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- Author or Editor: Robert J. Joly x
Writing is a powerful tool for thinking and for clarifying complex subjects. It's a much more physical activity than reading. It compels students to organize their thoughts and present them clearly and logically. They must continually reassess whether what they've written is really what they want to say. The focus of this presentation is on the impediments to implementing this approach in our teaching. Our objective is to seek methods for reducing the work load of instructors while maintaining the quality of learning that can occur in a writing-intensive course. Results of workshops conducted during the 1993 North Central Regional Teaching Symposium entitled “Writing to Learn in Science” will be discussed. The workshops were active, participatory sessions designed to elicit as many responses as possible to the question “How can we utilize writing, intensively, in our courses?” Five obstacles or barriers to implementation of writing were identified. These include (1) instructor anxiety, (2) students requiring individualized instruction, (3) time-consuming evaluation of student writing, (4) in-class time needed for writing instruction, and (5) lack of student motivation. A focused-discussion format was utilized in these sessions, and groups of participants were given responsibility to devise creative actions or strategies that could be utilized to meet the challenges noted above. More than forty “actions” were identified which could help to make this approach feasible in both graduate and undergraduate. courses. These are summarized within five broad strategic approaches.
Redbud (Cercis canadensis) is known to be very susceptible to injury by road de-icing salts. The purpose of these experiments was to measure the effects of sodium chloride on net CO2 assimilation (A), conductance (g), transpiration (E), and leaf area expansion (LAE) of hydroponically grown redbud seedlings. Eight week-old seedlings were exposed to 0, 4500, and 9000ppm NaCl in the hydroponic growth solution. A, g, E, and LAE were measured for seven consecutive days during treatment application.
A, g, E, an LAE all decreased with increased salt stress. By the seventh day, growth in NaCl at 4500 and 9000 ppm resulted in reductions in A from that that of the control by 34% and 63%, respectively. For the medium treatment, g and E had decreased by 70% over control rates, and by 85% over control for the high treatment. For the 0, 4500, and 9000ppm treatments, total leaf area increased by 68%, 46% and 28%, respectively, over the seven days of the experiment.
Further experiments will examine the effect of treatments on whole plant transpiration, water potential and osmotic potential and will measure the ability of seedlings to recover from treatments of various duration.
The principles of plant physiology are best learned in an environment where students are directly engaged in the process of scientific inquiry. Working from this assumption, we have developed a two-stage approach to laboratory instruction that fosters student-directed research within an undergraduate plant physiology course. During the first 10 weeks of a 16-week semester, students develop competency in measuring physiological variables by using an array of standard analytical techniques. A core set of 10 laboratory experiments provides structured instruction and teaches the principles of modern physiological analyses. During week 11, students observe a demonstration of a plant response, where the underlying cause of the phenomenon is not evident. Working together in groups of three or four, students hypothesize on the physiological mechanisms that may be involved. After submitting a statement of hypothesis and a plan of study, each group then requests the necessary instrumentation, plant material and greenhouse and/or growth chamber space to conduct their experiments. Results of their experimentation are presented during week 15 in both written and oral formats. The approach appears to help students to integrate and connect learnings from earlier in the semester to solve a defined problem. Further, students learn how to judge the reliability of experimental results and to evaluate whether conclusions drawn are justified by the data.
Diurnal variation in the chilling sensitivity of tomato seedlings was examined. Sensitivity to chilling in tomato seedlings is a response to light and not under the control of a circadian rhythm. Chilling sensitivity is highest in seedlings chilled at the end of the dark period, and these seedlings become more resistant to chilling injury upon exposure to the light. Diurnal variation in chilling sensitivity was associated with changes in catalase and superoxide dismutase activities. The results show an increase in catalase and superoxide dismutase activities at the end of the light period. The recovery of the net photosynthesis rate following chilling was faster in seedlings chilled at the end of the light period. It is suggested that an increase in catalase and superoxide dismutase activities at the end of light period before the chilling plays a role in the resistance to chilling stress in tomato seedlings. Forty-eight hours of 14°C acclimation or hydrogen peroxide pretreatment conferred chilling tolerance to tomato seedlings and were correlated with elevated catalase activity. Acclimated seedlings still exhibited diurnal variation in chilling sensitivity while hydrogen peroxide treated seedlings showed little evidence of a diurnal variation in chilling sensitivity. Transgenic tomato plants expressing an antisense catalase gene were generated. A several-fold decrease in total catalase has been detected in the leaf extracts of transformants. Preliminary analysis of these plants indicated that modification of reactive oxygen species scavenging in plant system can lead to change in oxidative stress tolerance.
