The effect of pot size on growth and transpiration of maize and soybean during water deficit stress

Many experiments are conducted in greenhouses or growth chambers in which plants are grown in pots. Considerable research has shown that pots can have a limiting effect on overall plant growth. This research was undertaken to examine the effects of pot size specifically on transpiration response of maize (Zea mays L.) and soybean (Glycine max L.) plants undergoing water-deficit stress. Maize and soybean experiments were conducted similarly, but as separate experiments. Maize plants were grown in 2.3, 4.1, 9.1, and 16.2 l pots sealed to prevent water loss except by transpiration. For each pot size, plants were divided into two watering regimes, a well-watered control and a water-deficit regime. Water deficits were imposed by simply not rewatering the pots. Soybean was examined in a similar manner, but only the three larger pot sizes were used in the experiment. For both maize and soybean, and in both watering regimes, there was a significant reduction of shoot dry weight and total transpiration with decreasing pot size. However, there were no significant differences among pot sizes in the fraction of transpirable soil water (FTSW) point at which transpiration began to decline (FTSW0.31 for maize and 0.35 for soybean) or in the overall relationship of transpiration rate to soil water content in response to water deficits. These results indicated that, regardless of pot size or plant size, the overriding factor determining transpirational response to drought stress was soil water content.     

Water Stress and Day-To-Day Variation in Apparent Photosynthetic Acclimation of Field-Grown Soybeans to Elevated Carbon Dioxide Concentration

Midday measurements of single leaf gas exchange rates of upper canopy leaves of soybeans grown in the field at 350 (AC) and 700 (EC) µmol(CO₂) mol^–1 in open topped chambers sometimes indicated up to 50 % higher net photosynthetic rates (P _N) measured at EC in plants grown at AC compared to EC. On other days mean P _N were nearly identical in the two growth [CO₂] treatments. There was no seasonal pattern to the variable photosynthetic responses of soybean to growth [CO₂]. Even on days with significantly lower P _N in the plants grown at EC, there was no reduction in ribulose-1,5-bisphosphate carboxylase/oxygenase, chlorophyll, or soluble protein contents per unit of leaf area. Over three years, gas exchange evidence of acclimation occurred on days when either soil was dry or the water vapor pressure deficit was high (n = 12 d) and did not occur on days after rain or on days with low water vapor pressure deficit (n = 9 d). On days when photosynthetic acclimation was evident, midday leaf water potentials were consistently 0.2 to 0.3 MPa lower for the plants grown at EC than at AC. This suggested that greater susceptibility to water stress in plants grown at EC cause the apparent photosynthetic acclimation. In other experiments, plants were grown in well-watered pots in field chambers and removed to the laboratory early in the morning for gas exchange measurements. In these experiments, the amount of photosynthetic acclimation evident in the gas exchange measurements increased with the maximum water vapor pressure deficit on the day prior to the measurements, indicating a lag in the recovery of photosynthesis from water stress. The apparent increase in susceptibility to water stress in soybean plants grown at EC is opposite to that observed in some other species, where photosynthetic acclimation was evident under wet but not dry conditions, and may be related to the observation that hydraulic conductance is reduced in soybeans when grown at EC. The day-to-day variation in photosynthetic acclimation observed here may account for some of the conflicting results in the literature concerning the existence of acclimation to EC in field-grown plants.     

Growth and transpiration of maize and winter wheat in response to water deficits in pots and plots

Pots used for experiments conducted on plants grown in them create rooting environments that are affected by limited soil volume, which can affect various physiological processes, including transpiration, and plant growth. However, the applicability of results from pot experiments to the field has received limited attention. The objective of this study was to compare the growth and transpiration of maize (Zea mays L.) and winter wheat (Triticum aestivum L.) when grown in pots and field plots under various constant water deficits. The experiments were conducted under similar environmental conditions for both pots and plots. Transpirational responses at both transient (RTTr) and daily (RDTr) time scales to a decreasing fraction of available soil water (FASW) were analyzed. For a comparable FASW, there was a significant reduction in shoot dry weight and total transpiration for plants in pots compared to those in plots. A parabolic relationship between shoot dry weight and total transpiration existed and was not influenced by soil volume or crop type. The plot experiment data for both crops was consistent with pot data for the response of RDTr and RTTr to changes in FASW, which was represented by a linear-plateau function. However, the threshold values were significantly different for the two time scales. The threshold values and slopes of the linear-plateau function for maize and wheat were not significantly different in the response of RTTr to FASW, but were significantly different in the response of RDTr to FASW. Therefore, the transpirational responses of the selected maize and winter wheat hybrids to soil drought were different at the daily and transient time scales.     

