B.L. McMichael

Applications and limitations of rhizotrons and minirhizotrons for root studies

This paper discusses two glass-wall techniques-rhizotrons and minirhizotrons. Rhizotrons are covered underground walkways with clear windows on one or both sides. Their design varies somewhat with the type of research to be conducted in them. A minirhizotron consists of a clear tube inserted into the soil then some type of viewing arrangement is lowered into the tube. In both techniques, roots and their rhizosphere organisms can be observed at the soil-wall boundary.

The impact of the soil environment on the growth of root systems

Abstract There are numerous environmental factors that can influence the growth and function of plant root systems. The impact of some of the major soil-related factors such as soil temperature, soil water, soil air, soil strength, and soil nutrient supply on the development of roots is reviewed. Emphasis has been placed on the interaction of these factors with each other and with the genetic diversity inherent in plant roots in determining the impact of the soil environment on root growth and ultimately on plant productivity.

Metabolic activity of cotton roots in response to temperature

Abstract Root growth, under genetic control, responds to numerous environmental stimuli. The occurrence of below optimal soil temperatures for root growth of cotton ( Gossypium hirsutum L.) at planting time may delay seedling establishment and reduce seasonal crop productivity. The present study assessed cotton root growth and metabolism at various temperatures to determine if observed temperature responses were related to developmental changes in seedling growth. Studies monitoring seedling root growth revealed distinct temperature optima for the cotton seedling. Analysis of the temperature characteristics of in vivo mitochondrial electron transport measured by 2,3,5-triphenyltetrazolium chloride reduction showed that the temperature optima of root metabolism at 10 days after planting (DAP) was lower than that obtained from the measure of accumulated root growth at 10 DAP. The differences in the temperature optima appear to be associated with dynamic changes in seedling development which may be related to changes in stored seed reserves. Metabolic temperature responses are broad during peak seed reserve mobilization and become narrow with the depletion of available reserves. Measurement of root length or root number at 10 DAP would reflect a composite of narrow and broad metabolic temperature sensitivities. Because root development is linked to this composite of metabolic temperature responses, the temperatures providing the maximum root size at 10 DAP are actually higher than the optimum temperature for metabolism under the non-saturating substrate levels associated with the majority of the growing season. Evaluation of cotton root growth responses to shoot and root temperatures within or below cottons thermal kinetic window revealed enhanced root growth when the roots temperatures were within the thermal kinetic window. These findings provide new insights for evaluation of the temperature characteristics of root growth.

GENETIC VARIATION FOR ROOT-SHOOT RELATIONSHIPS AMONG COTTON GERMPLASM*

Twenty-five cotton (Gossypium spp.) genotypes were used to evaluate the genetic variability in partitioning of biomass into roots and shoots when plants were grown under conditions of declining soil water and different atmospheric evaporative demands. The entries ranged from primitive race stocks to modern cultivars and were selected on field observations of growth under water stress conditions in the field. Seeds of each genotype were planted in soil (56 kg) in large containers in the greenhouse. Water was added to the soil to bring the water content to field capacity and the plants were allowed to grow with no additional water until they reached the permanent wilting point. Large fans were utilized in the second experiment to reduce the leaf boundary layer resistance and increase the evaporative demand. When the plants of each entry had reached the permanent wilting point, the plants were harvested, the roots washed free of the soil, and the dry weights of both roots and shoots were determined. Information on total water used and days to permanent wilting were also collected for each genotype. Differences were observed in partitioning of total biomass between roots and shoots between experiments and genotypes. There was no significant interaction, however, between entries and experiments. Root-shoot ratios increased in plants grown in the more stressful environment resulting from a significant increase in root dry weights with little change in shoot dry weights. The distribution for root-shoot ratios coincided in general with the distribution for root weights.among the entries, with a 59% decrease from the highest to lowest value. The exotic strains also in general had higher root-shoot ratios than the commercial varieties, the herbaceum species and the experimental strain (Lubbock dwarf). There was no direct relationship between shoot weights and root weights among the genotypes for either environment. Those plants that grew large tops did not necessarily grow correspondingly large root systems (e.g. T141 has a large root system with a very small shoot compared to the herbaceum species, which has a relative large shoot and a small root system). The lack of a correlation between shoot and root growth along with genotypic differences in changes in root-shoot ratios in response to environmental demand may provide an opportunity to exploit the observed variability to improve production for a wide range of growth conditions by altering the root development and function independent of shoot development.