Diurnal variation in the chilling sensitivity of `Rutgers' tomato (Lycopersicon esculentum Mill.) seedlings was examined. Chilling sensitivity was highest in seedlings chilled at the end of the dark period, and these seedlings became more resistant to chilling injury on exposure to the light. The development of chilling tolerance in tomato seedlings was a response to light and not under the control of a circadian rhythm. The recovery of leaf gas exchange following chilling was faster in seedlings chilled at the end of the light period. Diurnal variation in chilling sensitivity was associated with changes in catalase and superoxide dismutase activities. An increase in catalase and superoxide dismutase activities was observed at the end of the light period. Catalase activity was significantly higher in all stages of chilling following the light period compared to those chilled after the end of the dark period. Forty-eight hours of 14 °C acclimation or pretreatment with hydrogen peroxide conferred increased chilling tolerance to tomato seedlings. Hydrogen peroxide-treated seedlings showed little evidence of a diurnal variation in chilling sensitivity. These results support a role for light and oxidative stress in conferring increased chilling tolerance to tomato seedlings.
Honey locust (Gleditsia triacanthos var. inermis Wind.) and tree-of-heaven Ailanthus altissima (Mill.) Swingle] sometimes are exposed to high root-zone temperatures in urban microclimates. The objective of this study was to test the hypothesis that seedlings of these species differ in how elevated root-zone temperature affects growth, leaf water relations, and root hydraulic properties. Shoot extension, leaf area, root: shoot ratio, and root and shoot dry weights were less for tree-of-heaven grown with the root zone at 34C than for those with root zones at 24C. Tree-of-heaven with roots at 34C had a lower mean transpiration rate (E) than those grown at 24C, but leaf water potential (ψ1) was similar at both temperatures. In contrast, shoot extension of seedlings of honey locust grown with roots at 34C was greater than honey locust at 24C, E was similar at both temperatures, and ψ1 was reduced at 34C. Hydraulic properties of root systems grown at both temperatures were determined during exposure to pressure in solution held at 24 or 34C. For each species at both solution temperatures, water flux through root systems (Jv) grown at 34C was less than for roots grown at 24C. Roots of tree-of-heaven grown at 34C had lower hydraulic conductivity coefficients (Lp) than those grown at 24C, but Lp of roots of honey locust grown at the two temperatures was similar.
The planophile (horizontal) leaf presentation of closed cowpea (Vigna unguiculata Walp.) leaf canopies limits PAR absorption from overhead lamps to the top layer of overlapping leaves, resulting in suboptimal canopy photosynthesis and premature senescence and abscission of lower, shaded leaves. Very low crop yield rates have been obtained in growth chamber studies using dense cowpea stands compared to greenhouse and field studies using more widely spaced plants. Nine separate growth compartments were constructed in each of two growth rooms. Eight or sixteen 15-W fluorescent lamps were mounted horizontally or vertically in tiers within each compartment, remote from their ballasts, and which can be switched on or off separately according to different lighting strategies. Mylar sleeves around each tube prevents contacting leaves from overheating. Intracanopy lighting arrangements draw from 0.27 to 0.54 kW of power/m3 of growth volume, compared to 1.18 kW·m–3 for traditional overhead lighting. PPF within compartments varies from 80 to 280 mmol·m–2·s–1, depending on sensor location, lamp arrangement, and lamp number. Each compartment is equipped with a recirculating hydroponic system. One room is operated with overhead plus intracanopy lighting, whereas the other utilizes intracanopy lighting only. Cowpea canopies are being grown under different lighting strategies and compared for growth, yield, productivity, leaf orientation, and individual leaf gas-exchange rates. Electrical power draw and total electrical energy consumption are being compared among treatments.
