Larry Boersma

Partition of photosynthates between shoot and root in spring wheat (Triticum aestivum L.) as a function of soil water potential and root temperature

Low soil water potential and low or high root temperatures are important stresses affecting carbon allocation in plants. This study examines the effects of these stresses on carbon allocation from the perspective of whole plant mass balance. Sixteen-day old spring wheat seedlings were placed in a growth room under precisely controlled root temperatures and soil water potentials. Five soil water potential treatments, from −0.03 MPa to −0.25 MPa, and six root temperature treatments, from 12 to 32°C were used. A mathematical model based on mass balance considerations was used, in combination with experimental measurements of rate of net photosynthesis, leaf area, and shoot/root dry masses to determine photosynthate allocation between shoot and root. Partitioning of photosynthates to roots was the lowest at 22–27°C root temperature regardless soil water potential, and increased at both lower and higher root temperatures. Partitioning of photosynthates to the roots increased with decreasing soil water potential. Under the most favourable conditions, i.e. at −0.03 MPa soil water potential and 27°C root temperature, the largest fraction, 57%, of photosynthates was allocated to the shoots. Under the most stressed conditions, i.e. at −0.25 MPa soil water potential and 32°C root temperature, the largest fraction, more than 80%, of photosynthates was allocated to roots.

Effect of antecedent rainfall on runoff during low-intensity rainfall☆

The objective of this study was to determine the relative significance of several rainfall characteristics to the occurrence and magnitude of runoff from small (< 300 ha) watersheds during very low-intensity rainstorms (maximum rainfall intensities < 15 mm/hr). This was done by making hourly measurements of rainfall and runoff at 1-hr intervals for four years on five watersheds in western Oregon, U.S.A., an area with a modified marine climate. Normal annual precipitation at the site is 1.02m and occurs primarily as rain during the winter months. Rainfall intensities only rarely exceed 15 mm/hr. The watersheds (0.46 to 285 ha in size) were fall-planted (November) to wheat. The soils are predominantly Willakenzie series (Ultic Haploxeralfs, fine-silty, mixed, mesic). Rainfall characteristics were computed for each rainfall event, defined as a period of consecutive rainy hours separated by six or more consecutive dry hours. Included were magnitude, average intensity, maximum intensity and duration of rainfall during rainfall events, magnitude of rain during the 12, 48 and 120 hrs preceding events, the magnitude of rain since October 1 of each year and number of dry hours preceding the event. The amount of runoff was computed for each rainfall event. Stepwise discriminant analysis was used to determine those values and combinations of rainfall-event characteristics that were significant in predicting the occurrence and amount of runoff. Between 44% and 93% of the rainfall events caused no measurable runoff. Discriminant analysis indicated that the magnitude of rainfall during the 12 and 120 hrs preceding the events was more significant in determining the occurrence of runoff than either rainfall magnitude, average intensity or maximum intensity. The majority of rainfall events on all of the watersheds resulted in < 25% runoff. Discriminant analysis indicated that the amount of runoff during events was determined more by the magnitude of rainfall in the 12, 48 and 120 hrs preceding the events than by the other event characteristics. The results suggest that in climates characterized by low-intensity rainfall, antecedent rainfall is more important in controlling the occurrence and amount of runoff than rainfall magnitude or intensity. The results also suggest that more accurate predictions of runoff can be achieved by using values of antecedent-rainfall characteristics.

A stochastic cluster model for hourly precipitation data

Abstract The objective of this research was to develop a stochastic model of hourly precipitation that preserves the pattern of occurrence of precipitation events throughout the year as well as several characteristics of the duration, amount, and intensity of precipitation within events. In the developed model, the pattern of occurrence of precipitation events is described by a Poisson cluster process. The duration of events and the time between events within clusters are described with identical Logarithmic Negative Mixture distributions. The hourly precipitation amounts within events are described with a nonstationary, first-order autoregression model. The results of Monte Carlo simulations for Salem and Pendleton, Oregon (stations from very different climatic regimes) showed that the model accurately reproduces the seasonal pattern of event occurrence and the marginal and conditional distributions of the magnitude, duration, and intensity of precipitation during events. Autocorrelation functions for the historical and simulated data were also similar.