Effect of salt and soil water status on transpiration of Salsola kali L.

Abstract Transpiration of Salsola kali L. plants, grown in small pots under controlled environmental conditions, was followed through a drying cycle of the soil. Three different nutrient solutions were used during the preconditioning growth period: control (C), half-strength Hoagland's nutrient solution; C plus 150mol m⁻³ NaCl; and C plus 150mol m⁻³ KCl. Soil water content at saturation at the beginning of the drying cycle was 20% (w/w). Both NaCl and KCl treatments modified the plants' response to changes in soil water status. The control plants transpired twice as much (per unit leaf dry weight) as the salt-treated plants, even when the soil was at maximal water capacity. Transpiration of the control plants remained high, until the soil water content declined to 5%. After that stage the stomata of these plants closed abruptly. Transpiration of the salt-treated plants started decreasing when the soil water content was approximately 16%, and did so gradually until all the available water was depleted. When transpiration was plotted against soil water potential a sharp decline in the transpiration of control plants was observed with the soil water potential decreasing from -0.04 to -1.2MPa. Transpiration of the salt-treated plants decreased gradually over a wide range of soil water potential (−0.8 to −7.0MPa).     

Establishment of willow cuttings grown in porous membrane root envelopes

Small diameter fine, fibrous roots of willow are exceedingly difficult to remove from most soils. Where field retrieval of entire plants including clean roots may be important, porous membrane rooting envelopes are a method of choice.Purple osier willow (Salix purpurea L.) stem cuttings (clone SP3) were field rooted in buried porous membrane envelopes permeable to water and nutrients. Ramets were grown for two and four months, then separated into component parts for dry weight analysis.At two months, cuttings grown in rooting envelopes were no different in shoot dry weight than plants grown in clay pots or in soil. Tests indicated that rooting envelopes may be reused at least once without affecting shoot or total plant dry weight. Likewise, membrane pore sizes of 0.45, 1.2 and 3 m did not affect SP3 dry weight in four month tests. The importance of matching plant material and experiment duration to envelope size was illustrated by the limitation of growth by envelope edges at four months compared to two months.     

Speciation analysis of nickel in soil solutions and availability to oat plants

Soil solutions were collected for speciation analysis of nickel from a pot experiment with oats. Oat plants (Avena sativa L.) were grown on 3 soils differing in total amount and origin of nickel (Ni) (Luvisol, LS with 28 mg kg^-1; sludge amended Luvisol, LS+SS with 32 mg kg^-1; Cambisol, CS with 95 mg kg^-1). Results were compared with those for soil solutions obtained from corresponding unplanted pots. Separation methods were used for characterization of size and charge distribution and stability of the Ni species. In addition, short-term experiments were performed on the uptake rates of Ni by oat plants from the different soil solutions as well as from nutrient solutions with increasing concentrations of a synthetic chelator.The Ni concentrations in the soil solutions of unplanted soils increased in the order: LS5000 g mol^-1) was the predominant form, whereas in the other soils the low-molecular-size cationic and chelated Ni species (500–1000 g mol^-1) dominated in the soil solution. In the short-term uptake studies, the uptake rates of Ni from the solutions decreased in the order: nutrient solution > soil solutions, and in the latter in the order: LS>LS+SS>CS, which was inversely related to the concentrations of dissolved organic carbon in the soil solutions.The results demonstrate that Ni availability to plants is not only affected by total concentration of Ni in the soil solution and the rate of replenishment from the solid phase, but also by Ni species, which can differ considerably between soil types.     

