There is a striking similarity between the seasonal pattern of the endogenous IAA levels in the shoots that grow from the hanging and upright stem fragments and their seasonal rooting patterns. The lowest IAA levels for both stem types occurred in the spring, and the highest levels in late summer . At the time that the endogenous IAA levels are low, levels in the shoots from the upright stem fragments are significantly lower than in those from the hanging stems. As the IAA levels in the shoots increase with the progression of the growing season, the levels in the shoots from the upright stem fragments increase more than in those from the hanging stems, and the difference between the shoots of the two stem types disappears . The role of this IAA is unclear. Although studies have also shown more complicated mechanisms , IAA produced in the main stem apex plays an important role in the apical dominance, and the growth of side shoots. A difference between the rooting and the endogenous IAA patterns is that the seasonal pattern of endogenous IAA concentrations in the new shoots runs approximately one month behind that of the rooting success. This makes it less likely that the IAA concentrations in these new shoots, of which the growth is initiated prior to root growth, is the direct cause of the rooting of the stem fragments. It is possible that both the rooting pattern and the endogenous IAA pattern result from seasonal variation in another factor,veritcal hydroponic nft system possibly the concentrations of another plant growth regulator.
Preliminary enzyme kinetics results have shown higher NADP-dependent indole- 3-acetatealdehyde oxidase activity in the new shoots that developed from the hanging stems in April than in those from the upright stems . This indicates that the IAA measured in the new shoots may be the product of de novo synthesis in these shoots, rather than a trace of the IAA that may have been stored in the main stem fragment that was used in the experiment. The support for the formation of the vascular system by IAA and its promotion of the formation of adventitious roots may be related, but no causal relationship was determined in this study. The patterns of the IAA levels that developed in less than 10 days in the new shoots on the hanging and upright stem fragments may have resulted from at least two different factors. First, we hypothesize that the overall seasonal pattern resulted from the in situ temperature conditions at the time we sampled the main stem fragment. Secondly, the difference between IAA levels in the new shoots on the hanging and the upright stem fragments may have resulted from the effect of stem orientation on the inter- and intracellular distribution of plant growth regulators in the plant tissues. The near horizontal positioning of that part of the hanging stem where the side shoots were growing from the stem caused these side shoots to grow vertically, perpendicular to the direction of the original stem. This gravitropic response of the side shoots is the result of the different inter- and intracellular distribution of plant growth regulators that resulted from the horizontal orientation of the main stem. Rooting occurred for the intact meristems on both the intact stem fragments and those split lengthwise, with no significant difference in rooting percentages .
Zero rooting or shoot growth occurred from meristems that had been cut. Growth of the tissues with the highest CNC, the leaves, stopped earliest , but the leaves remained green. Their continued photosynthesis supplied the carbohydrates for the continued growth of the rhizomes , stems , and to a lesser extent that of the roots . Both below ground tissues, the rhizome and the roots, have a lower CNC level than the leaves. For those parts of the Arundo that is primarily responsible for growth, the leaves and the roots, their growth patterns mirrors their internal N:C ratio. As these tissues near their CNC, their sink strength for photosynthates, and therefore their growth is reduced. Both the leaves and roots have reached their CNC approximately since day 132, and their growth started tapering off since that time. If the experiment would have been continued longer, this would have shown better for the roots. After 60 days, the root masses of the individual A. donax plants could not be separated, and each plant was assigned a quarter of the root biomass, to show that overall root growth was tapering off near the end of the experiment. The tissue with the lowest CNC, was not, as expected, the rhizome which had a CNC of 0.030 g N/g C, but the stem, for which the N:C ratio went as low as 0.013 . Unlike the leaves, roots, and rhizome, which reached their CNC after approximately 130 days of growth, the N:C ratio did not reach its lowest levels until day 245. The rhizome of Arundo donax act as a storage tissue for reserves. The reserves stored will support stem regrowth from meristems on the rhizome in the spring . In addition to the rhizomes, spring regrowth is also supported by the stem tissues. Unlike the common reed, Phragmites australis, new side shoots grow from the upper section of Arundo stems in the spring .