Does hydraulic lift exist in shallow-rooted species? A quantitative examination with a half-shrub Gutierrezia sarothrae

Hydraulic lift occurs in some deep-rooted shrub and herbaceous species. In this process, water taken up by deep roots from the moist subsoil is delivered to the drier topsoil where it is later reabsorbed by shallow roots. However, little is known about the existence of hydraulic lift in shallow-rooted xeric species. The objectives of this study were 1) to ascertain whether hydraulic lift exists in Gutierrezia sarothrae (broom snakeweed), a widespread North American desert species with a shallow root system, grown in pot and field conditions and 2) if it does, how much water can be transferred from the subsoil to the 30 cm topsoil during the night. Snakeweed seedlings were transplanted in buried pots allowing the deeper roots to grow into the subsoil 30 cm below the surface. Soil water content inside and outside of the pot was measured seasonally and diurnally with time domain reflectometry technique (TDR). An increase in water content was detected in the pot after the plant was covered for 3 h by an opaque plastic bag during the day, suggesting hydraulic lift from deeper depths and exudation of water into the drier topsoil. Root exudation was also observed on native range sites dominated by snakeweed. Water efflux in the pot was 271 g per plant per night. which was equivalent to 15.3% of the extrapolated, porometer-derived whole-plant daily transpiration. Hydraulic lift observed in Gutierrezia improved water uptake during the day when evaporative demand is high and less water is available in the topsoil. We concluded that hydraulic lift might help snakeweed to alleviate the effect of water stress.

Methods for reducing the adverse effects of temperature stress on plants: A review

Abstract Thermal stresses adversely affect plant growth and development worldwide and the resultant reductions in yield limit profitability of agricultural production. The identification of an optimal thermal range provides a means for quantifying thermal stress experienced by a plant. The thermal dependence of apparent K m and variable fluorescence are two procedures for estimating optimal thermal ranges. The ability of a plant to resist thermal stress can be increased through alteration of plant temperature and/or alteration of the optimal thermal range. Optimization of temperature can be accomplished through alteration of canopy architecture, optimization of plant root systems, and irrigation management based on plant temperature. Alteration of optimal thermal range of a species may be accomplished through breeding or molecular engineering methodologies.

GENETIC VARIATION AMONG COTTON GERMPLASM FOR WATER-USE EFFICIENCY*

Abstract Cotton ( Gossypium spp.) genotypes including three species, five modern cultivars, one strain and 18 primitive race stocks were grown in two greenhouse experiments to determine differences in water-use efficiency (WUE). Water-use efficiency was defined as “the weight of total biomass (shoot and root) produced per unit of water transpired”. Plastic containers were filled with air-dried soil, and water was added until the soil reached field capacity. Plants were then grown (with further watering) until reaching the permanent wilting point. Data on plant biomass (shoot and root), water use, and days to permanent wilting were then collected. Differences occurred between the two experiments in water used prior to permanent wilting. No significant differences between genotypes were observed in the amount of water used, but differences did occur in biomass production and days to permanent wilting. Genotypes did not interact with environments for water used, biomass produced, or WUE. Differences did occur among genotypes between experiments for WUE. Genotypic means showed a 28% range in WUE between the highest vs the lowest entries and a 14% improvement above the best cultivar. G. herbaceum L. and G. barbadense L. fell within the distribution range for G. hirsutum L. Primitive race stocks of G. hirsutum were more efficient as a group in water use than were modern cultivars of the species.

Soil water extraction and photosynthesis in Gutierrezia sarothrae and Sporobolus cryptandrus.

