Typha Glauca Research Articles

Typha glauca Godron and its parental plants in Poland: taxonomic characteristics

Natural interspecific hybridization between Typha latifolia L. and Typha angustifolia L. was analyzed by morpho-anatomical and molecular methods to determine whether the hybrid Typha glauca Godr. is present in Poland and to identify the best diagnostic traits for its identification. Eighty-three samples of the Typha species were collected. Nine random amplified polymorphic DNA (RAPD) primers provided 12 fragments specific for T. angustifolia and eight fragments specific for T. latifolia. DNA of all sampled individuals was analyzed with 20 diagnostic RAPD markers. The morpho-anatomical variability of T. glauca F1 was found to be quite similar to that observed in parental plants. All of the 41 traits examined in the hybrids overlapped with those observed in the parents, however, the hybrids were visibly closer to T. angustifolia than to T. latifolia. The most discriminate characteristics were the length and pedicel width, as well as the epidermal cell thickness located above vascular bundles in leaf blades. Moreover, preliminary observations of seed sculpture showed that the length of testa cells could also be used to identify T. glauca. Clusters and the hybrid index (for molecular and morphological data) were highly coincident and support the hybridization hypothesis.

Estimating the breakdown and accumulation of emergent macrophyte litter: A mass-balance approach

Abstract Litter accumulation within emergent macrophyte marshes may significantly influence abiotic conditions and biota but litter is rarely considered in emergent macrophyte studies. Litter is defined here as the standing and fallen dead plant material that can be collected using harvest methods in the field. Litter accumulation can be predicted by combining annual production with litter breakdown rates. Breakdown rates are typically measured using litter bag studies but these rates may also be measured using a mass-balance approach. A five year study conducted in Delta Marsh, Manitoba measured annual standing crop and harvested accumulated litter of Phragmites australis (Cav.) Trin., Typha glauca Godr., and Scolochloa festucacea (Willd.) Link. These species differ in their level of refractory material and hence the amount of litter that is expected to accumulate. Using annual estimates of standing crop and litter mass, a mass-balance model was used to estimate the litter breakdown rate for each species...

An examination of hybridization between the cattail species typha latifolia and typha angustifolia using random amplified polymorphic DNA and chloroplast DNA markers

Typha glauca represents a significant portion of the biomass of the wetlands surrounding the Great Lakes, USA. It is generally accepted to be a form of hybrid between T. latifolia and T. angustifolia, which itself appears to be an exotic introduction from Europe. Based on morphological and isozyme data, conflicting theories have been proposed for the hybrid nature of T. glauca: it has been described as a hybrid swarm, a distinct hybrid species and an F1 hybrid. Therefore, we developed random amplified polymorphic DNA (RAPD) and chloroplast DNA markers, specific to the parental species, to assess hybrids. Ten RAPD primers gave 17 fragments specific to T. angustifolia and 13 fragments specific to T. latifolia. All of the interspecific hybrids contained each of the species-specific markers, indicating an F1 hybrid status. Furthermore, all hybrids tested contained the T. angustifolia chloroplast haplotype, which is consistent with differential interspecific crossing success found previously. Additional confirmation of an F1 hybrid status was gained by examining seedlings from T. glauca. These progeny were expected to be advanced-generation hybrids, as opposed to the F1 hybrid parent. Analysis of the seedlings revealed segregating marker patterns consistent with patterns observed in experimental advanced-generation hybrids, although these advanced hybrids do not appear to be a significant part of mature stands. Our data do not provide support for extensive gene flow between T. latifolia and T. angustifolia. However, our results suggest that hybridization between the native and introduced Typha species has impacted the native population through the spread of the F1 hybrid, T. glauca.