Enhanced Nematicidal Activity of Organic and Inorganic Ammonia-Releasing Amendments by Azadirachta indica Extracts

The nematicidal activities of ammonium sulfate, chicken litter and chitin, alone or in combination with neem (Azadirachta indica) extracts were tested against Meloidogyne javanica. Soil application of these amendments or the neem extracts alone did not reduce the root galling index of tomato plants or did so only slightly, but application of the amendments in combination with the neem extracts reduced root galling significantly. Soil analysis indicated that the neem extract inhibited the nitrification of the ammonium released from the amendments and extended the persistence of the ammonium concentrations in the soil. In microplot experiments, tomato plants were grown in pots filled with soils from the treated microplots. The galling indices of tomato plants grown in soil treated with ammonium sulfate or chicken litter in combination with the neem extract or a chemical nitrification inhibitor were far lower than those of plants grown in the control soil or in soil treated with chicken litter, neem extract or nitrification inhibitor alone. However, plants grown in the microplots showed only slight reductions in galling, probably because the soil amendments were inadequately mixed compared to their application in the pot experiments. The extended exposure of nematodes to ammonia as a result of nitrification inhibition by the neem extracts appeared to be the cause of the enhanced nematicidal activity of the ammonia-releasing amendments.     

Recovery of citrus surface roots following prolonged exposure to dry soil

The effects of prolonged exposure to dry surface soil on the capacity of roots to take up water and phosphorus were examined in mycorrhizal sour orange (Citrus aurantium L.) seedlings grown in pots with upper and lower portions separated hydraulically. In the first experiment, upper portions of the pots were either irrigated every 2-3 d, droughted for 14 d, droughted for 43 d, or droughted for 42 d followed by 8 d re-irrigation. Lower portions of the pots were irrigated and fertilized every 2-3 d. Phosphorus uptake capacity was estimated in excised roots using ³²P in aerated 50, 750, and 1500 M P solutions. Exposure to dry soil had no appreciable effect on P uptake capacity. In the second experiment, the ability of intact root to acquire water and P in the 8 d following rewatering after roots were exposed to localized drought for 14 and 43 d was examined. Roots were observed non-destructively using small transparent tubes (2 cm diameter) and a rigid borescope. Soil water depletion was monitored using time-domain reflectrometry. Phosphorus (³²P) was added at various depths in the soil in the upper compartment and uptake was assessed by non-destructively counting beta particle emissions from leaves using a scintillation probe. Similar to the first experiment, localized drought had no effect on P uptake and soil water depletion in citrus roots compared to continuously irrigated plants. Water and P uptake in the first few days apparently occurred from existing roots because of delayed production of new roots in the droughted treatment. Thus, citrus roots exposed to extended periods of dry soil apparently maintain or very quickly recover P and water uptake capacity. This behaviour is consistent with an overall rooting strategy where essentially no surface roots are shed following prolonged exposure to dry soil.     

The influence of temperature and nitrogen source on growth and nitrogen uptake of two polar-desert species,Saxifraga caespitosa and Cerastium alpinum

Polar-desert plants experience low average air temperatures during their short growing season (4–8 °C mean July temperature). In addition, low availability of inorganic nitrogen in the soil may also limit plant growth. Our goals were to elucidate which N sources can be acquired by polar-desert plants, and how growth and N-uptake are affected by low growth temperatures. We compared rates of N-uptake and increases in mass and leaf area of two polar-desert species (Cerastium alpinum L. and Saxifraga caespitosa L.) over a period of 3 weeks when grown at two temperatures (6 °C vs. 15 °C) and supplied with either glycine, NH₄ ⁺ or NO₃ ⁻. At 15 °C, plants at least doubled their leaf area, whereas there was no change in leaf area at 6 °C. Measured mean N-uptake rates varied between 0.5 nmol g⁻¹ root DM s⁻¹ on glycine at 15 °C and 7.5 nmol g⁻¹ root DM s⁻¹ on NH₄ ⁺ at 15 °C. Uptake rates based upon increases in mass and tissue N concentrations showed that plants had a lower N-uptake rate at 6 °C, regardless of N source or species. We conclude that these polar-desert plants can use all three N sources to increase their leaf area and support flowering when grown at 15 °C. Based upon short-term (8 h) uptake experiments, we also conclude that the short-term capacity to take up inorganic or organic N is not reduced by low temperature (6 °C). However, net N-uptake integrated over a three-week period is severely reduced at 6 °C. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of N-fertilizers,straw, and dry fallow on the nitrogen balance of a flooded soil planted with rice