Traditional overhead lighting of dense crop stands in controlled environments favors development of upper leaf layers to maximize interception of light incident at the top of the foliar canopy. The resultant mutual shading of lower leaves in the understory of the canopy can severely limit productivity and yield of planophile crops. Intracanopy lighting alleviated the effects of mutual shading in dense, vegetative stands of cowpea [Vigna unguiculata (L.) Walp ssp. unguiculata] growing in a controlled environment by sustaining irradiance within the understory throughout development of this edible-foliage crop. For an overhead lighting system, photosynthetic photon flux (PPF) in the understory was reduced to 1% of its initial value by 35 days of growth. PPF in an intracanopy-lighted stand remained within 30 μmol·m-2·s-1 of initial values throughout the 50-day cropping period. Spectral distribution of radiation within the intracanopy-lighted stand also remained relatively constant throughout canopy development. In the overhead-lighted stand, violet and blue radiation in the understory decreased as much as 60% from initial values. Stability of the radiation environment within the intracanopy-lighted stand delayed leaf senescence 27 days beyond when interior leaves of the overhead-lighted canopy began to turn yellow on day 16. The intracanopy-lighted stand produced twice as much edible biomass per unit electrical energy consumed by lamps as for the overhead-lighted system. The treatment differences were due to the continuous presence of understory irradiation when using intracanopy lighting but not when using overhead lighting, and they underscore the importance of the entire foliar canopy in realizing the full productivity potential of dense crop stands in controlled environments.
The difference between night and day temperature (DIF = day - night temperature) has been shown to affect plant height. A positive DIF (+DIF), cooler night than day temperature, increases stem elongation while a negative DIF (- DIF), warmer night than day temperature, decreases stem elongation. The physiological mechanism underlying the growth response to DIF is not understood, however, and the effects of day/night temperature differentials on root permeability to water and root elongation rate have not been studied. The objective of this study was to describe how +DIF and -DIF temperature regimes affect leaf water relations, root water flux (Jv ), root hydraulic conductivity (Lp ), and root elongation rates of `Boaldi' chrysanthemum [Dendranthema ×grandiflora Kitam. `Boaldi' (syn. Chrysanthemum ×morifolium Ramat.)] plants over time. Leaf turgor pressure (ψp) was 0.1 to 0.2 MPa higher in plants grown in a +6 °C DIF environment throughout both the light and dark periods, relative to those in a -6 °C DIF environment. Jv differed markedly in roots of plants grown in +DIF vs. -DIF environments. Rhythmic diurnal patterns of Jv were observed in all DIF treatments, but the relative timing of flux minima and maxima differed among treatments. Plants grown in positive DIF regimes exhibited maximum root flux at the beginning of the light period, while those in negative DIF environments had maximum root flux during the first few hours of the dark period. Plants grown in +DIF had significantly higher Lp than -DIF plants. Plants grown in +DIF and -DIF environments showed differences in the diurnal rhythm of root elongation. During the dark period, +DIF plants exhibited minimal root elongation rates, while -DIF plants exhibited maximal rates. During the light period, the converse was observed. In -DIF temperature regimes, periods of rapid root elongation coincided with periods of high Jv . Results of this study suggest that negative DIF environments lead to leaf turgor reductions and markedly alter diurnal patterns of root elongation. These changes may, in turn, act to reduce stem elongation.
Phosphorus is one of the essential but limiting nutrients in nature. In this study, we link the physiological changes occurring under phosphate (Pi) starvation to gene expression. Roots of aeroponically grown tomato (Lycopersicon esculentum L.) plants were sprayed intermittently with nutrient solutions containing varying concentrations of P. Decreasing the concentration of Pi in the nutrient solution resulted in reduced biomass production and altered the tissue concentration of nutrients in roots and shoots. Phosphorus starvation increased the root:shoot biomass ratio and decreased net CO2 assimilation and stomatal conductance. Phosphorus concentrations in roots and shoots decreased with decreasing concentration of Pi in the nutrient solution. Pi-deficient plants had a higher concentration of Ca in roots and Mg in shoots. Expression of the Pi starvation-induced gene, TPSI1, persisted even after 3 weeks of Pi starvation. The transcript accumulation in leaves was found to be a specific response to Pi starvation and not to the indirect effects of altered N, K, Fe, Mg, or Ca status. Accumulation of transcripts was also observed in stem and petioles, suggesting a global role for TPSI1 during Pi starvation response of tomatoes.