James R. Mahan

Maintenance of constant leaf temperature by plants—I. Hypothesis-limited homeothermy

Abstract The relationship between the temperature of a plant and the temperature of its environment is an important consideration in the study of thermal stress. Plants are generally assumed to be poikilotherms that exist in eurythermal environments and some variation in plant temperature is thought to be normal for the plant. Recent investigations in our laboratory have indicated that the thermal dependencies of enzymes from several plants are more characteristic of those from homeotherms than eurytherms. Thus, while it is assumed that thermal variation is normal for a plant, the thermal dependencies of the enzymes of the plant suggest that such variation may be stressful. We present a conceptual framework for the active maintenance of a normative, or characteristics, temperature by the plant. We propose that the lower limit of the temperature of a plant is controlled by its environment while the upper limit, even under a wide variety of conditions, can be controlled by the plant and maintained at a normative value. We suggest the term “limited homeothermy” to describe this type of thermal behavior by the plant. We propose three constraints on the maintenance of the normative temperature by the plant; (1) sufficient energy influx to raise its temperature to the normative value, (2) sufficient water supply for transpiration, and (3) humidity low enough to allow for cooling to the normative temperature.

Transgenic cotton (Gossypium hirsutum L.) seedlings expressing a tobacco glutathione S-transferase fail to provide improved stress tolerance.

Transgenic cotton (Gossypium hirsutum L.) lines expressing the tobacco glutathione S-transferase (GST) Nt107 were evaluated for tolerance to chilling, salinity, and herbicides, antioxidant enzyme activity, antioxidant compound levels, and lipid peroxidation. Although transgenic seedlings exhibited ten-fold and five-fold higher GST activity under normal and salt-stress conditions, respectively, germinating seedlings did not show improved tolerance to salinity, chilling conditions, or herbicides. Glutathione peroxidase (GPX) activity in transgenic seedlings was 30% to 60% higher under normal conditions, but was not different than GPX activity in wild-type seedlings under salt-stress conditions. Glutathione reductase, superoxide dismutase, ascorbate peroxidase, and monodehydroascorbate reductase activities were not increased in transgenic seedlings under salt-stress conditions, while dehydroascorbate reductase activity was decreased in transgenic seedlings under salt-stress conditions. Transgenic seedlings had 50% more oxidized glutathione when exposed to salt stress. Ascorbate levels were not increased in transgenic seedlings under salt-stress conditions. Malondialdehyde content in transgenic seedlings was nearly double that of wild-type seedlings under normal conditions and did not increase under salt-stress conditions. These results show that expression of Nt107 in cotton does not provide adequate protection against oxidative stress and suggests that the endogenous antioxidant system in cotton may be disrupted by the expression of the tobacco GST.

MAINTENANCE OF CONSTANT LEAF TEMPERATURE BY PLANTS--II. EXPERIMENTAL OBSERVATIONS IN COTTON

Abstract Plants are generally assumed to be eurythermal poikilotherms. Several species have, however, exhibited narrow temperature ranges for optimum enzyme function that are uncharacteristic of eurythermic organisms. In order to determine the extent to which cotton plants are eurytherms, the leaf temperatures of cotton (Gossypium hirsutum L.) grown in a fiberglass-covered greenhouse were monitored under eurythermal conditions. Leaf temperature, relative humidity, global and photosynthetically active radiation, air temperature and water use were measured continuously for 60 days. Homeothermic behavior by cotton plants was consistently observed when three environmental conditions were satisfied. These conditions were: (1) sufficient energy input to raise the leaf temperature to 27°C, (2) sufficient water available for transpiration, and (3) humidity low enough to allow evaporative cooling. Even with wide variations in air temperature (27–40°C), cotton maintained a normative plant temperature (Tn) of 27 ± 2°C. On the basis of this observation we conclude that cotton plants can function as limited homeotherms.

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.

