Root chemical concentrations were expressed per segment water volume based on root radius measurements at each location.The current study is one of the few that has determined both endogenous and exogenous patterns of nutrient supply and the only one that contrasts the two major inorganic N forms, NH4 + vs. NO3 – . A previous report estimated root net influx of exogenous N from the disappearance of N ions from the bathing solution; the current study calculated the endogenous rate at which the N ions are deposited locally with a continuity equation. Comparison of net influx to deposition rate indicates the extent to which the tissue is either retaining or exporting the N taken up from the bathing solution . If the deposition rate exceeds the exogenous net influx, then the difference shows the rate at which the tissue is importing the ion from older tissue or generating it metabolically.For roots receiving an exogenous NH4NO3, net NH4 + influx was fast enough to support the local deposition only in tissue basal to 5 mm . Because NH4 + deposition rate exceeded the exogenous net influx through the apical 5 mm, the meristem and apical half of the growth zone must retain most of the exogenous supply as well as import NH4 + or NH4 + precursors from more mature tissue. This conclusion is supported by the observation that similar NH4 + deposition rates occurred in the apical 6 mm when the source was Ca2 ; some combination of chemicaltransformation of NO3 – , import of NH4 + from more mature tissue, or deamination of amino acids must have occurred at these locations. With the exogenous Ca2 supply, the NH4 + deposition rate becomes small or even negative in the region basal to 6 mm. These conclusions confirm and, by showing the spatial profiles, extend the conclusions of Walter et al. , who found that as a whole,vertical rack the growth zone receives from non-growing tissue 31 nmol h 1 NH4 + while it exports 45 nmol h 1 NO3 – .
The excess of net influx over deposition of NH4 + and the co-occurring decline in tissue NH4 + in regions basal to 7 mm suggest that NH4 + absorbed near the apex remained unassimilated, whereas NH4 + absorbed in the region 7–60 mm from the apex was assimilated . Profiles of net NH4 + and H+ fluxes in the maize seminal root also support this interpretation . Assimilation of NH4 + produces H+ that roots rapidly excrete . The current study found that net NH4 + influx was greater at the root apex than at the regions 4–10 mm from the apex, but that net H+ efflux was greater 4–10 mm from the apex than at the apex, suggesting that NH4 + assimilation was more rapid in the more basal regions . This seems reasonable given the carbon/nitrogen balance in the root apex. The assimilation of NH4 + into glutamine is highly carbohydrate dependent, requiring carbon skeletons from 2-oxoglutarate and a respiratory expenditure of about 2 ATP equivalents per NH4 + . The root apex, however, lacks mature vascular tissue to facilitate carbohydrate translocation from more basal tissues, and nonvascular, symplastic diffusion of carbohydrates appears to be inadequate to meet the energy requirements of this tissue . Thus, the apex probably suffers from carbohydrate limitations. Maize seminal roots most likely store some of the NH4 + absorbed at the apex in vacuoles to avoid toxicity. Indeed, the location of maximum deposition of NH4 + coincides with the location where vacuoles in root cells are enlarging. Assimilation of NH4 + is greater in more basal regions where the phloem is more fully developed and capable of supplying sufficient carbohydrates. Although the NH4 + concentrations in the N-free and Ca2 treatments were significantly lower than those in the NH4NO3 and NH4H2PO4 treatments, the N-free and Ca2 treatments still showed small amounts of NH4 + in their root growth zones. Several explanations come to mind. First, the extraction and analysis protocols may have resulted in some deamination of amino acids. Second, some of the signal may have come from free amino acids in the root sample because free amino acids interfere with the NH4 + analysis technique, although the sensitivity of this analysis to amino acids is less than 10% of its sensitivity to free NH4 + . It is concluded that the apical 4 mm imported or produced free NH4 + through deamination during N-cycling even when NH4 + was not present in the bathing solution. When NH4 + was present exogenously, the presence of NO3 – in the medium had only a small effect upon NH4 + concentrations in the root tissue or in the xylem sap . This fits with earlier studies that demonstrated root NH4 + acquisition to be relatively independent of NO3 – or other anions .
