Phosphorus uptake rate and growth characteristics of wheat roots 1

Phosphorus uptake by plant roots is influenced by the plant root properties and solution P supply characteristics. These properties included (i) the relation between nutrient concentration and uptake rate, (ii) the change in uptake rate with plant age and with root age.

Information on the size of nutrient flux values and their change with increasing plant age can be used to determine the nutrient levels needed in the soil to supply nutrients rapidly enough to the root surface to minimize deficiencies. The objective of this research was to determine the relation between plant age and P absorption properties and root growth characteristics of wheat (Triticum vulgare L.) cv. Era.

Wheat was grown for periods up to 42 days in solution culture in a controlled climate chamber. Sequential harvests were made and P uptake and root morphology were measured. Shoot growth was exponential with time to 32 days and linear thereafter. Root dry weights increased linearly with time at a slower rate than shoot dry weights. Root length increased logarithmically with time (r² = 0.95; log y = 0.069x + 1.85).

With increasing plant age there was a reduction in average P uptake rate by wheat roots.     

Low-P solution culture can be used for screening root growth vigor in soil for high nutrient uptake of spring wheat varieties

Purpose: Root and root hairs of plants have been intensively studied in solution culture; however, correlation of such measurements in solution culture with development in soil is poorly understood. Therefore, the aim of this study is to study whether root and root hairs grown in solution culture can predict their behavior in soil and their correlation with macro- and micronutrients uptake of wheat genotypes.

Materials and methods: The growth of roots and root hairs as well as uptake of macro- and micronutrients of six spring wheat varieties was compared in solution culture under P stress and P abundance and in a low fertility soil.

Results and conclusions: Root length and surface area under P stress were significantly positively correlated with that in the low fertility soil, while no such correlation was apparent for root hair length and density. In absolute terms, the root length, surface area, root hair length and density of spring wheat varieties were substantially higher in soil than in solution culture, while the concentration and uptake of macro- and micronutrients in soil differed from solution culture in a complex way. The early uptake of macro- and micronutrients was intimately associated with root length and surface area as well as root hair length and density in soil but not in solution culture. Therefore, root length rather than root hair traits in low-P solution may be used to screen early root growth vigor in soil and thereby high nutrient uptake of wheat in low fertility soil.     

Effect of supplying P to a portion of the soybean root system on root growth and P uptake kinetics 1

Abstract

Knowledge of the effect of supplying P to portions of the soybean (Glycine max L. Merr) root system on P influx kinetics and root growth is important in developing P fertilizer placement practices for efficient fertilizer use. The objective of this research was to determine the effect of restricting P supply to portions of the root system on plant P status, root growth, and P influx kinetics. Two solution experiments were conducted in a controlled climate chamber. Phosphorus influx kinetics were determined on 25‐day‐old soybean plants that had been grown with 100, 75, 50, 25, and 12.5% of their roots initially exposed to P. Phosphorus influx kinetics were also measured on 25‐day‐old plants that had been P‐starved for the last 1, 2, 4, and 6 days prior to the determining P influx kinetics in order to relate plant P status to P influx kinetics.

Reducing the portion of the roots supplied with P reduced P uptake. This resulted in a reduction in plant P concentration and was related to a 3.41‐fold increase in maximum P influx measured on 25‐day‐old plants. Restricting the proportion of roots supplied with P had no significant effects on the Michaelis‐Menten constant or on the concentration in solution where net influx was zero. Root growth rate of the roots in the P containing solution was not significantly different from those in the ‐P solution.

Phosphorus uptake was correlated with final root surface area exposed to P (r² = 0.88??). Starving the plants for P reduced P concentration in the shoot and root and this resulted in as much as a 1.68‐fold increase in maximum influx.     

Phosphorus (P) Uptake mechanisms of crops grown in soils with low P status

In our previous studies, pigeonpea (Cajanus cajan L.), groundnut (Arachis hypogaea L.), and rice (Oryza sativa L.) were found to have a higher ability to take up Fe- or Al-bound phosphorus (P) than soybean (Glycine max L.) and sorghum (Sorghum bicolor L.). Phosphorus absorption characteristics like I _max, K _m, C _min, and Fe^III reduction activity of roots, and root exudates in various crops were examined with a view to analyzing the mechanisms of P uptake. Phosphorus uptake ability was largely unrelated to variations in I _max, K _m, C _min, and Fe^III reduction activity of roots. Phosphorus-solubilizing activity in anionic fractions of root exudates was detected in pigeonpea but not in rice or groundnut. Malonic acid was the major component followed by oxalic and piscidic acid. These organic acids were able to release P from FePO₄ and A1PO₄. The higher P uptake ability of pigeonpea in soils with low P fertility presumably depends on the secretion of such organic acids from roots.     