Summary Nitrogen balance studies were made on rice (Oryza sativa) grown in flooded soil in pots. A low rate of fertilizer (5.64 mg N. kg⁻¹ soil) did not depress the N gain, but a high rate (99.72 mg N. kg⁻¹ soil) elminated the N gain. Soil N loss was negligible since¹⁵N applied as ammonium sulfate and thoroughly mixed with the soil was recovered from the soil-plant system after 3 crops. The observed N gain, therefore, was caused by N₂-fixation, not by a reduction of soil N loss. Straw enhanced N gain at the rate of 2–4 mg per g straw. However, this gain was not observed when soil N availability was high. Dry fallow between rice crops decreased the N gain.     

Root to Shoot Communication in Maize Plants of the Effects of Soil Drying

Seedlings of Zea mays L. (John Innes hybrid) were grown withroots divided between two containers such that part of the rootsystem could reduce the water potential of the soil in its immediatevicinity while the rest of the root system was well suppliedwith water. When compared to plants rooted in two pots of moistsoil, drying of part of the root system resulted in partialclosure of stomata, even though leaf water potential, turgorand abscisic acid (ABA) content remained unaffected. When leafpieces were removed from the two groups of plants and incubatedunder conditions favourable for stomatal opening, stomata ofthe ‘half-watered’ plants still showed restrictedapertures. Incubation in kinetin (10 mmol m^–3) or zeatin(100 mmol m^–3) reversed the closure of stomata stimulatedby soil drying. These results suggest that a continuous supplyof cytokinin from roots may be necessary to sustain maximalstomatal opening and an interruption of this supply due to soildrying may act as an indicator of inhibited root activity, resultingin restricted stomatal opening and thereby restricted wateruse. Key words: Zea mays L., Soil drying, Stomata, Roots     

Nonhydraulic signalling of soil drying in mycorrhizal maize

Our objectives were to (1) verify that nonhydraulic signalling of soil drying can reduce leaf growth of maize, (2) determine if a mycorrhizal influence on such signalling can occur independently of a mycorrhizal effect on leaf phosphorus concentration, plant size or soil drying rate, and (3) determine if leaf phosphorus concentration can affect response to the signalling process. Maize (Zea mays L. Pioneer 3147) seedlings were grown in a glasshouse with root systems split between two pots. The 2 x 3 x 2 experimental design included two levels of mycorrhizal colonization (presence or absence of Glomus intraradices Schenck & Smith), three levels of phosphorus fertilization within each mycorrhizal treatment and two levels of water (both pots watered or one pot watered, one pot allowed to dry). Fully watered mycorrhizal and nonmycorrhizal control plants had similar total leaf lengths throughout the experiment, and similar final shoot dry weights, root dry weights and leaf length/root dry weight ratios. Leaf growth of mycorrhizal plants was not affected by partial soil drying, but final plant leaf length and shoot dry weight were reduced in half-dried nonmycorrhizal plants. At low P fertilization, effects of nonhydraulic signalling were not evident. At medium and high P fertilization, final total plant leaf length of nonmycorrhizal plants was reduced by 9% and 10%, respectively. These growth reductions preceded restriction of stomatal conductance by 7 d. This and the fact that leaf water potentials were unaffected by partial soil drying suggested that leaf growth reductions were nonhydraulically induced. Stomatal conductance of plants given low phosphorus was less influenced by nonhydraulic signalling of soil drying than plants given higher phosphorus. Soil drying was not affected by mycorrhizal colonization, and reductions in leaf growth were not related to soil drying rate (characterized by time required for soil matric potential to drop below control levels and by time roots were exposed to soil matric potential below typical leaf water potential). We conclude that mycorrhizal symbiosis acted independently of phosphorus nutrition, plant size or soil drying rate in eliminating leaf growth response to nonhydraulic root-to-shoot communication of soil drying.Abbreviations and Symbols ANOVA analysis of variance - Cs stomatal conductance(s) - med medium - P probability - matric potential(s) - water potential(s) This work was supported by the U.S. Department of Agriculture grant No. 91-37100-6723 and a University of Tennessee Professional Development Research Award to R.M.A. We thank Angela Berry for the graphics.     

