Surviving the hot zone.
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Emergency Medical Services↗
Biomedical subjects
L R Parsons
Publications and source records attributed to L R Parsons.
Changes in Frost Hardiness of Stem Cortical Tissues of Cornus stolonifera Michx. after Recovery from Water Stress.
Moderate water stress increases frost hardiness in many woody plants but little attention has been given to changes in hardiness after recovery from water stress. Tests were carried out to examine how much water stress-induced frost hardiness remained when plants were rewatered under different day length regimes. Red osier dogwood plants (Cornus stolonifera Michx.) were water-stressed at normal growing temperatures in long day (LD) or short day (SD) conditions, exposed to 6 nights of freezing temperatures, and then returned to normal growing conditions with full water supply. Water-stressed plants gained an additional 8 to 10 C of hardiness. The amount of freeze-induced hardiness in both stressed and control plants was not significant (approximately 2 C) and was not affected by photoperiod. When plants were kept in or transferred to LD, they lost nearly all of their water stress-induced hardiness within 7 days after rewatering. Water-stressed plants in SD lost the least amount of hardiness (5 C) when rewatered. In dogwood, water stress is an effective way to increase hardiness temporarily, but the photoperiod has a large effect on the retention of the acquired hardiness.
Journal Article↗
Water Relations, Stomatal Behavior, and Root Conductivity of Red Osier Dogwood during Acclimation to Freezing Temperatures.
Red osier dogwood (Cornus stolonifera Michx.) was artificially acclimated by exposing plants to 8-hour short days (SD) and low (15/5 C) temperatures for 54 to 63 days. Several factors including transpiration rate, stomatal resistance, and root conductivity were correlated so that the rate of water loss in acclimating plants was higher during the first 30 to 40 days of the acclimation sequence. Six days after transferring plants to SD conditions, the stomatal resistance (r(8)) decreased significantly below the r(8) of the 16-hour long day (LD) control plants at the same temperature. Transpiration rate increased by approximately 20 to 30% in the plants transferred to SD. After the initially higher transpiration rate and greater stomatal opening, the stomates closed tightly during the last 2 weeks of acclimation and the transpiration rate of the SD plants dropped to well below the LD control plants. By the end of the acclimation sequence, root conductivity to water uptake was two to three times lower in the SD plants. Leaf xylem water potentials were similar or slightly lower in the plants kept under SD conditions during the first 5 to 7 weeks of the acclimation sequence. During the last 10 to 15 days of acclimation when the stomates closed, SD leaf water potential rose significantly above the plants in the LD conditions. During acclimation, stem water content decreased by 40 to 50%. Changes in tissue hydration can be indirectly related to plant hardiness and may be affected by alteration of stomatal resistance, transpiration rate, and root conductivity during acclimation.
Journal Article↗
Soil temperature, nitrogen concentration, and residence time affect nitrogen uptake efficiency in citrus.
We try to elucidate which environmental and soil factors control nitrogen uptake efficiency in citrus. Effects of residence time and nitrogen (N) concentration (three 500-mL applications of 7 mg N L(-1), representative of reclaimed water used for citrus irrigation in central Florida, or one 150-mL application of 70 mg N L(-1)) on nitrogen uptake efficiency (NUE) of young citrus seedlings were studied. Increasing residence times from 2 to 8 h increased NUE from 36 to 82% and from 17 to 34% for high and low application frequencies, respectively. We developed a model to predict N uptake based on root density, N concentration, and soil temperature (Ts). Assuming a base temperature (Tb) of 10 degrees C, N uptake temperature sum (UTS) = sigma(Ts - Tb)/24 (degrees CdN, degree day units of N uptake). To eliminate the risk of N leaching for young seedlings, minimum uptake periods of 5 and 16 degrees CdN were required at initial soil N concentrations of 0.9 and 2.5 mg N L(-1), respectively. After correcting for differences in root length, this information was then used to predict the effect of irrigation practices on N uptake from reclaimed water for mature trees. Applying 2500 mm yr(-1) vs. 400 mm yr(-1) reclaimed water reduced the NUE of N in this water from 100 to 63% during the summer and from 100 to 28% during the winter. Reductions in NUE at higher irrigation rates appeared to be related to N displacement below the root zone prior to complete N uptake.
Citrus↗