Allocations to root and grain usually were greatest at ambient CO2, and those to chaff and shoots at either sub-ambient or elevated CO2. Grain typically contained the largest proportion of total N, P, Zn, and Cu, although the organ with the largest percentage of Cu varied with CO2 treatment among NO− 3 -supplied plants. Plants at sub-ambient and elevated CO2 allocated more Cu to the grain, while those at ambient CO2 allocated more to the roots. In general shoots received the majority of K, S, B,Ca, and Mg for all N and CO2 treatments. Ammonium-supplied plants allocated slightly more Mn to the roots at sub-ambient CO2, but allocated increasing amounts to the shoots at the expense of the roots as CO2 concentration increased. In contrast, NO− 3 -supplied plants allocated most of the Mn to the shoots. Ammonium-supplied plants typically allocated more resources to the chaff while NO− 3 -supplied plants allocated a greater percentage of elements to the roots.No other study to our knowledge has examined the influence of N form on plant nutrient relations at three different atmospheric CO2 concentrations. Overall, N form affected growth, total plant nutrient contents, and nutrient distribution in senescing wheat shoots, grain, and roots. The influence of NH + 4 and NO− 3 on growth and nutrient status were so distinct that they should be treated as separate nutrients and not bundled into a general category of N nutrition. Wheat size and nutrition at senescence responded to CO2 concentration in a non-linear manner. As was previously shown , we found that plants supplied with NH4 + were more responsive to CO2 concentration than those supplied with NO− 3 . Although not explicitly addressed here because of the heterogeneity of variances,vertical grow racks interactions between CO2 and N treatments likely existed for a number of the biomass and nutrient measures.
Most nutrient concentrations were generally higher in NH4 + – supplied plants, with the exceptions of NO− 3 − N, Mg, B, and Mn, which were generally higher in NO− 3 -supplied plants. Phytate, which hinders human absorption of Zn and Fe , showed little variation at ambient and elevated CO2 between NH4 + and NO− 3 -supplied plants, which, in conjunction with the observed greater bio-available of Zn in NH + 4 -supplied plants, may have consequences for human nutrition. Distribution of nutrients to the shoots, roots, chaff, and grain in response to CO2 concentration and N form was also non-linear and varied by nutrient. The data support our hypothesis that NO− 3 -supplied plants would show a more limited biomass and yield enhancement with CO2 enrichment than NH4 + -supplied plants. Nevertheless, mean biomass and yield decreased from ambient to elevated CO2 in both NO− 3 – and NH4 + -supplied plants in contrast to biomass increases in prior work on wheat seedlings . NO− 3 – supplied plants allocated more biomass to roots and had larger root:shoot ratios than NH4 + -supplied plants regardless of CO2 concentrations as has been reported previously , but increased root mass at elevated CO2 concentration for NO− 3 -supplied plants reported previously were not observed here. The shoot biomass data suggest that growth differences measured early in the lifespan of wheat supplied with NH4 + or NO− 3 or NH4 + do not necessarily carry through to senescence. This may be due in part to a shift in NO− 3 assimilation to the root , allowing NO− 3 -supplied plants to compensate for the decrease in shoot NO− 3 assimilation that occurs at elevated atmospheric CO2 concentrations . The decrease in yield and biomass measures at elevated CO2 concentrations does not agree with field observations where wheat yields as well as overall biomass increased with elevated CO2 . Similarly, our results that the greatest values for other yield measures occurred at ambient CO2 concentrations varies from the literature. Conflicting results, however, have also been reported .
Many of the field and open top chamber studies were grown under natural light and thus received substantially greater photosynthetic flux density than our chamber-grown plants. These higher light conditions would be more favorable to biomass accumulation. Also, these studies typically applied high amounts of mixed N fertilizer , and yields and biomass have been found to be greater under mixed N nutrition than under either NH4 + or NO− 3 alone . Finally, the wheat cultivar we used is a short-statured variety that has rarely been used in other studies and may have accounted for some of the differences between our study and other published data. Our results that NH4 + -supplied plants had greater yield and yield components than NO− 3 -supplied plants at ambient CO2 have been observed previously . Wang and Below observed greater numbers of kernels head−1 and KN in plants supplied NO− 3 that was not observed here. Their study, however, supplied NH4 + at relatively high levels . Several studies have found that incipient NH4 + toxicity can start appearing at N levels as low as 0.08–0.2 mM NH4 + , although the onset of NH4 + toxicity depends on light level and solution pH . The poorer performance of the NH4 + treatment in Wang and Below , therefore, might derive from NH4 + toxicity. We have previously determined that the 0.2 mM NH4 + -supplied to our plants to be sufficiently high for normal growth, but low enough to avoid toxicity problems under our experimental conditions .Our second hypothesis, that nutrient concentrations are differentially affected by the inorganic N form supplied to the plants and CO2 enrichment, was supported by our data. CO2 concentration and N form interactions may alter tissue demands for nutrients. For many nutrients, ratios between different elements are typically maintained within a narrow range . CO2 concentration and N form may disturb the balance between different nutrients, leading to a cascade of changes in demand, accumulation, and allocation among the different plant tissues .Some portion of the greater response of NH4 + -supplied plants to CO2 derived from a dilution effect from the greater biomass at ambient CO2 concentrations .