Effect of phosphorus fertilization on water stress in Douglas fir seedlings during soil drying

A growth chamber experiment was conducted to determine if P fertilization to enhance the P nutrition of otherwise N and P deficient Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings reduces water stress in the seedlings during drought periods. Seedlings were grown in pasteurized mineral soil under well-watered conditions and fertilized periodically with a small amount of nutrient solution containing P at either of three levels: 0, 20, or 50 mg P L-1. By age 6 mo, leaf nutrient analysis indicated that N and P were deficient in control (0 mg P L-1) seedlings. The highest level of P fertilization, which doubled leaf P concentration, did not affect plant biomass, suggesting that N deficiency was limiting growth. When these seedlings were subjected to drought, there was no effect of P fertilization on leaf water potential or osmotic potential. Furthermore, P fertilized seedlings had lower stomatal conductance and net photosynthesis rate. These results indicate that enhanced P nutrition, in the presence of N deficiency, does not reduce water stress in Douglas fir seedlings during drought periods.

Comparison of biomass allocation in ectomycorrhizal and nonmycorrhizal Douglas fir seedlings of similar nutrition and overall size

Biomass allocation in 6-month-old ectomycorrhizal Douglas fir seedlings was compared to that in nonmycorrhizal seedlings of the same age, nutrient status and total biomass. Seedlings colonized by Rhizopogon vinicolor had the same distribution of biomass between roots, stems and needles, but only 56% of the total length of roots (including mycorrhizal branches) compared to nonmycorrhizal seedlings. Laccaria laccata had no effect on distribution of biomass or root length of seedlings. The results for Rhizopogon provide direct evidence that the process of ectomycorrhizal colonization can significantly affect plant biomass allocation by one or more mechanisms not directly related to altered nutrition or overall plant size.

Dispersion at the field scale resulting from spatial variability of the hydraulic conductivity field

Abstract A two-dimensional mathematical model for simulating the transport and fate of chemicals in aquifers with spatially heterogeneous but isotropic fluid flow properties was developed, as part of an evaluation of bioremediation technologies that include injection of reactants into the aquifer. An important physical process is dispersion of the injected reactant in the aquifer. Dispersion was simulated with the model which has user-prescribed hydraulic pressure fields at the inflow and outflow boundaries. The model accounts for the major physical processes of dispersion and advection and several fundamental chemical and biological processes, including linear equilibrium sorption, irreversible sorption and/or dissolution into an organic phase, microbial degradation, radioactive decay, and other irreversible processes. The chemical may be released internally via distributed leaks from sources that do not perturb the flow field, from fully penetrating injection wells, or it may enter at the inlet boundary. The chemical transport and fate equations are solved in terms of user-stipulated initial and boundary conditions. Simulations were made to evaluate dispersion resulting from spatial variability in the hydraulic conductivity ( K ). Flow fields were divided into regions with hydraulic conductivities of 0.2, 2.0 and 20 m day −1 . Distributions of chemical, initially present as a distributed source near the inflow boundary, were obtained for several geometries of the hydraulic conductivity regions. Results show the influence on distributions of regions of low, medium, and high K ; they demonstrate the importance of knowledge about the hydraulic conductivity field, both for interpretation of sampling data and for prediction of plume behaviour in terms of direction of movement, dispersion and rate of travel.

Plant responses to drought and salinity stresses

The distribution of vegetation over the earth’s surface is controlled more by the availability of water than by any other factor. Production of crops requires a reliable supply of water from either rainfall or irrigation. Increasingly agriculture, industry, and cities are competing for scarce water resources. Thus, there is great interest in developing technologies which would make it possible to use water of low quality, currently considered to be unsuitable for irrigation, such as water with high salt content. Such technologies would make it possible to raise crops on large areas where none can be raised now.

Osmotic Adjustment in Plant Cells Exposed to Drought and Temperature Stress: Can a Cause and Effect Hypothesis be Formulated and Tested?

The study of plant response to environmental stresses has developed rapidly during the past several decades. Attention has increasingly been devoted to the biochemical, biophysical, and physiological processes involved in the changes in solute concentration and content of cells which occur in response to changes in the external water potential. One aspect of these changes is osmotic adjustment.