Development of the endodermis and hypodermis of Typha glauca Godr. and Typha angustifolia L. roots

The development of the endodermis and hypodermis in adventitious roots of Typha angustifolia L. and Typha glauca Godr. was followed from the apical meristem to full maturity. The endodermis was typical, developing a thin Casparian band near the root tip, followed by suberin lamellae and asymmetric, secondary, lignified walls (C-type at maturity). Passage cells were present at an intermediate stage but eventually disappeared when all cells developed lamellae and secondary walls. The hypodermis was multiple (four to six layers at maturity) and began differentiating near the root tip. Here, the radial and transverse walls of the outermost layer did not dissolve in strong acid and the former were wavy in the longitudinal direction, both features characteristic of a Casparian band, but these walls were permeable to berberine. No other indication of a wall modification was seen for 3 weeks, at which time the root had become determinate and aerenchyma was beginning to form in the midcortex. Casparian bands, which were impermeable to berberine, matured in the hypodermis; thus, it proved to be an exodermis. Different forms of Casparian band were detected: one was typical and occupied the radial and transverse walls of the outermost layer, but others were novel and included tangential walls, often forming an H-shaped structure (as seen in cross section of the root). We propose calling the latter type an H-type Casparian band. It functioned as an apoplastic barrier to berberine applied either externally or internally by injection into the cortical aerenchyma. Following maturation of the Casparian band, the outer two layers of the exodermis soon produced suberin lamellae. These continued to be deposited in a centripetal pattern until they were found in all layers of the multiple exodermis. Development of the suberin wall modifications correlates with the development of the aerenchyma and may play a role in preventing gas exchange between the root and the rhizosphere. Later, all exodermal cells produced lignified, secondary walls. These were asymmetric in the outermost and innermost layers of the hypodermis (like the C-type endodermis); eventually, all layers had Casparian wall materials. Thus, the mature hypodermis consisted of two to six layers of exodermis, except at the tips of determinate roots where the exodermis was uniseriate with typical Casparian bands.Key words: cattail, endodermis, exodermis, hypodermis, roots, Typha.

Decomposition of emergent macrophyte roots and rhizomes in a northern prairie marsh

Dry mass, nitrogen and phosphorus content in belowground litter of four emergent macrophytes ( Typha glauca Godr., Phragmites australis (Cav.) Trin., Scolochloa festucacea (Willd.) Link and Scirpus lacustris L.) were followed for 1.2 years in a series of experimental marshes, Delta Marsh, Manitoba. Litter bags containing roots and rhizome materials of each species were buried in unflooded soil, or soil flooded at three water depths (1–30, 31–60, > 60 cm). There were few differences in dry mass loss in unflooded or flooded soils, and depth of flooding also had little effect on decomposition rates. In the flooded sites, Scolochloa and Phragmites roots lost more mass (48.9–63.8% and 59.2–85.5%, respectively) after 112 days than Typha and Scirpus (36.3–43.6 and 37.0–47.2%, respectively). These differences continued through to the end of the study, except in the shallow sites where Scirpus roots lost more mass and had comparable mass remaining as Scolochloa and Phragmites. In the unflooded sites, there was little difference between species. All litters lost nitrogen (22.9–90.0%) and phosphorus (46.3–92.7%) during the first 112 days, then levels tended to remain constant. Decay rates for our belowground root and rhizome litters were comparable to published literature values for aboveground shoot litter of the same species, except for Phragmites roots and rhizomes which decomposed at a faster rate (− k = 0.0014−0.0032) than shoots (− k = 0.0003−0.0007, [van der Valk, A.G., Rhymer, J.M., Murkin, H.R., 1991. Flooding and the decomposition of litter of four emergent plant species in a prairie wetland. Wetlands 11, 1–16]).

Changes in atmospheric circulation patterns affect midcontinent wetlands sensitive to climate

Twenty‐seven years of data from midcontinent wetlands indicate that the response of these wetlands to extremes in precipitation—drought and deluge—persists beyond the extreme events. Chemical changes transcend such simple relations as increased salinity during dry periods because drought provides mechanisms for removal of salt by deflation and seepage to groundwater. Inundation of vegetation zones including rooted or floating mats of cattail (Typha glauca) can stimulate sulfate reduction and shift the anion balance from sulfate to bicarbonate dominance. Disruptions in the circulation of moisture‐laden air masses over the midcontinent, as in the drought of 1988 and the deluge of 1993, have a major effect on these wetlands, which are representatives of the primary waterfowl breeding habitat of the continent.

