment chambers set at 23/20˚C day/night at 60–70% relative humidity with a photoperiod of 15 h. The photosynthetic flux density was 375µmol m−2 s −1 at plant height. Plants were subjected to one of three CO2 concentrations: “sub-ambient” , “ambient” , and “elevated” . Sub-ambient CO2 concentrations were maintained by passing air that entered the growth chamber through wet soda lime, a mixture of KOH, NaOH, and Ca2 that was replaced as needed. The elevated CO2 conditions were maintained in an environmental chamber equipped with non-dispersive infrared analyzers for CO2 and valves that added pure CO2 to the incoming air stream to hold the chamber concentration at 720 ppm. The wheat was grown until all above ground parts turned completely yellow. Plant matter was sorted into grain, chaff, shoots, and roots and dried for 48 h at 55˚C. Data on kernel number , kernel mass, number of heads, kernels head−1 , and HI were collected prior to sample preparation for nutrient analysis. A portion of the grain was analyzed for phytate using a modification of the method as described by Haug and Lantzsch . The remainder of the grain as well as the shoots and chaff was bulked into five repetitions per treatment and sent to the UC Davis Analytical Laboratory for nutrient analysis.
The roots of plants for each CO2 × N treatment became entangled within the same tub; therefore, we were unable to separate the roots of the individual plants for analysis. Root data are thus presented as means for each treatment with no standard errors or confidence intervals. Data were analyzed using PROC MIXED . Nitrogen form and CO2 factors were treated as fixed independent variables. We used the Tukey–Kramer Honestly Significant Difference test for mean separation. Probabilities less than 0.05 were considered significant. Because some of the transformed variables did not meet the assumption of homogeneity of variances,vertical farming in shipping containers but one-way ANOVAs met the ANOVA assumptions, we analyzed the results via one-way ANOVAs to gain some information on the interactions between CO2 and N form.We used a database derived from the United Nation’s Food and Agriculture Organization ’s national food balance sheets to estimate the average daily per capita dietary intake of zinc and phytate from 95 different food commodities in each of 176 countries. This database combines FAO data on per capita intake of food commodities with USDA data on the nutrient or phytate content of each of these commodities. More detailed discussion of the creation of this database for the International Zinc Collaborative Group may be found in Wuehler et al. . Using this database, we produced two data sheets: one containing per capita daily dietary intake of zinc from each food commodity for each country and another containing per capita phytate intake from each food commodity for each country. To calculate total dietary zinc and total dietary phytate per country, we summed across the rows of all food commodities for each respective country. To determine the proportion of a population at risk for zinc deficiency from a hypothetical least developed country , we first calculated TDP and TDZ values for a set of 44 countries defined by the United Nations as being least developed. We took the mean TDP and TDZ values for these countries to represent a hypothetical “less developed country.” To calculate the bio-available zinc portion we used the Miller equation . Mean TDZ and TDP values were converted to mg mmol−1 and put into the Miller equation to compute the average per capita TAZ in our hypothetical LDC. The variables TDZ, TDP, and TAZ are described above, and Amax, KP, and KR are constants as described in Miller et al. . We made an assumption that our hypothetical LDC receives half of its phytate and half of its zinc from wheat, which is roughly consistent with many of the LDCs in the FAO database. We analyzed the effect of elevated carbon dioxide levels on TDP, TDZ, and TAZ concentrations in a hypothetical LDC population for both NH4 + and NO− 3 -supplied wheat. To calculate a new TAZ for wheat grown under elevated CO2 conditions, we first calculated the percent change in TAZ from ambient to elevated levels for wheat receiving NH4 + or NO− 3 .
This computed percent change was then applied to half of the hypothetical TDZ and TDP; meanwhile, the other half of the hypothetical TDZ and TDP remained unmodified. Thus, the total new TDP and TDZ is the sum of the unmodified and modified portions. These new TDP and TDZ values for both NH4 + and NO− 3 -supplied wheat were then put into the Miller equation to compute new hypothetical TAZ values for an LDC. Differences and corresponding percent changes between the new TAZ values and the original TAZ value for a LDC were computed to determine the overall affect of elevated CO2 on TAZ in NH4 + and NO− 3 -supplied wheat for an average developing world population. TAZ, TDP, and TDZ concentrations can only be compared within a single N form across the CO2 concentrations due to methodological constraints of the model. Plants supplied NH4 + vs. NO− 3 nutrition reacted differently to CO2 enrichment . Plants supplied NH4 + differed across CO2 treatments for most of the yield and biomass measurements. The greatest values typically were found at ambient CO2 concentrations. Shoot, chaff, grain yield, number of heads, and KN were greatest at ambient CO2 levels. Individual kernel mass was greatest under both ambient and elevated CO2 treatments. HI and kernels head−1 showed no change across CO2 treatments. In contrast, biomass and yield measures of NO− 3 -supplied plants did not differ among the three CO2 concentrations. At sub-ambient CO2, differences between the NH4 + and NO− 3 treatments occurred in shoot biomass and three of the yield components: kernel mass, head number, and kernels head−1 . Ammonium-supplied plants had a larger number of heads while NO− 3 -supplied plants had greater shoot biomass, kernel mass, and kernels head−1 . At ambient CO2, NH4 + -supplied plants had a greater number of heads and greater chaff biomass. Plants supplied NO− 3 had a larger number of kernels head−1 . At elevated CO2, biomass and yield measures did not differ with N treatment. The distribution of nutrients and micro-nutrients among plant parts followed similar patterns in both the NH4 + and NO− 3 – supplied plants, although the NH4 + -supplied plant distributions were slightly more variable .