The effect of salinity and iron application on growth and mineral uptake of sunflower (helianthus annuus L.)

In a greenhouse experiment, the effect of salinity and Fe chelate on growth and mineral uptake of sunflower (Helianthus annuus L. c.v. Record) was studied.

Sunflower plants were grown in nutrient solution with four levels of salinity (0, 1.5, 3.0 and 4.5 atm), induced by NaCl and four rates of Fe chelate (0, 0.5, 1.0 and 1.5, ppm Fe) as FeEDDHA. The experiment was a completely randomized design with treatment combinations arranged in a factorial manner with three replications.

Dry matter yield, shoot‐root ratio, leaf area, plant height and transpiration decreased as salinity increased, the effect of salinity being depressed by iron applications. Salinity reduced P, K, Ca and Mg uptake by roots as well as that of N, P, K, Ca, Mg by shoots, while Fe applications increased uptake of these elements in roots and shoots. Both salinity and iron applications increased Cl, Na and Fe uptake by roots and shoots, as expected. In most instances salinity reduced uptake of Fe, Mn and Zn by the plants while iron applications improved uptake of these elements.

The sunflower plant used in this experiment was found to be, at least partly, tolerant to salinity and decreased water availability as well as toxicity of ions. Nutritional disorders were the cause of decreased plant growth by increasing salinity of the nutrient solution. The decreased plant growth and mineral uptake, induced by salinity, were partially offset by increased iron levels in the nutrient solution.     

Growth and uptake rates of P,K, Ca and Mg in wheat 1

The relation between plant age and nutrient absorption properties of red winter wheat (Triticum aestivum L.) roots were investigated. Understanding the change in ion uptake parameters with increasing plant age is helpful in devising efficient fertilization systems. Such information can be used to determine the nutrient levels needed in the soil to supply nutrients rapidly enough to the root surface to minimize deficiencies. Wheat was grown for periods up to 40 days in solution culture in a controlled climate chamber. Sequential harvest and nutrient influx measurements were made. Shoot growth was exponential with time to 30 days and linear thereafter. Root dry weight increased linearly with time at a slower rate than shoot dry weight. Root length increased linearily with time. With increasing plant age there was a reduction in average P and K uptake rate while average uptake rates for Ca and Mg remained relatively unchanged. With increasing plant age, the maximum influx, I_max. for P and Mg remained constant, but for K and Ca, there was a decrease. For the Michael is constant, Km, no change was observed for P, an increase occurred for K, and a decrease for Ca and Mg, as the wheat plant grew from 5 to 40 days.     

The Effect of Mycorrhiza Inoculation and Phosphorus Application on Phosphorus Efficiency of Wheat Plants

The effect of indigenous soil and selected mycorrhizal inoculation and phosphorus (P) applications on wheat yield, root infection and nutrient uptake was monitored for two successive years under field conditions. In addition, phosphorus efficiency and inoculation effectiveness (IE) were determined. Wheat (Triticum aestivum L.) plants were used as host plants in a Menzilat soil series (Typic Xerofluvents) in the Mediterranean coastal region of Turkey. Three levels of phosphorus were applied with Glomus mosseae to wheat plants over two successive years. Mycorrhizal inoculation significantly increased root colonization. G. mosseae-inoculated plants in both years exhibited a two-fold higher root colonization than the indigenous mycorrhizal colonization. Compared with non-inoculated plants, mycorrhizal inoculation increased wheat yield for both years. In addition, increasing P fertilizer levels enhanced the wheat grain yield. In both years, the inoculum efficiency (IE) decreased with increasing P level addition. Phosphorus efficiency is higher under low P application than the higher P application. However, with mycorrhizal inoculation P efficiency is higher than the non-inoculated treatment.