Broom snakeweed (Gutierrezia sarothrae Shinners), a C3 evergreen half-shrub, is a formidable competitor of grasses in the semiarid southwestern rangelands. Sand dropseed (Sporobolus cryptandrus (Toff.) Gray), perennial C4 bunchgrass, is the most drought resistant species in the short-grass prairie. A comparative study on soil water extraction patterns, photosynthesis, and canopy development in both species during spring-summer growing season of 1991 was conducted in pot- and field-grown plants. Sand dropseed extracts water at depths between 0 and 30 cm more effectively than broom snakeweed. In contrast, broom snakeweed can take up more water from the subsoil (30-60 cm) than sand dropseed. Photosynthesis in sand dropseed was more affected by soil water deficit than was broom snakeweed, which was related to their water extraction patterns. Leaf area accumulation of broom snakeweed was not affected by spring drought, but that of sand dropseed was reduced. Because of greater water extraction from the wetter subsoil by broom snakeweed during drought, it can assimilate more carbon and, therefore, prevail in a competitive relationship with sand dropseed.

Lateral root development in exotic cottons

Abstract Lateral root development and vascular bundle arrangements of young 7-day-old seedlings of 120 exotic cotton ( Gossypium hirsutum L.) strains were evaluated to determine the relationship between the number of vascular bundles in the taproots and the observed lateral root branching. Seeds were germinated and seedlings were grown in polyethylene growth pouches at constant temperature (30°C) in the dark for 7 days. Taproot length, lateral root length and number of lateral roots were evaluated on a daily basis during the 7-day measurement period. The vascular bundle arrangement of each taproot was determined by examining a segment of the taproot 5 cm from the root apex with a light microscope at the end of the 7-day period. Significant differences in the lateral root development among strains were noted. An in-depth presentation of the root development of four strains (T25, T169, T256 and T165) indicated that partitioning of total root length into lateral root was (Anov and Duncans Multiple Range Test) significantly higher for the T25 and T256 strains relative to the T169 and T165 strain. Branching intensity was shown to be dependent on the vascular bundle arrangements as indicated by the Chi-square analysis of the relationship between the number of vascular bundles present in the aproots and the branching intensity (No. lateral roots per cm of taproot). As the number of vascular bundles increased, the proportion of plants with high branching intensities also increased. This information may possibly be useful in genetically altering plant rooting patterns and improving water and nutrient utilization under adverse growing conditions.

Root vascular bundle arrangements among cotton strains and cultivars

Abstract Vascular bundle arrangements of young primary roots of exotic cotton (Gossypium hirsutum L.) strains T25 and T169, and commercial cultivars ‘Acala SJ5’, ‘Coker 5110’, ‘Deltapine 16’, ‘Paymaster 266’, ‘Stoneville 213’ and ‘Tamcot 788’ were studied to determine if differences occurred in internal root anatomy. Seeds of each strain or cultivar were germinated and seedlings were grown in 17.4-cm long × 16.4-cm wide polyethylene growth pouches in a constant temperature (30°C) incubator without light. Hoaglands nutrient solution was added to each pouch prior to ‘planting’, and the seed were kept moist by a paper insert within the pouch. When the roots were 7 days old, samples of primary or ‘tap’ roots (5 cm from apex) were collected and examined with a light microscope. Measurements of primary root length, lateral root length, and total lateral root numbers were made daily on the T25 and T169 plants for a 7-day period. The vascular ‘bundle’ arrangement in all the commercial entries and in the T169 strain were tetrarch (four distinct bundles), while the vascular bundle arrangement of strain T25 was pentarch (five distinct bundles). A stereological analysis of the root vascular systems showed that the total cross-sectional vessel area per root was 45% greater in T25 than in T169. These differences were primarily because of an increased number of vessel elements as a result of an additional vascular bundle. There was an increase in lateral root development (increased number and length) in T25. The increased number of vessel elements in T25 suggested a decrease in root axial resistance to water flow. The internal root anatomy of T25 may be associated with characteristics of drought tolerant cotton and further research will begin to quantify these relationships.