Effect of 6-benzyladenine and Casein Hydrolysate on Micropropagation of Amorpha fruticosa

Pisum sativum L. cv. Phenomen plants were grown in pots in greenhouse and their growth, and ATP and chlorophyll (Chl) a and b contents were assessed after 9-d exposure to sodium arsenate [0.04 and 0.07 mmol kg^–1(soil)], or to lead acetate [2.0 and 4.0 mmol kg^–1(soil)], or zinc acetate [5.3 and 9.3 mmol kg^–1(soil)]. The luciferin-luciferase method was used for ATP analyses. Soil pollution reduced significantly the growth, but the low toxicant concentrations elevated the cotyledon and shoot ATP concentrations per fresh matter content. The ATP/Chl ratio was increased in the zinc-treated seedlings as compared with the respective controls. The ATP concentration and a number of growth parameters were negatively correlated, and thus the high ATP content might contribute to the significantly reduced growth of seedlings.     

Genotypic variability among peanut (Arachis hypogea L.) in sensitivity of nitrogen fixation to soil drying

Peanuts (Arachis hypogea L.) are often grown on sandy soils and drought stress can be a major limitation on yield. In particular, loss in nitrogen fixation activity associated with soil drying might be limiting due to the need for high nitrogen amounts in both vegetative tissues and seeds of peanut. This study examined the response of nitrogen fixation of intact plants of seventeen peanut genotypes when subjected to soil drying in pots over approximately a 2-wk period. A large range in the sensitivity of nitrogen fixation to soil drying was observed among the seventeen genotypes. Genotype ICGV86015, in particular, was found to have nitrogen fixation that was especially tolerant of soil drying. Significant positive (P?<?0.0001) correlation was found between the soil water content at which nitrogen fixation began decreasing and the amino acid concentration in the leaves of severely stressed plants.     

Drought resistance of wheat plants inoculated with vesicular-arbuscular mycorrhizae

Summary Drought resistance of wheat (Triticum aestivum L.) as influenced by two vesiculararbuscular mycorrhizal (VAM) fungi,Glomus fasciculatum ¹⁰ andGlomus deserticola ¹⁹, was evaluated. Soil columns 0.15 m diam. by 1.20 m length were used to reduce the influence of limited rooting space. With initial soil water at 0.5 MPa (0.145 kg kg^–1), plants were subjected to low-level water stress throughout the experiment and severe water stress for 24 h at one (55 days after transplanting, Feekes scale 10.1) two (55 and 63 days, Feekes 10.1 and 10.2), or three (55, 63, and 70 days, Feekes 10.1, 10.1, and 10.2) periods. After each stress period, one set of plants was watered and grown to maturity without subsequent water stress. A second set of plants was harvested 1 week after stress.G. fasciculatum-inoculated plants harvested 7 days after stress at 55 days had greater leaf area and leaf, total plant, and root weight than non-VAM plants.G. deserticola-inoculated plants had greater leaf area and leaf weight than non-VAM plants. After stress at 55 and 63 days, leaf area, and leaf and total dry weight were again greater for VAM than for non-VAM plants. However, after stress at 55, 63, and 70 days, differences in aboveground biomass between VAM and non-VAM plants were not significant at P=0.05. Aboveground biomass was not affected by VAM species in plants stressed at 55 or 55 and 63 days, butG. fasciculatum-inoculated plants produced more tillers atter stress at 55 days. When grown to maturity, VAM plants which had undergone three stress periods had twice the biomass and grain yield as non-VAM plants subjected to the same stress. The three stress periods reduced number of heads and kernel numbers of weight of non-VAM plants compared to VAM plants.G. fasciculatum-inoculated plants consistently had increased root weight and rooting depth.Contribution from the Agricultural Research Service, USDA, in cooperation with the Nebr. Agric. Exp. Stn., Univ. Nebr.-Lincoln, Lincoln, Nebr. Published as Paper No. 7571 Journal Series, Nebr. Agric. Exp. Stn.     