Experimental evaluation of realized niche models for predicting responses of plant species to a change in environmental conditions

Abstract. We estimated, using logistic regression techniques, the realized niches of the four dominant species in an experimental marsh complex located in the Delta Marsh, Manitoba, Canada. These models were then used to predict the probability of occurrence of these species in selected elevation ranges when water levels were raised in 1985 either 0, 30 or 60 cm above the long‐term normal water level. These realized‐niche models were calculated using elevation and species data collected in 1980. After having been eliminated by two years of deep flooding, the emergent vegetation in this complex had been re‐established during a drawdown beginning in either 1983 or 1984. Our hypothesis was that from 1985 to 1989 the frequencies of occurrence of species in selected elevation ranges would converge to their probabilities predicted from the 1980 logistic models.This was not borne out by our results. Actual frequencies and predicted probabilities of occurrence of a species were similar at best less than 40% and then mostly in the control (0 cm) treatment. The realized‐niche models were not adequate to predict the distribution of emergents after an increase in water level in the short term because the emergent species did not migrate upslope. Emergent species in the medium and high treatments either (1) died out ‐ Scolochloa festucacea and Scirpus lacustris ‐ after 3 yr because they could not survive permanent flooding, (2) stayed where they were ‐ Phragmites australis ‐ because they were unable to move upslope through clonal growth, or (3) became more widespread ‐ Typha glauca only because of the expansion of small local populations already established in 1985 in areas dominated formerly by other species.

Effects of prolonged flooding on the distribution and biomass of emergent species along a freshwater wetland coenocline

Water levels were raised 1 m for two years in 10 cells of an experimental wetland complex located in the Delta Marsh, Manitoba, Canada. The mean area covered by each of the 5 dominant emergent species in these cells declined significantly during the first year of flooding. There was no significant difference in the total acreage of ?mergents between flooding years. Three species were completely {Carex atherodes Spreng, Scolochloa festucacea (Willd.) Link) or almost {Scirpus lacustris L. spp. glaucus (Sm.) Hartm.) eliminated from the cells. Typha glauca Godr. and Phragmites australis (Cav.) Trin. still covered in the second year of flooding about 40 and 25%, respectively, of the area that they had occupied in the cells in the preflooding year. The mean elevation at which the two surviving species were found both increased 0.1 and 0.2 m over preflooding elevations during the first and second year of flooding, respectively. There is no evidence that during the flooding years any species migrated upslope toward more optimal water depths. Aboveground standing crop for all five species declined significantly during the first year of flooding, but was not significantly different between the two flooding years. Total belowground biomass did not differ between spring and fall, but declined significantly (about 20%) during the first year of flooding and remained about at that level during the second year. In the spring following the two years of flooding, mean total belowground biomass was only 40 g m-2, less than 10% of the belowground biomass dur ing the preflooding year.

Loss and assimilation of 15N-nitrate added to a North Dakota cattail marsh

Field 15N balance studies were conducted to determine the fate of K 15NO 3 added during summer to surface water in 0.27 m 2 plots within a North Dakota cattail (a mixture of Typha glauca Godr. and Typha angustifolia L.) marsh. In a 1989 study, 50.4 mmol 15NO 3 −-N (approximately 1.1 mmol NO 3 − 1 −1) disappeared within 8 days after a temporary build-up of NO 2 −-N. Approximately 42% of the applied 15N remained in the marsh. This residual N was found in cattail roots and rhizomes (9.5%), floating plants ( Lemna minor L. and Spirodela polyrhiza (L.) Schleiden) (8.9%), soil (8.3%), litter (6.9%), surface water (4.6%) and cattail shoots (4.1%). The missing 15N (58%) was presumably lost as a result of denitrification. In a 1991 experiment, 50.4 mmol of K 15NO 3 was added to plots which had previously received additions of deionized water or 153 mmol of unlabeled KNO 3. All the labeled NO 3 − again disappeared within 8 days. The unaccounted-for 15N was approximately 56% and 82% of the added K 15NO 3-N in plots without and with the antecedent unlabeled KNO 3 treatment, respectively. The difference in recovery (26%) was highly significant. Stimulation of dissimilatory NO 3 − and NO 2 − reductases and/or suppression of dissimilatory reduction of 15NO 3 − to 15NH 4 + are postulated as likely factors for the higher apparent denitrification loss associated with the antecedent KNO 3 treatment.