Both tissues are originally stem tissues, and both play a role in the spring regrowth of the Arundo plant. Both the Arundo tissues that support spring regrowth, stems and rhizomes, have CNC levels below that of the leaves of Arundo. The difference in CNC between the stems and the leaves is larger than between the rhizome and the leaves. This resulted in significantly more stem growth than rhizome growth, with a final stem biomass of 1190 ± 95 g, and a final rhizome biomass of 171 ± 79 g after 334 days of development. For Ipomoea batatas these patterns were the opposite, because the CNC of the reserve storage tissue, the storage roots, was significantly lower than that of the stems . The CNC of both tissues was lower than that of the Ipomoea leaves. The N:C ratio of the leaves was 0.045 ± 0.0014, of the stem 0.017 ± 0.000006, and that of the storage roots was 0.013 ± 0.0011. For this species, as for Arundo, the biomass of the tissue with the lowest CNC, the storage roots, was significantly higher at 181.8 ± 23.8 g DW, than that of the tissue with a higher CNC, the stems, which reached 28.9 ± 7.5 g DW, when the plants had matured.Cerium oxide nanoparticles are highly stable nanoparticles that undergo very limited transformation in different environmental media.However, recent studies using synchrotron spectroscopic techniques showed that plants could enhance CeO2-NPs reduction [Ce → Ce] to as much as 23% Ce in soil,and 34% Ce acetate, 22% Ce carboxylates, 40% Ce phosphate, 40% Ce oxalate in hydroponic culture.Table 1). These enhanced transformations have been attributed to root exudates that plants release when exposed to CeO2-NPs.Previously, we showed peculiar differences on the uptake and translocation of Ce in rice, wheat and barley cultivated to full maturity in CeO2-NPs amended soil: rice and barley accumulated Ce in plant tissues and grains,nft hydroponic system whereas wheat did not transport Ce to the above ground tissues and grains.These plants were cultivated in similar growth conditions following soil exposure to CeO2-NPs. Therefore, it could be inferred that the discrimination on Ce uptake could have occurred through the roots wherein chemical environment and nanoparticle behavior in the root-soil interface controlled the differences on plants’ ability to transport Ce into the roots and aerial parts. Recently, we reported a follow-up study to understand the transformation and uptake of CeO2-NPs in wheat. Synchrotron micro X-ray fluorescence and micro Xray absorption spectroscopy techniques revealed the limited transformation of CeO2-NPs in full profile intact root-soil system with no evidence of plant uptake or accumulation;a finding that corroborated the results from our previous study in wheat.
In view of these findings, assessing the speciation of Ce in the root-soil system of barley exposed to CeO2-NPs may yield information on how Ce is taken up by the roots and translocated to the grains in barley as opposed to being immobilized in wheat roots. This study investigated the chemical fate of CeO2-NPs in the rhizosphere of barley and its possible role on Ce accumulation inside the roots. Since Ce was detected in the aerial parts of barley, it was hypothesized that the behavior of CeO2-NPs in the rhizosphere influenced the uptake of Ce in roots. Thus, this study focused on the speciation of CeO2-NPs in root-soil interface using synchrotron spectroscopy.This study is a continuation of research on long-term impacts of CeO2-NPs in barley. The CeO2-NPs are rods with primary size of × nm , particle size of 231 ± 16 nm in DI water, surface area of 93.8 m2 /g, and 95.14% purity.The preparation of nanoparticle suspension and plant cultivation were described in previous studies and given in the SI.Briefly, barley seedlings were cultivated in soil for 60 days in growth chamber with conditions maintained at 16-h photoperiod, 20/10˚C, 70% humidity, 300 μmol/m2 -s for the first 40 days, after which the light intensity was increased to 600 μmol/m2 -s. One hundred mL of Yoshida nutrient solution was added to the pots on the day the seedlings were transplanted. Samples for synchrotron analysis was prepared following the method described previously.Briefly, soil core was collected in a 2.5 cm × 6 cm aluminum cylinder, wrapped in plastic, and kept frozen . The cores were embedded with Spurr’s Resin, cut in half along the long axis, and 3–5 cm by 7 cm glass slides were glued to the cut surface to cover the entire cut surface on one half of each core. Further processing produced intact root/soil thin-sections with a thickness of ~100 μm. The μXRF mapping and μXANES analysis of Ce at the L3 edge in the barley root-soil thin sections was performed at 10.3.2 X-ray Microprobe Beamline at the Advanced Light Source at Lawrence Berkeley National Laboratory following the method described in Rico et al.and given in the SI. μXANES data for spots of interest as well as reference spectra were taken in fluorescence, using the QXAS flying-energy-scan mode of data acquisition. Reference standard μXANES spectra for linear combination fit were obtained from Ce oxide nanoparticles, Ce acetate, Ce carbonate, Ce oxalate, and Ce phosphate.The standards were prepared by blending a 1:1 ratio of the standards and boron carbide with a clean agate mortar and pestle. Small amounts of these mixtures were bound on Kapton tape and presented to the μXRF beamline. The beam energy was calibrated so that the first peak for CeO2-NPs was at 5730.39eV. Data were taken with a fine spacing near 5848.6eV, where a monochromator Bragg glitch served as an internal energy calibrant for each spectrum. The short dwell time in this region is the source of the noisiness of all spectra there. Because the white-line intensity is high, the fluorescence spectra for the Ce references are very sensitive to over absorption.Therefore, we took spectra at places where the intensity was high, for good signal, and at tiny particles, where the spectra were noisy but the same for a range of particles which yielded different count rates. We thus considered that these particles were small enough to avoid over absorption, and adjusted the spectra from the stronger-signal areas using a simple model for over absorption with the amount of over absorption varied so the spectral shapes for the strong-signal areas matched those for the tiny particles. This procedure gives us the signal quality from the strong-signal areas and the freedom from over absorption found with small particles. The μXANES spectra from the reference compounds were presented in Figure 1. LCF values obtained were not significantly different from each other so that it was difficult to determine the preferred Ce species. Thus, LCF from one Ce species was used . The μXRF elemental maps from intact root-soil rhizosphere of treated barley revealed a heavy presence and wide distribution of Ce in soil. Figure 2A depicts the thin section of intact rhizosphere and the area where elemental and chemical maps were acquired.