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.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,hydroponic growing 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 .

Synthesis of malate, however, may unduly tax a carbohydrate-limited root apex. Indeed, Ca2 treatment, which accumulated more NO3 – than the other treatments , contained negligible amounts of malate . In conclusion, NH4 + and NO3 – differentially affect the finescale spatial patterns of uptake, export, assimilation, and carbohydrate content along root apices. Moreover, although NO3 – levels are maintained low in the meristem and the apical part of the growth zone, NO3 – clearly needs to be considered as a significant component of the osmotic pool supporting expansion at the base of the growth zone Although conditions on early Earth are still a matter of much debate, Pasek and co-workers have argued that reduced phosphorus compounds, in particular phosphite, were abundant when life first emerged during the Archean period . They note the fact that most meteorites contain phosphide minerals , such as schreibersite , which can abiotically corrode in the presence of water to release reduced P compounds such as phosphite, hypophosphite , and phosphine gas  . Due to the heavy bombardment believed to have occurred 4.5-3.8 Gya, up to 1018 kg may have been derived from meteorite impacts . Given that schreibersite corrosion occurs fairly rapidly at geological timescales and phosphite can account for >50% of the total soluble reduced P produced, meteoritic impacts would have deposited a substantial quantity of phosphite on the early Earth . Some additional phosphite could also have been derived from lighting discharges associated with volcanic activity since phosphite is known to occur when lightning strikes phosphate–containing minerals and volcanic ash . Since phosphite is very kinetically stable it would have had a half-life of 108 -1010 years under the reducing conditions of the Archean and could therefore have accumulated in the early ocean to concentrations of up to 10 mM . The recent detection of phosphite at relatively high proportions in 3.5 billion-year-old marine carbonate rocks appears to support this scenario .

The idea that reduced phosphorus compounds may have been involved in the development of early life was first proposed by Gulick in the 1950s . He reasoned that phosphate would have been a poor substrate for the phosphorylation of prebiotic organic molecules due to its low solubility and reactivity, whereas reduced P species such as phosphite and hypophosphite, which are significantly more soluble and more reactive towards organic carbon and nitrogen compounds, could have facilitated the emergence of phosphorylated biomolecules . Gulick’s theory was dismissed at the time because there was no known source of reduced P that could account for the proposed reactions, but in light of recent evidence for the prevalence of phosphite on early Earth, Pasek and co-workers have revisited this idea . In a series of experiments, they showed that schreibersite corrosion in water not only produces phosphite and hypophosphite but can also lead to the phosphorylation of simple organic molecules like acetate and ethanol . Based on these findings it seems plausible that phosphite could have played a key role in the emergence of life,mobile vertical farm although further work is needed in order to establish the relevance of these reactions within the context of protobiotic chemistry. It had been previously assumed that reduced P compounds present on early Earth would have been gradually oxidized to phosphate after the Great Oxygenation Event and therefore phosphite should be a negligible component of modern environments . However, phosphite has recently been detected in various environments including rivers, lakes, swamps, and geothermal pools . The phosphite concentrations measured in these studies ranged from 0.1 to 1.3 µM and accounted for 1 to 33% of the total dissolved P in the systems. Although phosphite tended to be more abundant under more reducing conditions, concentrations of up to 1 µM were observed even in some surface water samples . The presence of micromolar amounts of phosphite in oxygen-exposed environments is unexpected given that phosphite reacts with oxygen fairly rapidly at geologically timescales . As noted by Pasek and coworkers, meteorite strikes and lightning discharges are relatively rare on modern Earth, making it unlikely that these processes by themselves can account for the amounts of phosphite detected in surface waters . Some of this observed phosphite might be of anthropogenic origin since it can be a byproduct of the industrial production of phosphonates , which are used as herbicides, detergents, and chelating agents . Additionally, phosphite itself is used as a reducing agent in some industrial metal electroplating processes and as a fungicide in agriculture . Phosphite can therefore be a component of industrial waste as well as agricultural runoff and has in fact been detected in the influent of wastewater treatment plants .

Han and coworkers have also observed higher phosphite concentrations at heavily polluted lake sites compared to less impacted areas . In pristine environments, geothermal activity may potentially serve as an alternate source of phosphite via the formation and subsequent corrosion of metal phosphides . Like other reduced P compounds, phosphide minerals are unstable in the presence of oxygen at geological timescales and are therefore rare on the Earth’s surface . The deposition of extraterrestrial schreibersite by meteorites and the reduction of phosphorus impurities in iron ore during industrial smelting are typically cited as the only significant sources of phosphides on Earth . Nonetheless, natural terrestrially produced schreibersite has been found in iron-rich basalts in Greenland , in ultramafic rocks uncovered during continental drilling in China , and in pyrometamorphic rocks in the Levant . Britvin and coworkers cite these findings as evidence for “the occurrence of geologically juvenile terrestrial phosphides” and outline the four conditions necessary for the formation of these compounds: the presence of phosphorus, the presence of transition metals such as Fe or Ni, a highly reducing geochemical environment, and temperatures high enough to sustain the reduction process . Based on these criteria it is likely that metal phosphide formation occurs in the subsurface due to the geothermal reduction of phosphate. Indeed, Glindemann and colleagues have noted that the strong reducing conditions observed within the Earth’s crust should be conducive to the reduction of phosphate minerals to phosphides . The average elemental proportion of phosphorus in the Earth’s crust is thought to be about 0.1% although it may be higher in the oceanic crust due to phosphate deposition into porous sub seafloor basalts during hydrothermal circulation of seawater along mid-ocean ridge flanks . Sub seafloor basalts are also rich in Fe and other reduced chemical species such as H2, H2S, and CH4 . Furthermore, temperatures at the contact zone between mantle-derived magma and seawater at mid ocean ridge spreading zones can be as high as 400o C . Reduction of phosphate in the presence of metal salts to produce metal phosphides is known to occur within hours at temperatures as low as 400o C in the presence of hydrogen gas . The sub seafloor crust therefore appears to satisfy all the requirements for the formation of metal phosphides, which would subsequently react with seawater at short geological timescales to release phosphite and other reduced P compounds. Since phosphite is highly soluble and kinetically stable it would likely diffuse up through the porous basalt into the cooler upper layers of the sub seafloor and possibly into the water column before being re-oxidized by dissolved oxygen in the ocean. Phosphite may also be derived from biological processes, such as phosphonate degradation . Phosphonates are organic compounds with C-P bonds, as opposed to the C-O-P esters found in organophosphate compounds, and they have a P oxidation state of +3, as in phosphite . They can account for up to 25% of the dissolved organic P in some marine environments . Some of this environmental phosphonate may be derived from industrial processes, but there are also biological routes for phosphonate production. Various organisms can incorporate phosphonates into their cell membranes as phosphonolipids or secrete antibiotic phosphonate compounds such as fosfomycin . Although the biosynthetic pathways for these compounds have not been well characterized, the conversion of phosphoenolpyruvate to phosphonopyruvate by the enzyme PEP mutase is thought to be a common initial step in the production of phosphonates .