The effects of mycorrhizal inoculation on plant nutrient concentrations were determined: mycorrhiza-inoculated plants exhibited a higher zinc (Zn), manganese (Mn), copper (Cu), iron (Fe) nutrients concentration than non-inoculated plants. After two years of field experiments, it is concluded that mycorrhizal inoculation can be used in large arable areas; however, it is also very important to manage the indigenous mycorrhiza of arable land.     

Dinitrogen fixation and soil n uptake by soybean as affected by phosphorus availability

The effects of phosphorus supply (0, 30, and 90 mg P kg^‐1) on growth, N₂ fixation, and soil N uptake by soybean (Glycine max (L.) Merr.) were studied in a pot experiment using the ¹⁵N isotope technique. Phosphorus supply increased the top dry matter production at flowering and the dry matter production of seeds, straw, pod shells, and roots at late pod filling of inoculated soybeans. Phosphorus supply reduced the N concentration of plant tops at flowering, but increased the amount of N accumulated at both flowering and late pod filling. In inoculated soybeans total N accumulation paralleled the dry matter production. The P concentration in above‐ground plant parts of nodulated soybeans was not affected by P application. At flowering only 18 to 34% of total N was derived from N₂ fixation, whereas as much as 74% was derived from N₂ fixation at late pod filling. Only the addition of 90 mg P kg^‐1 soil significantly increased the amount of N₂ fixed at the late pod filling stage. Phosphorus supply did not influence the uptake of fertilizer or soil N in soybeans, even if the root mass was increased up to 60% by the P supply.     

Root cation exchange capacity of cotton cultivars in relation to aluminum toxicity

Root cation exchange capacity (CEC) was analyzed for four cotton cultivars (Pima S‐5, Stoneville 825, Deltapine 41 and Auburn 56) within tvo species (Gossypium barbadense and G. hirsutum) grown in control (O Al) and Al (1.5 mg/l) solution. Pima S‐5, a G. barbadense variety, had significantly (P < 0.10) lower root CEC than G. hirsutum cultivars in control (O Al) solution. Root CEC of Stoneville 825 was numerically but not significantly lower than Auburn 56 and Deltapine Al in control solution. Root CEC was significantly reduced in all cultivars when grown in Al solution. Compared to controls, Pima S‐5 and Stoneville 825 had either numerically or significantly less reduction in root CEC than Auburn 56 or Deltapine 41 in Al solution. Aluminum content of roots after CEC analysis was numerically greater in the former cultivars than the latter.

The lower root CEC of Pima S‐5 and Stoneville 825 in non‐toxic conditions could provide an initially greater Al tolerance when roots grow into marginally Al toxic soil. Under sustained, Al toxic conditions, root CEC becomes altered and is more of an indirect indicator of root growth as affected by as yet undetermined Al tolerance mechanism(s).

The steady‐state technique to determine root CEC virtually eliminated the inherent problems of CO. effects on pH and titrating to an end point in a specific period of time in a dynamic system.     

Simulation of effects of soil bulk density and p addition on k uptake by soybeans

Abstract

Increasing soil bulk density has been shown to reduce root growth and decrease K uptake by soybeans (Glycine max L. Merrill). Changing soil bulk density also affects soil buffer power, b, and effective diffusion coefficient, De, which affect K influx. The relative decrease in K uptake due to reduced root growth as compared to reduced K influx is not known. Addition of P may affect root growth and P influx properties of plant roots. The objectives of this paper were (1) to use the Cushman mechanistic model to simulate the effect of changing soil bulk density and soil P on K uptake by soybeans, and (2) to determine the parameters that are changed by changes in bulk density and added P and their effect on K uptake. Plant and soil data of an experiment where Williams soybeans were grown for 21 days in pots of Raub (Aquic Argiudoll) silt loam with factorial treatments of two rates of K (0 and 100 mg K kg^‐1 soil), two rates of P (0 and 100 mg P kg^‐1 soil), and two bulk densities (1.25 and 1.45 g cm^‐3 ) were used to verify the model. Plant and soil parameters for the model were measured independently of the verification experiment. Predicted K (y) uptake agreed with observed uptake (x) (y = 1.09x‐0.19; r = 0.97) for the P x K factorial and (y = 1.19X‐0.22; r = 0.90) for the K x soil bulk density factorial treatments. In a sensitivity analysis, the model predicted a maximal K influx at a soil bulk density of 1.38 g cm^‐3. The greatest effect of soil bulk density on K uptake was due to reduction of root growth. Increased K uptake as a result of P addition was because of the effect on root growth.     