Water-depth tolerances of the dominant emergent macrophytes of the Delta Marsh, Manitoba

The growth (shoot height, cumulative shoot length, shoot density, above- and below-ground biomass) of seven emergent species growing at five different water depths was measured for 2 years. These species belonged to three different ecological classes: (i) upper marsh species (Carex atherodes, Scolochloa festucacea, and Phragmites australis) that occupy sections of the water-depth gradient that are only seasonally flooded in the Delta Marsh; (ii) lower marsh species (Typha glauca and Scirpus lacutris spp. glaucus) that occupy permanently flooded areas; and (iii) drawdown species (Scirpus lacustris spp. validus and Scirpus maritimus) that become established temporarily during drawdowns. Upper marsh species could not adjust their shoot length if they were growing in water deeper than 20 cm. Lower marsh species were able to do this in water up to 70 cm deep. All three types survived for 1 or 2 years in water too deep for long-term persistence. Scirpus species survived as tubers in areas with water too deep for them to grow. The distributions of the seven species in the experiments overlapped considerably, and all species cooccurred at water depths to 70 cm after 2 years of flooding. The predicted distribution in the four dominant species along a water-depth gradient were consistent with their actual distributions in the Delta Marsh, but their predicted distributions overlapped much more than is the case in the field. Key words: emergent vegetation, experiment, water-depth tolerance, plant growth, distributions.

Effect of water depth on population parameters of a Typha glauca stand

The response of a Typha glauca stand to a water depth gradient was studied in a small marsh pond in Delta Marsh, Manitoba. Weekly density counts and height measurements were made from May to October 1986 in permanent quadrats at five depths from 25 to 100 cm. Shoot mass was estimated from shoot height using a regression model based on destructive analyses. Shoot density declined significantly from 41 shoots/m2 at 25 cm to 12 shoots/m2 at 85 cm but increased at 100 cm to 38 shoots/m2. Shoot mass increased in shoots growing at water depths from 25 to 65 cm, resulting in relatively constant stand biomass over this depth range. Stand biomass declined at 85 cm and reached its maximum (1789.8 g/m2) at 100 cm. Frequency distributions of shoot size categories based on height deviated from normality and were negatively skewed at all depths, with the greatest skewness occurring at 100 cm. These population parameters were interpreted as evidence of a plastic population response to water depth. Key words: clonal macrophyte, frequency distributions, plasticity, shoot density, Typha glauca, water depth.

A field study of the effects of water depth, order of emergence and flowering on the growth of Typha glauca shoots using the Richards model

The effects of water depth, order of emergence and flowering on growth curves of hybrid cattail ( Typha glauca Godr.) shoots growing in permanent quadrats were analyzed using Ebert's formulation of the Richards function. Although estimate variability was high and heterogeneous, the model produced a good fit when applied to height measurements of individual shoots. The logistic ( n=1) described the typical growth curve of vegetative shoots, with a grand mean of n=1.15. Although maximal size increased with depth in the 25–65 cm range, the shape parameter ( n) of early emerging vegetative shoots was not substantially altered by position along the water depth gradient. The mean n value for the first vegetative cohort at 25–65 cm was 1.54. There was a substantial increase in estimate variability in the second vegetative cohort, suggesting that delayed emergence imposes a greater stress on T. glauca shoot growth than water depth per se. The mean n value for all depths declined to n=0.55. Floral shoots had a lower mean n value ( n=0.28), coupled with low estimate variability and high weighted relative growth rates. These shoots exercise priority within the clonal mass, diverting resources from vegetative expansion.

Hydraulic Conductivity and Nitrogen Removal in an Artificial Wetland System

AbstractA greenhouse study using aquatic macrophytes rooted in sand was conducted to test if the presence of belowground plant material increases or maintains permeability in horizontal subsurface flow systems constructed for wastewater treatment. Nutrient solutions low in suspended solids (SS) and biochemical oxygen demand (BOD) were substituted for actual wastewater. Three batch applications of NH4‐N and three of NO3‐N were added for 48‐h periods at concentrations of approximately 20, 55, and 80 mg L−1 to assess the ability of these systems to remove N. Saturated hydraulic conductivity, Ks, was monitored over 236 d in boxes of sand planted with two different aquatic macrophytes—Typha glauca Godr. and Scirpus acutus Muhl. The Ks decreased over time in the vegetated and nonvegetated (control) systems. The decrease was significantly greater in the vegetated systems (55%) than in the control (41%). Despite these decreases, Ks at the end of the study was still high (0.62 cm s−1) even in the vegetated systems. The vegetated systems were significantly more effective than the nonvegetated at removal of NH4‐N and NO3‐N. No significant difference was noted in the N removal efficiency of the two plant species. Our results suggest that artificial wetlands using sand can be used to remove N from wastewater, but decreased soil permeability may occur.

