Dan R. Upchurch

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.

Crop water stress index relationships with crop productivity.

SummaryField experiments between 1983 and 1987 were used to study the effect of crop development on crop water stress index (CWSI) parameters and the relationship of CWSI with the yield of cotton and grain sorghum. The absolute slopes of nonstressed baselines (NSBL) generally increased until canopy cover reached 70% (Table 1). NSBL derived from data collected when canopy temperature exceeded 27.4 °C had greater absolute slopes and higher R2-values than NSBL that included all diurnal measurements (Table 1). Average CWSI values of cotton and grain sorghum grown under varying soil water regimes were negatively correlated with yield. Grain sorghum yield was more sensitive to CWSI values than was cotton lint yield (Figs. 1 and 2). Multiyear data analysis indicated that yields from cotton that experienced a completely stressed condition during part of each day during the boll setting period would be 40% of those from completely nonstressed cotton (Fig. 3). Negative values of CWSI computed for cotton growing under non-water stressed conditions were associated with uncertainties in calculations of aerodynamic resistance (raand in estimating canopy resistance at potential evapotranspiration (rcp).

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.

Accounting for humidity in canopy-temperature-controlled irrigation scheduling

Abstract High moisture content in the air surrounding crop canopies can reduce transpiration and increase canopy temperature (Tc) independently of soil moisture. Humid conditions can affect the accuracy of irrigation signals produced by a canopy-temperature-based irrigation scheduling procedure that uses a time threshold (TT), which is the daily summation of time above the temperature threshold (To) defined as the midpoint of the crops optimum temperature range. Because historical crop canopy temperature data were unavailable, an energy balance model was used to simulate time threshold values for different climates. A limiting relative humidity (LRH) algorithm was added to the model to estimate whether canopy temperatures that exceed the (To) were affected by high humidity. The LRH was computed from Ta and δT, denoted as (To) - T wb ∗ , where T wb ∗ is the highest wet bulb temperature that does not increase Tc. Time periods of restricted transpiration were identified by calculating ambient relative humidity (RH) and comparing it to the LRH value. If RH > LRH, canopy temperature was assumed to be increased by a reduction in transpiration. In a humid climate the LRH criterion reduced the simulated average TT value by 27%, 51%, and 69%, respectively, for δT values between 3°C and 5°C. This same LRH reduced the TT values by 16%, 32% and 36%, respectively, in a semiarid climate. The LRH criterion had no effect on the average TT value in the and climate. Estimated TT values had the lowest variability among years for a AT value of 4°C in the humid and semiarid climates. A generalized curve described the TT versus ΔT relationship across a wide spectrum of climates. The LRH procedure produced consistent adjustments to TT; however, further refinements may be needed to improve the accuracy of estimating daily TT when weather conditions are highly variable.

Soil temperature derived prediction of root density in cotton

Abstract Soil temperature has a significant impact on the development of plant root systems. Root growth increases with an increase in soil temperature until an optimum is reached, with decreased growth occurring as the temperature continues to rise. Soil temperature change during the season as a result of changes in agronomic factors such as row spacing and irrigation. A model was developed to test the hypothesis that the growth of the root systems of cotton seedlings growing in controlled temperature environments coupled with information on changes in soil temperature could be used to predict the development of the root systems in the field. Cotton (Gossypium hirsutum L. cv ‘Paymaster HS-26’) seedlings were grown in polyethylene growth pouches at various temperatures (10–40°C) and root development evaluated as a function of temperature. Information obtained from a field experiment where plants were grown at two irrigation levels (irrigated vs. no irrigation) and two row spacings (76 and 100 cm) was also utilized. Soil temperatures were measured at various depths and root cores were taken at the end of the experiment to determine the root length density profiles. The model was then used to determine the root development for the day the root cores were taken and compared with the actual data for each treatment. In general, the temperature driven model estimated the relative rooting density variations with depth when water was not limiting (irrigated treatment) but more closely estimated the actual values for rooting density at the various depths under dryland conditions. Root length density was also more closely estimated by the temperature model for the 100 cm row spacing compared with the 76 cm spacing (R2=0.91 and 0.71 vs. 0.91 and 0.42 for each irrigation level, respectively). The temperature modes may provide the opportunity to determine root system characteristics on a dynamic basis throughout the growing season.

Concepts in Deficit Irrigation: Defining a Basis for Effective Management

Deficit irrigation can be defined as an agricultural water management system in which less than 100% of the potential evapotranspiration can be provided by a combination of stored soil water, rainfall and irrigation, during the growing season. As water supplies decline and the cost of water increases, it is clear that producers are being driven toward deficit irrigation management. The implication of this management system is that some level of plant water stress is unavoidable. The challenge is to define a management system that will minimize the negative impact of the expected stress. Irrigation management requires choosing the timing and amount of water to be applied. Deficit irrigation management requires optimizing the timing and degree of plant stress, within the restriction of available water. This third, critical, concept greatly increases the complexity of the decision process. This presentation will incorporate a wide discussion of deficit irrigation concepts, with a focus on emerging technologies that can be applied to the detection and management of plant stress, within production environments.

Crop Water Status Control With Temperature-Time Threshold Irrigation

Center pivot and subsurface drip irrigation systems can accurately apply variable quantities of irrigation. A robust irrigation timing protocol that can function in variable environments is needed to maximize crop use of available water. Canopy temperature (TC) has been successfully used to time irrigation applications for well-watered crop growing conditions. The cumulative daily time that TC exceeds a crop specific temperature threshold, designated as stress time (ST), is used to indicate the need for irrigation. The ST value that generates the irrigation signal is the time threshold (TT). Manipulation of the TT value changes irrigation frequency and seasonal irrigation quantity. This paper describes a procedure for estimating the relationship between TT for cotton and quantity of water input. Data were analyzed from multiple studies that included a common control TT of 330 min/day above a canopy temperature threshold of 28 °C as the irrigation signal criteria for well-watered cotton. Daily ST averaged over the irrigation season was correlated with cotton yield and water input. A procedure for identifying control TT to establish different crop water status levels is described. Average TT values were 408, 468, and 528 min./day, which corresponded with control TT values of 330, 390, and 450 min/day. These control TT should result in different water application amounts during the growing season and produce different cotton yields.

Horizontal Alignment of Drip Lines Relative to Beds in SDI: Effects on Cotton Growth and Yield

A subsurface drip irrigation system with drip lines below alternating furrows was used to establish three irrigation treatments designated as HW, MW, and LW applied 1.0*PET, 0.6*PET, and 0.5*PET. By mid July a pattern of alternating rows with tall and short plants (row type) was visible. A study was initiated to quantify the variability of cotton growth and yield between adjacent rows. The position of irrigation laterals and flow rate of emitters was measured. Plant size and lint yield were measured in the two row types. The drip line moved closer to one of the adjacent beds as distance increased from the header line. Water flow was uniform among emitters along the drip lines. Plant height decrease along the row was greater for short rows rather than tall rows. Cotton yields were higher in tall rows than short rows. Short rows in all water levels had a decreasing yield trend with distance from the header line. Tall row yields increased down the row in the LW and MW water levels, but decreased in the HW water level. Difference in plant height and yield between row types was attributed to water supply differences caused by drip lines being closer to tall rather than short rows. The simultaneously decreasing trend of plant height in all water levels in both row types and HW treatment yield were likely caused by reductions in soil nutrient levels.