Dual inoculation withRhizobium sp. andGlomus fasciculatum enhances nodulation,yield and nitrogen fixation in chickpea (Cicer arietinum Linn.)

Summary Seed inoculation with Rhizobium and soil inoculation withGlomus fasciculatum increased nodulation, nitrogen and phosphorus concentration in plants and yield of chickpea (Cicer arietinum) var. BG 212 in pots containing unsterilized soil especially with 50kgP₂O₅ ha⁻¹ in the form of superphosphate. Inoculation with Rhizobium orG. fasciculatum separately or in combination significantly increased the N₂ fixed in straw and grain than uninoculated controls as determined by¹⁵N atom percent excess of plants grown in soil amended with labelled ammonium sulphate (¹⁵NH₄)₂SO₄) at the rate of 20kg N ha⁻¹. These increases were most pronounced when P was applied at 50kgP₂O₅ ha⁻¹.     

Assimilation of exogenous 2′-14C-indole-3-acetic acid and 3′-14C-tryptophan exposed to the roots of three wheat varieties

This study was conducted to determine if plants can assimilate indole-3-acetic acid (IAA) from rooting media and if exogenous L-tryptophan (L-TRP) can be assimilated and converted by plants into auxins. The addition of 2-¹⁴C-IAA (3.7 kBq plant^-1) to wheat (Triticum aestivum L.) seedlings of three varieties grown in nutrient solution resulted in the uptake (avg.=7.6%) of labelled IAA. Most of the label IAA was recovered in the shoot (avg.=7.2%) with little accumulation in the root (avg.=0.43%). A portion of the assimilated IAA-label in the plant was identified by co-chromatography and UV spectral confirmation as IAA-glycine and IAA-aspartic acid conjugates. Little of the assimilated IAA label was found as free IAA in the wheat plants. These same assimilation patterns were observed when 2-¹⁴C-IAA was added to wheat plants grown in sterile and nonsterile soil. In contrast, the wheat varieties assimilated considerably less (avg.=1.3%) of the added microbial IAA precursor, 3-¹⁴C-L-TRP (3.7 kBq plant^-1) and thus much lower amounts of IAA conjugates were detected. Glasshouse soil experiments revealed that 2 out of 3 wheat varieties had increased growth rates and increased yields when L-TRP (10^-5 and 10^-7 M) was added to the root zone. It is surmised that this positive response is a result of microbial auxin production within the rhizosphere upon the addition of the precursor, L-TRP. The amino acid composition of the root exudates plays a critical role in microbial production of auxins in the rhizosphere. This study showed that wheat roots can assimilate IAA from their rooting media, which will supplement the endogenous IAA levels in the shoot tissue and may positively influence plant growth and subsequent yield.     

Mineralization of15N-labelled legume residues in soils with different nitrogen contents and its uptake by Rhodes grass