The profile of NO3 – concentration in the growth zone was the same with Ca2 as with NH4NO3 , but the way in which the patterns were produced varied with the form of nitrogen supplied. With the Ca2 treatment,NO3 – influx exceeded the net deposition rate at all locations, but especially in the parts of the root basal to the growth zone where the influx was 4- to 5-fold greater than the deposition rate. The patterns with the NH4NO3 treatment were quite different: in the meristem and beyond 12 mm NO3 – influx slightly exceeded the deposition rate, but throughout the rapid growth zone the deposition rate exceeded influx. Thus, with the Ca2 treatment, the entire root tip absorbed more NO3 – than it deposited; indeed, the NO3 – influx greatly exceeded the amount that remained in the tissue. In contrast, with NH4NO3 treatment, most of the growth zone was importing NO3 – from mature tissue and NO3 – influx greatly exceeded that which remained in the tissues only in the more basal regions . Thus when NH4 + was present in the medium, NO3 – absorbed near the root apex was stored in the tissue and negligible amounts were assimilated or translocated. It is concluded that the more mature tissues were assimilating and exporting much of the NO3 – absorbed. This is a reasonable conclusion because NO3 – influx exceeded the deposition rate in these tissues and NO3 – in the xylem sap doubled .During its development, an individual tissue element is displaced from the root meristem through and then beyond the growth zone. The position of the basal and apical ends of the segment can be tracked to find the location and length of the segment over time. The growth trajectory gives the time course of the element position and can be calculated by integrating the displacement velocity over time or by counting the number of cells to the position of interest and multiplying by the ratio of mature cell length to root elongation rate . This paper is interested in calculating the total uptake of NO3 – into the tissue element as it expands and moves farther from the apex. To model the ‘potential uptake’ of NO3 – that results from influx,vertical farming hydroponic this study assumes no assimilation or translocation and then considers what the NO3 – content would be in the moving tissue element. Before calculating the total uptake in the developing tissue element, two extreme cases are considered: localized influx only in the apical 3 mm; and uniform influx along the growth zone. In the first case, with influx occurring only near the apex , then the NO3 – content would first increase and then decrease .
Where growth continues after influx ceases, then NO3 – would decrease more rapidly with position. Where influx and growth have both stopped, NO3 – would remain uniform in the absence of assimilation and translocation. Therefore, if influx is restricted to the apical 3 mm, the potential uptake into the older root segment would be quite small. In the second case, with influx occurring throughout the root, then the potential uptake would increase slowly as the element moves through the growth zone and more rapidly after growth has ceased in the tissue element . NO3 – is in fact taken up throughout the root . Fig. 9B shows that the total NO3 – uptake slightly exceeds the observed content while the tissue element is moving through in the growth zone, and the total uptake vastly exceeds the content when the tissue element is in the 10–20 mm region. Comparisons of the influx and deposition rates are most useful to analyse the physiology and biochemistry of the local nitrogen transformations and to determine the source–sink relations. It is also instructive, however, to compare the total uptake to the content , to appreciate the amount of the influx that has been retained in the tissue element over time.Glucose and fructose contributed about half of the osmolarity in the zone of elongation . Sucrose was undetectable . Other studies estimated sucrose in the maize root apex from tissues extracted with 80% ethanol at 80 C and estimated sucrose after its chemical or biochemical conversion to glucose , procedures that can overestimate sucrose and underestimate glucose and fructose . The current study directly measured glucose, fructose, and sucrose via HPLC immediately after boiling water extraction to inactivate any enzymes that might hydrolyse sucrose. K+ and its counter-ions contributed the other half of the osmolarity in the zone of elongation . Previous studies on maize seminal roots have not addressed the issue of counter-ions for K+ . This study found that the counter-ions for K+ included malate and NO3 – , but these could balance less than half of the K+ . The nutrient solution also contained H2PO4 – and SO4 2–. Walter et al. measured H2PO4 – and SO4 2– along the apical 10 mm of maize seminal roots receiving NH4NO3 and found their concentrations to be less than a third of the NO3 – concentrations. Most likely a combination of these anions, organic anions other than malate and citrate, and an increase in cellular pH accounted for the remainder of the counter-ions . Osmolarity remained high in the more basal zones of the root despite a substantial decline in glucose, fructose, and K+ concentrations . NO3 – accumulated in these more basal regions, as discussed above, but NO3 – and its counter-ions such as K+ contributed less than half of the observed osmolarity . Unfortunately, previous studies have not analysed solute concentrations in these more basal regions or have analysed only the soluble sugars .These results indicate that when both NH4 + and NO3 – were available in the rhizosphere, maize roots absorbed both forms, but preferentially assimilated NH4 + and stored NO3 – . Assimilation of NO3 – to glutamine expends 12 ATP equivalents versus only 2 ATP equivalents for NH4 + to glutamine . For the root apex, which may be carbohydrate-limited, a 6-fold difference in energy requirements was obviously critical. When NO3 – was the sole N-source, the root stored about the same amount of NO3 – in its tissues, while apparently importing or assimilating some NO3 – to support the rapid protein synthesis in the meristem and translocating a large portion of the NO3 – from the young mature tissues to the shoot. Shoots can use surplus light to assimilate NO3 – so that the large energy demands of this process do not detract from growth . The storage of substantial quantities of NO3 – at the base of the growth zone and in the young mature root tissues argues that NO3 – may serve as a metabolically benign osmoticant to balance other ions in plant tissues . Zhen et al. , using intracellular NO3 – -selective microelectrodes, found that most of the NO3 – in the epidermal and cortical cells of barley roots was stored in the vacuole and at levels that varied between 50 and 100 mol m 3 . Here, accumulation of hexoses and K+ in root cells of the elongation zone sustained root expansion, and malate served as counter-ions to K+ , as it does in other tissues .