Progress 09/15/00 to 09/14/05
Outputs
The overall goal of the project is to understand the spatial and temporal aspects of pH patterns that form around the growing tips of roots, to begin to catalog the effects of environmental variation on the patterns, and to understand the implications of the pH patterns for nutrition of growing tissue. The first two years of the project saw the completion of two major goals. The effect of soil pH on growth and cation deposition in seedling roots of maize was determined. Over a wide range (pH 4 to pH 8), growth was unaffected by pH of the sandy soil. The shape of the developmental pattern of nutrient accumulation (characterized by plots of element tissue concentration and element deposition rate versus distance from the root tip) was unchanged by pH variation in the surrounding soil. However, the magnitudes of cation deposition rates, particularly for Ca, are strongly increased at low pH. An important goal of the project is testing the convection-diffusion model. Using
microelectrodes on growing roots, we verified that in sandy soil the root acidifies the surrounding soil as far as one mm from the root surface the growth zone, and this pattern is steady (in the moving coordinate reference frame attached to the root tip) over an extended time period. We also confirmed that in agar, the root acidifies the surrounding medium for several cm; this pattern changes over a time period of an hour. The features of our convection-diffusion model were supported by the empirical tests. We were disappointed in our attempt to use a genotype reported to have enhanced proton secretion from the primary root. The seedling had developmental abnormalities (especially positive geotropism!) and moreover produced acidification of the growth media indistinguishable from that of the control variety. An unexpected result arose from the work initiated by A. Walter in the Silk laboratory. For many major nutrients, seedlings grown in water had the same nutrient accumulation and
deposition rate as seedlings grown in complete nutrient solution. This result draws attention to the importance of the seed as the source of nutrients to the young maize root. It also suggests that the seedling is not the best system for determination of the role of proton secretion and rhizosphere modification on nutrient uptake. Therefore, in the Silk sabbatical project with Dr. Michelle Watt, we began studies on wheat plants at the four-leaf stage. The transport of apoplastic solutes down the soil profile by the growing root tip was calculated from growth analyses and observed by time lapse photography of marked roots in a stationary reference frame. Calculated and observed downward displacements were shown to be small (less than four millimeters) except for substance in root cells located in the apical millimeter of the root tip. Time lapse records were communicated at the International Symposium on Plant Biomechanics at East Lansing, summer 2003. An important concept is
rhizosphere development, and numerical methods were developed to elucidate the experience of particles on or adjacent to the growing root tip.
Impacts
The convection-diffusion model is of general interest. For any substance that is secreted or taken up by growing tissue and that moves down a concentration gradient in the soil, the model can be used to characterize the spatial and temporal distribution patterns within the rhizosphere. Thus from an ecological perspective, our model gives insight into many interactions between moving growth zones and the soil. Four students received masters degrees with funding from this grant and secured employment. Seven undergraduate interns also received training in the laboratory. A postgraduate researcher supported on the grant went on to a career in college teaching.
Publications
- Walter, A., Schurr, U. and W. K. Silk. 2000. Effect of soil pH on growth and cation deposition in the root tip of Zea mays L. Journal of Plant Growth Regulation 19: 65-76. Baum, S.F., Tran, P.N. and W.K. Silk. 2000. Effects of salinity on xylem structure and water use in growing leaves of sorghum. New Phytologist 146: 119-127.
- Nichol S.A. and W.K. Silk. 2001. Empirical evidence of a convection-diffusion model for pH patterns in the rhizospheres of root tips. Plant, Cell and Environment 24: 967-974.
- Walter, A., Feil R., and Schurr, U. 2003. Expansion dynamics, metabolite composition and substance transfer of the primary root growth zone of Zea mays L. grown in different external nutrient availabilities.
- Silk, W.K. Kinematics of Primary Growth. IN eds. Y. Waisel and A. Eshel Plant Roots: the Hidden Half new edition (2003) Marcel Dekker, New York.
- Boyer J.S. and W.K. Silk. 2004. Hydraulics of plant growth. Functional Plant Biology 31: 761-773.
- Girousse, C., Moulia, B., Silk, W.K. and J-L Bonnemain. 2005. Aphid infestation causes different changes in carbon and nitrogen allocation in alfalfa stems as well as different inhibitions of longitudinal and radial expansion. Plant Physiology 137: 1474-1484.