Absorption kinetics of Ca,Mg, Na and P by intact corn and onion roots

The influence of Ca on the influx kinetics of Mg and Na, and Mg effects on Ca influx kinetics were evaluated in intact corn roots. Increasing levels of Mg (25 to 75 μM) reduced the Imax for Ca. However, in absence of Mg, increasing levels of Ca (10 to 30 μM) increased the Imax and Km for Ca. Higher levels of Mg interfered with Ca influx. Increasing levels of Ca promoted the Mg influx rates and increased Km as well. Absence of Ca reduced the magnitude of Imax and Km for Mg. At an initial level of 50 μM Ca, increasing Na levels from 50 to 100 μM enhanced the Imax and Km for Na. In the absence of Ca, higher Imax were recorded for Na. Calcium reduced the Na influx rates.

Ion influx kinetics for Ca, Mg, Na and P were determined with the two morphologically differing intact root systems of corn and onion. The rate of influx for Ca, Mg and Na in corn were lower than onion roots. The Imax for P in corn were slightly higher than in onion. The Km vaues for Ca, Mg and Na in corn were also higher than onion indicating a lower affinity by the carrier sites for these ions in corn roots.     

Establishing sorption isotherm to meet phosphorus requirement for tomato seedling growth 1

An Indiana silt loam soil was equilibrated with various amounts of Ca(H₂PO₄)₂ H₂O and a 0.01 M CaCl₂ solution to construct its phosphorus sorption isotherms. Using the isotherms, the P buffering capacity of the soil was calculated and amounts of fertilizer P necessary to give several levels of P in the soil solution, for experiments conducted over a 2 year period, were determined. Twenty‐four day old tomato seedlings were grown and measured for leaf area, root length, dry weights and P concentrations in leaf, petiole, stem and root.

Phosphorus concentration in soil solution increased slowly with the first increment of P added to the soil. Subsequent P additions increased the P concentrations in solution exponentially. The maximum P absorption by the soil was 324 μg P/g soil and the constant related to P binding energy was 1.37. In addition, the soil buffering capacity decreased with an increase in the amount of P in the soil solution.

Plant shoot dry weight increased linearly with P increase in the concentration range 0.65 to 6.5 μM P in soil solution. However, beyond this level the response was low. The leaf area rate of increase in the 0.65 to 6.5 μM P solution concentration range was 75 times that in the 6.5 to 84 μM P. The root length: shoot dry weight ratio decreased with increasing P supply in the soil solution. P uptake by the plants increased with increased P concentration in soil solution. At soil solution concentrations above 6.5 μMP the rate of P uptake in the shoot was 20 times less than the rate for concentrations below 6.5 μM P. Of the P taken up by tomato seedlings about 65% was in the leaf, 13% in the stem, 13% in the petiole and 9% in the root.     

Determining the Moisture and Plant Effect on Nutrient Release,and Plant Nutrient Uptake Using Ion Exchange Resin Membrane

Traditional soil testing has a limited predictability about available nutrients for plant uptake. Potential of ion exchange resin membrane (RM) or plant root simulator probe is evaluated to determine the effect of moisture on nutrient availability and uptake by corn (Zea mays L.), under greenhouse condition. Available nutrient concentrations measured by RM in two soil series at three soil moisture levels (40%, 60%, and 80% of field capacity) with (W) and without (W/O) the plant at V3 and V7 stages were compared with plant nutrient content at the V7 stage. Soil moisture did not influence RM-extracted nutrient concentrations (except for N at V3). Concentrations of nitrogen (N), phosphorus (P), potassium (K), sulfur (S), and iron (Fe) from RM at the V3 stage significantly correlated with shoot uptake. The presence of plant (W- vs. W/O-plant) significantly influenced RM-nutrient concentration at both stages. RM can predict crop nutrient requirements.