A field study of the morphometric response of Typha glauca shoots to a water depth gradient

In the Delta Marsh, Manitoba, the hybrid cattail Typha glauca Godr. forms large monodominant stands over a wide range of water depths (25–100 cm). Despite a considerable body of research concerning the origin and development of this important species, its ecology has received little attention. In field studies conducted in two freshwater wetland sites in the Delta Marsh, mean height and biomass of vegetative shoots of T. glauca increased significantly along a gradient of increasing water depth. Product–moment correlations between height and biomass were [Formula: see text]. Flowering shoots, in contrast, were less flexible in their response to depth. No significant differences associated with depth were found in reproductive biomass, height, spike width, spike length, or gap length of flowering spikes harvested in August. Differences in size of vegetative shoots were interpreted as a plastic response that may play a role in adaptation to a range of water depths. By adjusting the length of underwater shoot tissue, the ramet maintains relatively constant aerial shoot dimensions. Growth of shoots in deep water is enhanced by an intrinsic factor (juvenility) and an environmental factor (edge effect). Key words: Typha glauca, water depth, growth, plasticity, field study, morphometric response.

Seasonal comparisons of weight loss for two types of Typha glauca Godr. leaf litter

Samples of living (2.8% N, 0.3% P) and naturally senesced (0.6% N, 0.05% P) leaves of cattail ( Typha glauca Godr.) were collected from a shallowly flooded stand during July 1979 and February 1980, respectively. Decomposition patterns of the two litter types were compared by adding and removing litter samples from the Delta Marsh, Manitoba, Canada at various intervals during the growing season. Green leaf material, collected just before flowering, decomposed more rapidly than senesced litter during all time periods. Simple exponential decomposition rates differed significantly; −0.0241 day −1 for green litter vs. −0.0110 day −1 for senesced litter. Initial rates of organic dry wt. loss were not different between seasons for either litter type, but initial rates declined faster later in the summer. Less organic weight was lost during late summer after 2 and 30 days of incubation for both types of litter. However, seasonal effects could not be detected after longer treatment periods. During late summer, siltation of wind-suspended sediments apparently impeded decomposition by depressing oxygen levels and invertebrate activity in the litter samples. We estimate that up to 21 and 36 times more N and P, respectively would be released from litter toppled by muskrat feeding activity than by natural toppling of dead plant material during the growing season.

Responses by macroinvertebrates to cattail litter quality and timing of litter submergence in a Northern Prairie Marsh

An experiment was conducted in the Delta Marsh in south-central Manitoba to assess the effects of initial litter quality, duration of litter submergence, and season of submergence on invertebrate abundance and biomass. Samples of two types of hybrid cattail (Typha glauca) litter were added to the marsh throughout the spring and summer and subsequently collected at monthly intervals. Green cattail litter gathered during the preflowering stage contained higher levels of N and P than senesced litter collected from the same stand later in the year. Although the green litter contained higher nutrient levels throughout the season, there was no preference by invertebrates between the two litter types provided. There was a tendency for higher invertebrate densities, especially collectorfilterers, to be associated with litter that had been submersed longer. Examination of seasonal patterns of invertebrate functional group abundance showed scrapers with early spring peaks in abundance and biomass. Shredder populations were at peak levels slightly later in the season than scrapers. As the season progressed, shredder and scraper populations declined to relatively low levels. Collector-filterers and predators dominated the invertebrate fauna during August and September.

Origin and Development of Lateral Roots in Typha glauca

Lateral roots of Typha glauca arose from the pericycle of the parent adventitious root. Periclinal divisions of the pericycle gave rise to two layers; the outermost initially produced the ground meristem and protoderm, and the innermost produced the procambium. The immature endodermis of the parent root contributed to the early stages of the root tip as an endodermal covering. Prior to emergence, the ground meristem/protoderm produced cells into the endodermal covering. After emergence, the endodermal covering was replaced by a calyptrogen, which was derived from the ground meristem/protoderm and which, in turn, formed the rootcap. A typical monocotyledonous three-tiered meristem was then produced. An outer ground meristem also arose before emergence to form a hypodermis in many lateral roots; in these, crystalliferous cell production began in midcortex cells before emergence, and a small aerenchyma developed in their cortices. The rootcap columella stored small amounts of starch shortly after emergence. Lateral roots of T. glauca were smaller than their parental adventitious roots; they normally had only two to six poles of xylem and phloem, and the cortex was less than six cells across. During 1–3-cm elongation, the lateral root apical meristem and mature regions narrowed, stored starch disappeared, fewer crystals formed, aerenchyma production ceased, and the roots stopped elongation.