Deficit irrigation in a production setting: canopy temperature as an adjunct to ET estimates

Water available for agricultural use is declining worldwide as a result of both declining water resources and increasing application costs. Managing crop irrigation under conditions where the water need cannot be fully met represents the future of irrigation in many areas. On the southern high plains of Texas there is interest among producers to reduce the amount of water applied to cotton. In this study, a producer’s efforts to reduce water application to a cotton crop were assessed in terms of a comparison between evapotranspiration, rainfall, and irrigation that is widely used in the region. The producer was able to reduce water application to meet intended reductions relative to the evapotranspiration estimates but, depending on the method used for calculating the crop water need, he tended to over water the crop in two out of three intended deficit irrigation regimes. Analysis of continuously monitored canopy temperatures provided verification of over-irrigation. Continuously monitored canopy temperature is proposed as a useful adjunct to evapotranspiration approaches to deficit irrigation management.

Physiology and transcriptomics of water-deficit stress responses in wheat cultivars TAM 111 and TAM 112

Hard red winter wheat crops on the U.S. Southern Great Plains often experience moderate to severe drought stress, especially during the grain filling stage, resulting in significant yield losses. Cultivars TAM 111 and TAM 112 are widely cultivated in the region, share parentage and showed superior but distinct adaption mechanisms under water-deficit (WD) conditions. Nevertheless, the physiological and molecular basis of their adaptation remains unknown. A greenhouse study was conducted to understand the differences in the physiological and transcriptomic responses of TAM 111 and TAM 112 to WD stress. Whole-plant data indicated that TAM 112 used more water, produced more biomass and grain yield under WD compared to TAM 111. Leaf-level data at the grain filling stage indicated that TAM 112 had elevated abscisic acid (ABA) content and reduced stomatal conductance and photosynthesis as compared to TAM 111. Sustained WD during the grain filling stage also resulted in greater flag leaf transcriptome changes in TAM 112 than TAM 111. Transcripts associated with photosynthesis, carbohydrate metabolism, phytohormone metabolism, and other dehydration responses were uniquely regulated between cultivars. These results suggested a differential role for ABA in regulating physiological and transcriptomic changes associated with WD stress and potential involvement in the superior adaptation and yield of TAM 112.

Thermal environment of cotton irrigated using canopy temperature

The threshold canopy temperature method for controlling a drip irrigation system includes a physiologically based threshold temperature and irrigation application rate that responds to the environment. Energy input from the environment causes canopy temperature to exceed the threshold value and irrigation is then applied. This study evaluated temperature distributions, amount of optimum time, and the amount of irrigation control time for cotton where irrigation scheduling was controlled by different threshold temperatures during the years 1988 to 1991. Optimum time for cotton growth was defined as the accumulated time that canopy temperatures were between 25 and 31 °C and the time accumulated above different threshold temperatures was designated as irrigation control time. Threshold temperatures over a 26 to 32 °C range altered the frequency distribution of temperature within the optimum temperature range (25–31 °C) by reducing temperatures above the threshold. Frequency of canopy temperatures of a 28 °C threshold temperature treatment decreased in the 28 to 29 °C increment and then remained below air temperature. Irrigation control time was more sensitive than optimum time to changes in threshold temperature between 26 and 31 °C. Optimum time and irrigation control time of the 28 °C threshold temperature varied by 37% and 29%, respectively. Lint yields in 1988 and 1990 were high while those in 1989 and 1991 were low because of unfavorable weather. Irrigation amounts applied during DOY 198–273 that were above 20 cm in high yield years or 12 cm in low yield years did not increase yield.

Thermal dependence of bioengineered glufosinate tolerance in cotton

Abstract Tolerance to glufosinate has been bioengineered into cotton through the expression of a gene encoding the enzyme phosphinothricin acetyl transferase (PAT). Studies were conducted to determine thermal limitations on herbicide efficacy in bioengineered cotton. The 50% inhibition (I50) of glufosinate of the target-site enzyme glutamine synthetase was thermally dependent with the lowest values between 25 and 35 C. Larger values of I50 were measured above and below the 25 to 35 C range. The apparent Michaelis constant KM of the enzyme PAT was relatively stable from 15 to 30 C and increased more rapidly from 30 to 45 C. The two components in combination suggest the aggregate tolerance to glufosinate would not be thermally limited between 15 and 45 C. The thermal dependence of the aggregate tolerance in cotton suggests that glufosinate would not damage the crop over a range of temperatures. This prediction is in agreement with the results of field studies carried out over a number of years, which showed the glufosinate-tolerant cotton to be undamaged by glufosinate over a wide range of temperatures. Nomenclature: Glufosinate; cotton, Gossypium hirsutum L. ‘SeedCo 9023’, ‘DeltaPine 458’.