Summary Soil was collected from pots that had grown 1,3 or 6 soybean (Glycine max) or Siratro (Macroptillium atropurpureum) crops that had received organic residue returns from each crop.¹⁵N-labelled residues were added to half the pots in the experiment and the other half left unamended. Half of each group was then sown to Rhodes grass (Chloris gayana) which was grown, under glasshouse conditions, for 12 weeks.Ten grams of organic matter residues were added to each pot (1.5 kg soil) and the pots subjected to two wetting and drying cycles. At the end of the second wet cycle, soil mineral N values ranged from 6 to 64 ppm in unamended soils and from 19 to 177 ppm in amended soils. These levels generally declined over a 12 week period both in the presence and absence of sown grass.Nitrogen uptake by the grass increased with the number of previous cycles and was higher in Siratro than soybean soils. Recovery of¹⁵N by plant growth from the incorporated soybean residues was little effected by previous crop history and averaged 15.4%. On the other hand, Siratro recoveries were 13.7, 42.4 and 55.5% from soils that had grown 1, 3 and 6 previous Siratro crops, respectively.The addition of organic residues stimulated the release of native organic N (positive priming effect) on all soils.These results show that the turnover rate of nitrogen from organic residues can be high and the net result of these additions depends on the nature of the organic residues and the soil system to which they are added. These data emphasise the need to consider the rate of nutrient turnover from organic sources rather than concentrate on the nature and size of the resident nutrient pools.     

The effects of elevated atmospheric CO2 and soil P placement on cotton root deployment

Root proliferation into nutrient rich zones is an important mechanism in the exploitation of soil nutrients by plants. No studies have examined atmospheric CO₂ effects on cotton (Gossypium hirsutum L.) root distribution as affected by localized phosphorus (P). Cotton plants were grown in a Troup sand (loamy, thermic Grossarenic Kandiudults) using 17.2-l containers placed in open top field chambers (OTC) under ambient (360 mol mol^–1) or enriched (720 mol mol^–1) atmospheric CO₂ concentrations for 40 days. Equivalent amounts of P were added (150 mg P per kg of soil) to 100, 50, 25, 12.5, and 6.25% of the total soil volume; control containers with no added P were also included. Under extremely low P (controls), cotton was unresponsive to CO₂ enrichment. In treatments with both fertilized and unfertilized soil volumes, root proliferation was greater in the unfertilized soil under elevated CO₂ conditions. Stimulation of root growth occurred in the P-fertilized soil fraction; the pattern of stimulation was similar under both CO₂ levels. Under ambient CO₂, cotton plant response was positive (shoot mass, and total root mass and length) when soil P was confined to relatively small proportions of the total soil volume (6.25 and 12.5%). However, elevated CO₂ grown plants tended to respond to P regardless of its distribution.     

EDTA-Facilitated Phytoremediation of Soil with Heavy Metals from Sewage Sludge

The objective of this research was to determine the effect of the chelate EDTA (ethylenediaminetetraacetic acid), which is used in phytoremediation, on plant availability of heavy metals in liquid sewage sludge applied to soil. Sunflower (Helianthus annuus L.) was grown under greenhouse conditions in a commercial potting soil; the tetrasodium salt of EDTA (EDTA Na₄) was added at a rate of 1 g kg^-1 to half the pots. Immediately after seeds were planted, half of the pots with each soil (with or without EDTA) were irrigated with 60 ml sludge, and half were irrigated with 60 ml tap water. For the subsequent five irrigations, plants in soil with EDTA received either sludge or tap water containing 0.5 g EDTA Na₄ per 1000 ml, and plants in soil without EDTA received sludge or tap water without EDTA. Of the four heavy metals whose extractable concentrations in the soil were measured (Cu, Fe, Mn, and Zn), only Zn had a higher concentration in sludge-treated soil with EDTA compared to sludge-treated soil without EDTA. The concentrations of Fe, Cu, and Mn were similar in sludge-treated soil with and without EDTA. Of the three heavy metals whose total concentrations in the soil were measured (Cd, Pb, Cr), Pb (<10 mg kg^-1) and Cd (< 1 mg kg^-1) were below detection limits, and Cr was unaffected by treatment. The concentration of all measured elements in plants (Cd, Cu, Fe, Zn, Pb) was higher than the concentrations measured in the soil. With no EDTA, sludge-treated plants had a higher concentration of the five heavy elements than plants grown without sludge. Cadmium was lower in sludge-treated plants with EDTA than plants with EDTA and no sludge. After treatment with EDTA, the concentrations of Cu, Fe, and Zn were similar in plants with and without sludge. Lead was higher in plants with EDTA than plants without EDTA, showing that EDTA can facilitate phytoremediation of soil with Pb from sewage sludge.