Abbreviations: Ion exchange resin membrane (RM); nitrogen (N); phosphorus (P); potassium (K), sulfur (S), field capacity (FC)     

Effect of one phosphorus rate placed in different soil volumes on p uptake and growth of wheat

Abstract

The volume of soil treated with P fertilizer affects P uptake by the crop. Earlier studies have shown that the stimulation of root growth in P‐fertilized soil was similar for both corn (Zea mays L.) and soybean (Glycine max L. Merr). The objective of this research was to determine the effect of fertilizer P placement on P uptake and shoot and root growth of spring wheat (Triticum vulgare L.). Wheat was grown for 34 days in Raub silt loam (Aquic Argiudolls) in a controlled climate chamber. One rate of phosphate per pot, 150 mg P per three kg of soil, was mixed with 2, 5, 10, 20, 40 and 100% of the soil in the pot. The P was equilibrated with moist soil for 5 days at 70°C followed by 21 days at 25° C before transplanting 8‐day‐old wheat plants into each 3 L pot. The P stimulation of root growth in the P‐treated soil was similar to that for corn and soybeans. The effect could be described by the equation y = x^0.7 where y is the fraction of the root system in the P‐fertilized soil where P is mixed with x fraction of the soil. The greatest P uptake and plant growth occurred when added P was mixed with 20% of the soil.     

A method for determining Michaelis-Menten kinetic parameters of nutrient uptake for plants growing in soil

The relationship between nutrient influx (I_n) and solution concentration at the root surface (C_lo) has not yet been determined for roots growing in soil because of difficulties in measuring C_lo. Corn was grown on two soils with 12 and 21% clay. Each soil had five K levels ranging from low to very high. Potassium influx (I_n) was determined from K uptake between two harvests and root length. C_lo was then calculated from the average soil solution concentration and I_n by assuming that diffusion is the main transport mechanism for K to the root. Potassium influx plotted against C_lo showed a saturation curve, typical of a Michaelis-Menten kinetic relationship. The Michaelis-Menten uptake parameters, maximum influx (I_max) and Michaelis constant (K_m), were obtained by the “Hanes” plot. There was close agreement, without lack of fit, between calculated and observed data. The proposed procedure therefore appears to be suitable for estimating the uptake kinetics of roots growing in soil. Requirements for applications of the method are discussed.     

The relative efficiency of ionic iron(III) and iron(II) utilization by the rice plant

Uptake of iron by rice plants was equally rapid when supplied as ionic iron(II) or iron(III) at pH 3 and 4. Iron(III) uptake was reduced at pH 5 and uptake of iron when supplied as FeEDTA was relatively low at all three pH levels.

At pH 4 in the presence of plant roots, reduction of iron(III) to iron(II) occurred as indicated by Fe²⁺ BPDS formation. BPDS in a 3:1 ratio to iron(III) suppressed iron uptake by about 70%. The reduction was observed to be located in the endodermis of young roots and exodermis of older roots.

A capacity to oxidize iron(II) at the root surface was also observed under local anaerobic and relatively high pH conditions.

The significance of these two counteracting processes in affecting the oxidation state of iron at the root surface is discussed.     

Phosphatversorgung von Sommerweizen (Triticum aestivum L.) in Mischkultur mit Weißer Lupine (Lupinus albus L.)

Phosphorus nutrition of spring wheat (Triticum aestivum L.) in mixed culture with white lupin (Lupinus albus L.). Spring wheat (Triticum aestivum L. ?Schirokko”?) and white lupin (Lupinus albus L.) were grown in mixed culture in Mitscherlich pots with 20 kg of soil in a green house. The soil used was a B_t of a Parabraunerde-Pseudogley from loess low in available P and limed from pH 4.6 to pH 6.5. Phosphorus was added as phosphate rock. In half of the pots cylinders of stainless steel screen prevented intertwining of the roots of the plant species. Independent of P addition, white lupin had higher dry matter production and P uptake than wheat, even although wheat had thinner roots and higher root densities than lupin, factors which favour the utilization of soil and fertilizer P. The higher P efficiency of white lupin was due to higher P uptake rates per unit root length mainly through mobilization of P especially in the rhizosphere of the proteoid roots. When the roots of the two species were allowed to intertwine, shoot dry matter production of wheat was nearly double because of improved tillering. Higher P concentrations and a more than 2-fold higher P uptake indicated that the increase in dry matter production of wheat was due to improved P nutrition. Nitrogen concentrations, however, remained unaffected at sufficient levels. An increased P uptake rate per unit root length was responsible for the better utilization of P by wheat, rather than the increase in total root length, due to the extended root volume. White lupin was able to mobilize P in the rhizosphere in excess of its own requirements. Thus mobilized P may be available to less P-efficient plants grown in mixed culture.     