A thermal application range for postemergence pyrithiobac applications

Abstract Pyrithiobac control of Palmer amaranth on the Texas Southern High Plains was correlated previously with temperature at the time of application. In the present study, the thermal dependence of pyrithiobac efficacy was used to define a thermal application range (TAR) for postemergence pyrithiobac applications. Several years of temperature data from four cotton-growing regions of the United States were analyzed with respect to the TAR to determine the extent to which temperature limitations could affect pyrithiobac applications. Temperatures outside the TAR occurred in all years and regions analyzed. Analyses of four geographic regions utilizing 4 to 11 yr of data for each region indicated the following percentages of hours inside the TAR: Lubbock, TX, 54 to 94%; Maricopa, AZ, 27 to 33%; Raleigh-Durham, NC, 70 to 97%; and Jackson, MS, 81 to 99%. A detailed analysis of the frequency and duration of the TAR in Lubbock, TX, showed that, periodically, temperatures outside the TAR may limit the efficacy of postemergence pyrithiobac applications for several consecutive days. Finally, the TAR was shown to be useful as a postapplication diagnostic tool for evaluating herbicide applications that resulted in poor efficacy. These results suggest that long-term evaluation of historic temperatures with respect to the TAR for a given herbicide may provide insight into the potential limitations of herbicide efficacy and underscore the potential utility of developing TARs based on field and laboratory analyses of herbicide thermal dependence. Nomenclature: Pyrithiobac; cotton, Gossypium hirsutum L.; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA.

Nitrate reductase activity during desiccation and rehydration of the desiccation-tolerant moss Tortula ruralis

Abstract Water stress reduces the activity of many plant enzymes. When the stress is alleviated there is generally a delay in the recovery of metabolism to unstressed levels. The enzyme nitrate reductase (EC 1.6.6.1) declines in plants experiencing water deficits and upon the alleviation of water stress its activity recovers to control levels in 1–7 days. In desiccation-tolerant plants, metabolic recovery can occur within hours following rehydration. It has been proposed that metabolic characteristics of desiccation-tolerant plants are a source of information that can be used to improve the performance of crop plants under water deficits. In this study, the effect of desiccation and hydration on nitrate reductase activity in the desiccation-tolerant moss Tortula ruralis has been investigated. It was expected that the activity of nitrate reductase would decline during desiccation and recover rapidly following rehydration. Inclusion of nitrate in the hydration medium increased the activity in the tissue. Nitrate reductase activity increased with nitrate concentration in the bathing medium up to levels of 200 mM. The highest activity measured in rehydrated moss samples was 5.23 nkat g−1 dry weight (dw). The activity declined rapidly during dehydration and was not detectable in dried moss samples after 24 h of dehydration. The recovery of activity following rehydration was dependant on the rate of the preceding dehydration. In slowly dried moss, the activity recovered to control levels in less than 8 h while rapidly dried samples required 24 h for full recovery. Nitrite accumulated during slow dehydration but did not accumulate when desiccation was rapid. Following rehydration of slowly dried moss, the amount of nitrite declined and reached a control level within 1 h. It was proposed that nitrite accumulation might provide a source of nitrogen for metabolism during the time required for nitrate reduction to resume following rehydration. Attempts to measure immunologically reactive nitrate reductase protein levels or nitrate reductase mRNA levels using heterologous DNA probes were unsuccessful, suggesting that the Tortula nitrate reductase may be significantly different from that found in algae and higher plants.