Total amounts of nutrients tended to decline with CO2 enrichment for NH4 + -supplied plants, which had the greatest amounts of macro/micro-nutrients at sub-ambient CO2 . These results have not been observed in other published studies . Growth chamber studies, however, tend to have more exaggerated differences among treatments than field and greenhouse experiments , and N source cannot be well-controlled in field and greenhouse experiments. The observed increase in NO− 3 −N concentration with CO2 concentration in NO− 3 -supplied plants has been reported previously , and adds further support to the hypothesis that elevated CO2 concentrations and the resulting decrease in photorespiration inhibit shoot NO− 3 photoassimilation. Nevertheless, tissue NO− 3 − N concentrations observed here were substantially lower than those in the earlier study . Again, this may derive from difference in life stages in the two studies. Most of the N available to the plant for grain filling comes from N translocation rather than uptake from the substrate . Probably, the plants continued to assimilate plant NO− 3 using a non-photorespiratory dependent process such as root assimilation after root N uptake slowed or stopped. Loss of NO− 3 through root efflux to the nutrient solution also may have contributed to the lower concentration of NO− 3 − N. The partitioning and accumulation of all mineral elements was affected in some manner by the CO2 treatment and N form supplied to the plants. Observations that cation concentrations decrease under NH4 + supply relative to NO− 3 supply were not apparent in this study. Again, this could be partly due to the relatively low concentration of NH4 + -supplied in our study, the age of the plants at harvest, and differences among wheat cultivars. Allocation of nutrients within the plant followed similar trends for both N forms,vertical hydroponics with the exceptions of Mn and Cu . Interestingly, in NO− 3 -supplied plants, shoot Mn concentrations increased slightly with CO2, and these plants allocated far more Mn to the shoots than NH4 + -supplied plants at all CO2 concentrations. Manganese has been found to activate Rubisco in place of Mg2+ and the Rubisco-Mn complex has been observed to decrease Rubisco carboxylase activity while minimally affecting or even enhancing oxygenase activity . The slight increase in shoot Mn with CO2 corresponded to a large 23% decrease in Mg concentration. Manganese, which can act as a cofactor for glutamine synthetase , was also the only nutrient that NH4 + -supplied plants allocated agreater percentage to the roots at the expense of the shoots. NO− 3 – supplied plants typically allocated a higher percentage of most nutrients to the roots, as has been reported previously .
Phytate, which forms complexes with divalent cations, has been found to hinder human Zn and Fe absorption during digestion and thus has been labeled an “anti-nutrient.” It may serve a number of valuable functions, however, including roles as an anti-oxidant and anti-cancer agent . Phytate is also the major repository of grain P, and variation in P supply to the developing seed is the major determinant of net seed phytate accumulation . To our knowledge, no published studies have explicitly looked at how phytate is affected by CO2 concentration. Elevated CO2 has been found to have a much larger negative impact on Zn and Fe concentrations than on P in wheat . Several studies have observed that P increases slightly with CO2 concentration, and because the majority of P is tied up in phytate, this may cause increases in grain phytate concentrations as atmospheric CO2 rises. As a result, bio-available Zn and Fe–Zn and Fe not bound to phytate – is expected to decrease even further . Nonetheless, we did not observe such trends in macro- and micro-nutrient concentrations in this study. The mechanism behind these contrasting results is not clear, although the environmental conditions and nutrient solution in which the plants were grown likely had some role. The modeled data demonstrated only a small negative impact of CO2 concentration on bio-available Zn concentrations , which was unexpected. Indeed, the grain from NO− 3 -supplied plants actually showed a slight increase in bio-available Zn between ambient and elevated CO2. These results combined with the differences in grain bio-available Zn between NH4 + and NO− 3 -supplied plants demonstrates that N form may differentially affect the nutritional status of this important nutrient, especially in less developed countries that might be more dependent on phytate-rich grains for their Zn nutrition . The milling process removes some, if not most, of the phytate and grain mineral content with the bran fraction of the grain . Regardless, with over 50% of the human population suffering from Zn deficiencies, even small increases in bio-available Zn would be beneficial . This modeling exercise, however, is not a prediction of how increasing CO2 will affect wheat nutrition so much as illustrates that N source may mediate, to some extent, the effects of CO2 on phytate and bio-available Zn, and that N source will become an even more important agricultural consideration in the future. In summary, both CO2 concentration and N form strongly affect biomass and yield in hydroponically grown wheat, as well as nutrient concentrations in above- and below ground tissues. Interactions among plant nutrient concentrations,CO2 concentrations, and N form are complex and non-linear. The impact of N form and CO2 concentration on the mechanisms affecting nutrient accumulation and distribution requires further research and extension to more realistic and agriculturally relevant growing conditions found in greenhouse and field studies. Of course, in greenhouse and field studies, control of N source is limited and control of atmospheric CO2 concentration is expensive. The effects of CO2 and N form on agriculture and human nutrition observed here are interesting and suggest a new area of research on mitigating the effects of climate change on agriculture. The supply of fertilizers or addition of nitrification inhibitors that increase the amount of available NH4 + may have beneficial effects for human nutrition, particularly in regards to micro-nutrient deficiencies such as Zn and Fe that currently affect billions of people worldwide.