Interrelationships between growth and nutrient uptake in alfalfa and corn

Nutrient requirements o£ plants during their various phases of growth are affected by several internal and external factors. The changes in rate of uptake by root with age are an important factor to meet the increasing plant demand for nutrients. Nutrient culture experiments were carried out under controlled greenhouse conditions with corn (Zea Mays L.) and alfalfa (Medicago sativa L.) to investigate the relationship of stage of growth to changes in plant parameters and nutrient uptake properties. With advancement of age. both plant species increased their ambient growth medium pH towards neutrality. With increasing age in alfalfa there was very little change in observed S:R ratio and root growth rate. On the other hand in corn plants the S:R ratio increased and growth rate for root and shoot decreased with age. Alfalfa contained higher concentrations of N, K, Na, and Ca than corn; while ion concentrations in both crops decreased with plant age. At all stages of growth, alfalfa absorbed less nutrients than corn. The rates of nutrient influx, I_n in both the crops showed various degrees of correlation with age and rate of shoot growth. In corn. I_n for ions reached a maximum at 25 days growth; whereas, in alfalfa, I_n reached maximum at 30 days of growth. The differences in influx rates for different ions in the two species are probably due to the difference in development of shoot and root parameters and shoot demand for the ions.     

Iron nutrition of sunflower(Helianthus annuus L.) as affected by various levels of p and ZN

The effects of various P and Zn levels on iron nutrition of sunflower (Helianthus annuus L.c.v. Record) were studied in two separate experiments in nutrient solution under greenhouse conditions.

In the first experiment, sunflower was grown in nutrient solutions containing four levels of P(1.5, 2.5, 3.5 and 4.5 mM/l) and three levels of Fe(0.25, 0.75, and 1.5 ppm) as FeCl₃ or FeEDDHA. In the second experiment (following the first experiment), the treatments were three P levels (0.75, 1.50 and 3.00 mM/l), three Fe levels (0.25, 0.75 and 1.5 ppm) as FeEDDHA and three Zn levels (0.1, 0.2 and 0.4 ppm).

The plants receiving Fe‐chelate, except for 0.25 ppm Fe, showed no symptoms of iron chlorosis. With inorganic Fe treatments, iron chlorosis appeared after 7–10 days depending on P level, but except for 0.25 ppm Fe which remained chlorotic, plants recovered completely within 3–4 days thereafter due to pH regulating mechanism of sunflower under iron stress condition. With both sources of Fe, chlorosis was associated with high P:Fe ratio.

Increased P and Fe levels in nutrient solution resulted in general increases in the dry weights of roots and shoots. The Fe concentration of shoots, except in few instances, was not affected by P levels, indicating that the sunflower cultivar used in this experiment could utilize inorganic Fe as well as Fe‐chelate under our experimental conditions.

Increasing P levels caused significant increases in Mn content of the shoots as 0.25 and 0.75 ppm inorganic Fe³⁺. Increased Fe levels increased shoot Mn content with inorganic Fe and decreased it with Fe‐chelate. The effects of P, Fe and Zn on sunflower indicated an antagonistic effect of Zn on 1.5 ppm Fe for all P levels. Increased Zn levels in nutrient solution generally increased Zn content of the shoots without having any marked effect on their Mn content.     

Growth and nutrient influx patterns during early pumpkin development

Pumpkin specie Cucurbita moschata cv ‘Libby‐Select’ was grown in nutrient solution in the greenhouse to characterize growth and ion uptake for the period up to 56 days. Pumpkin relative growth rate was highest between 26 and 32 days, just after vines began to run. Dry matter accumulation was highest between 38 and 50 days. Root growth in terms of dry weight and total length generally kept pace with shoot growth up until 26 days. Thereafter, root growth increased linearly but at a slower rate than shoot growth. Significant differences in influx (uptake rate per length of root) of P, K, Ca, and Mg occurred during the growth period. Influx rates were generally highest between 26 and 32 days. For each nutrient, the relative absorption rate exceeded plant growth rate. Ion influx parameters (I_max, K_m, C_O) were determined at 18, 28, 40, and 48 days from depletion measurements. For each nutrient, I_max and max C_O tended to decrease as plant age increased. K values were generally variable.