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Progress 01/01/04 to 12/31/04
Outputs
Laboratory studies were undertaken to analyze the relative importance of soil acidity and plant developmental stage in controlling the copper content of native grasses exposed to acid mine drainage. Copper accumulation was shown to depend primarily on leaf developmental stage in California brome grass. A pilot study showed that amending soil with small amounts of lawn compost can detoxify mine soils by improving soil acidity and adsorbing copper. Time lapse photography and root length density counts revealed that roots grow well in the amended mine soil.
Impacts
This study has immediate relevance for revegetation of hostile soils. A larger scale modeling effort would be useful to determine the dosing requirements for amendments in the face of continuing acid and copper release from seeps in metalliferous soils.
Publications
- No publications reported this period
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Progress 01/01/03 to 12/31/03
Outputs
The ability of the root to change the pH (acidity) of the nearby soil affects the uptake of both beneficial and toxic nutrients and thus regulates the rate of introduction of mineral elements into the food chain. This year experiments were conducted to confirm that lower soil pH increases calcium content of the growth zone of primary roots of maize. The effect is due to a pH related increase in the calcium content of the soil solution. Barley, wheat and maize were all shown to have growth zones that acidify their rhizospheres for sandy soils with initial pH near 6.0. Preliminary determinations of phosphorus content of roots grown at different soil acidities produced unexpectedly low phosphorus levels within the growth zones. Work is continuing to characterize the phosphorus uptake process and to determine whether phosphorus uptake, like that of calcium is favored by acidification of many soils.
Impacts
Knowledge of the ability of different plant varieties to modify the pH of the soil surrounding the root will enable growers and horticulturalists to select plants to increase at least calcium and iron content, and possibly phosphorus uptake. There is also the potential to enhance uptake of toxic ions such as copper if soil purification by plants ("phytoremediation") is desired, or to exclude toxic ions if the plants are to be used as food.
Publications
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Progress 01/01/02 to 12/31/02
Outputs
The ability of the root to change the pH (acidity) of the nearby soil affects the uptake of both beneficial and toxic nutrients and thus regulates the rate of introduction of mineral elements into the food chain. This year experiments were conducted to determine the relationship between nitrogen uptake and pH of the incubation medium for duckweed. Results showed that acidity decreases as duckweed takes up ammonium and then decreases as the plants take up nitrate. In other experiments, we found phosphorus content of the growth zone of maize roots increases at lower soil pH, whether the acidified soil surrounds the growth zone or forms a layer above the growth zone. A pilot project was conducted to determine copper uptake as a function of developmental stage in monkey flower plants collected from an abandoned copper mine. Copper was found to be more concentrated in older leaf tissue. A paper reviewing growth patterns in primary roots was written and published.
Impacts
Knowledge of the ability of different plant varieties to modify the pH of the soil surrounding the root will enable growers and horticulturalists to select plants to increase at least calcium and iron content, and possibly phosphorus uptake. There is also the potential to enhance uptake of toxic ions such as copper if soil purification by plants ("phytoremediation") is desired, or to exclude toxic ions if the plants are to be used as food.
Publications
- No publications reported this period
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Progress 01/01/01 to 12/31/01
Outputs
The ability of the root to change the pH (acidity) of the nearby soil affects the uptake of both beneficial and toxic nutrients and thus regulates the rate of introduction of mineral elements into the food chain. This year experiments were conducted to confirm that lower soil pH increases calcium content of the growth zone of primary roots of maize. The effect is due to a pH -related increase in the calcium content of the soil solution. Barley, wheat and maize were all shown to have growth zones that acidify their rhizospheres for sandy soils with initial pH near 6.0. Preliminary determinations of phosphorus content of roots grown at different soil acidities produced unexpectedly low phosphorus levels within the growth zones. Work is continuing to characterize the phosphorus uptake process and to determine whether phosphorus uptake, like that of calcium is favored by acidification of many soils.
Impacts
Knowledge of the ability of different plant varieties to modify the pH of the soil surrounding the root will enable growers and horticulturalists to select plants to increase at least calcium and iron content, and possibly phosphorus uptake. There is also the potential to enhance uptake of toxic ions such as copper if soil purification by plants ("phytoremediation") is desired, or to exclude toxic ions if the plants are to be used as food.
Publications
- No publications reported this period
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