ORIGIN AND DEVELOPMENT OF LATERAL ROOTS IN TYPHA GLAUCA

Lateral roots of Typha glauca arose from the pericycle of the parent adventitious root. Periclinal divisions of the pericycle gave rise to two layers; the outermost initially produced the ground meristem and protoderm, and the innermost produced the procambium. The immature endodermis of the parent root contributed to the early stages of the root tip as an endodermal covering. Prior to emergence, the ground meristem/protoderm produced cells into the endodermal covering. After emergence, the endodermal covering was replaced by a calyptrogen, which was derived from the ground meristem/protoderm and which, in turn, formed the rootcap. A typical monocotyledonous three‐tiered meristem was then produced. An outer ground meristem also arose before emergence to form a hypodermis in many lateral roots; in these, crystalliferous cell production began in midcortex cells before emergence, and a small aerenchyma developed in their cortices. The rootcap columella stored small amounts of starch shortly after emergence. Lateral roots of T. glauca were smaller than their parental adventitious roots; they normally had only two to six poles of xylem and phloem, and the cortex was less than six cells across. During 1–3‐cm elongation, the lateral root apical meristem and mature regions narrowed, stored starch disappeared, fewer crystals formed, aerenchyma production ceased, and the roots stopped elongation.

Effects of nutrients and water levels on emergent macrophyte biomass in a prairie marsh

Nitrogen and phosphorus fertilizers were added over two growing seasons to marshes dominated by whitetop grass (Scolochloa festucacea) or cattail (Typha glauca) in a prairie lacustrine marsh to assess nutrient limitation and the interaction of nutrient limitation with water depth. For each species, stands were selected at the deep and shallow extremes of its water depth range. Water levels were high during the first year of fertilization and low during the second year, exposing the fertilized stands to a variety of water depths. Nitrogen limited growth in whitetop and cattail marshes. Water level, by controlling whether the soil was flooded or the water table was below the soil surface, affected growth and the degree of nitrogen limitation. In whitetop marshes, nitrogen increased biomass more when the soil was flooded or when standing water was deeper and in cattail marshes, it increased biomass more under intermediate water depths (approximately 0–20 cm) than under more deeply flooded (20–40 cm) or dry conditions. Nitrogen reduced biomass in whitetop marshes the second year, apparently because growth was inhibited by fallen litter from the previous year. Nitrogen did not limit cattail marsh biomass in the driest locations during a year of low water levels. Phosphorus caused a small increase in growth of both species after 2 years. Changes of nitrogen limitation with flooding suggest that annual water level fluctuations, by creating alternating flooded and dry conditions, may influence the primary production of emergent macrophytes through effects on nitrogen cycling.

Effect of long-term flooding on root metabolic response in five freshwater marsh plant species

Five freshwater marsh plant species exhibited different root metabolic responses when flooded to three water depths in field macrocosms. The capacity for alcoholic fermentation (as indicated by alcohol dehydrogenase activity) increased and remained at a relatively high level in the roots of the least flood-tolerant species, Scolochloa festucacea, but was not stimulated significantly or only temporarily in the more tolerant species, Scirpus acutus, Scirpus validus, Typha glauca, and Phragmites australis. During the first month of flooding, alcohol dehydrogenase activity showed a positive relationship with flooding depth and a negative relationship with soil redox potential. Malate accumulated in the roots of S. acutus, S. validus, and to a lesser extent in P. australis in response to flooding; concentrations showed a significant positive relationship with water depth and a significant negative relationship with soil redox potential during the first month of flooding. Differences in root metabolism among the five species were still evident after 1 year of continual flooding. Root specific gravities and air space cross-sectional volumes demonstrated potential species differences in root resistance to oxygen movement and root oxygen volume, respectively. The results suggest that the observed metabolic response reflected differential aeration of the roots resulting from differences in root structure and soil oxygen demand (reducing power).