Dennis C. Gitz

How do UV photomorphogenic responses confer water stress tolerance

Abstract Although ultraviolet-B (UV-B) radiation is potentially harmful, it is an important component of terrestrial radiation to which plants have been exposed since invading land. Since then, plants have evolved mechanisms to avoid and repair UV radiation damage; therefore, it is not surprising that photomorphogenic responses to UV-B are often assumed to be adaptations to harmful radiation. This presupposes that the function of the observed responses is to prevent UV damage. It has been hypothesized that, as with blue light, UV-B provides a signal important for normal plant development and might be perceived within developing plants through nondestructive processes, perhaps through UV-specific signal perception mechanisms. UV signal perception can lead to photomorphogenic responses that may confer adaptive advantages under conditions associated with high-light environments, such as water stress. Plant responses to UV radiation in this regard include changes in leaf area, leaf thickness, stomatal density, photosynthetic pigment production and altered stem elongation and branching patterns. Such responses may lead to altered transpiration rates and water-use efficiencies. For example, we found that the cumulative effect of ambient UV-B radiation upon stomatal density and conductance can lead to altered water-use efficiencies. In field settings, UV might more properly be viewed as a photomorphogenic signal than as a stressor. Hence, it might be insufficient to attempt to fully evaluate the adaptive roles of plant responses to UV-B cues upon stress tolerance by the simultaneous application of UV and drought stress during development. We propose that rather than examining a plants response to combinations of stressors one might also examine how a plants response to UV induces tolerance to subsequently applied stresses.

Soil water content on drip irrigated cotton: comparison of measured and simulated values obtained with the Hydrus 2-D model

Crop irrigation with subsurface drip (SDI) is increasing in the semiarid Texas High Plains (THP). Information on drip-tubing positioning, irrigation strategies, and wetted soil area is needed to increase rainwater effectiveness when well capacities are inadequate to meet full irrigation requirements. Time and resources necessary to test SDI strategies for different conditions through field experimentation is too large. However, a mechanistic model such as Hydrus-2D can quantify the effect of different installation geometries and irrigation strategies. Our objective was to experimentally validate the Hydrus-2D in an Amarillo soil in THP so that the model can be used to evaluate different irrigation frequency and timing strategies for SDI cotton. Results showed that Hydrus-2D simulated volumetric soil water content within ±3% of measured values, and simulation bias represented the smaller portion of the simulation error, indicating that the model can be used to evaluate irrigation strategies.

Coupling short-term changes in ambient UV-B levels with induction of UV-screening compounds.

A substantial number of studies have been conducted over the last several decades to assess the potential impacts of long‐term increases in ultraviolet‐B radiation (UV‐B between 280 and 320 nm) that will result from continued depletion of stratospheric ozone. However, seasonal changes, tropospheric chemistry and cloudiness are the dominant factors controlling ambient UV‐B levels on a short‐term or daily basis. The effects of short‐term changes in UV‐B on plant growth, phytochemistry and physiological processes have received relatively little attention. The USDA UV‐B Monitoring and Research Program provides an excellent network of stations that provide an opportunity to monitor long‐term changes in solar UV‐B radiation and evaluate the responses of plants to short‐term variation in UV‐B levels on a near‐real‐time basis. In this study barley (Hordeum vulgare L.) and soybean (Glycine max [L] Merr.) were used as model systems. Emerging seedlings of these species were grown under either near‐ambient levels of UV‐B or under reduced levels (ca 90% reduction) in the field. Periodic measurements of foliar UV‐screening compounds were made on separate groups of seedlings planted at intervals over the growing season during contrasting periods of ambient levels of UV radiation. The levels of UV‐screening compounds correlated with UV‐B levels in both species and with UV‐A in soybean but the sensitivity of the response differed between the two species and among the soybean cultivars. Response differences among species may be related to unique secondary chemistry of each species, so one response estimate or action spectrum may not be appropriate for all species.

Comparison of deficit irrigation scheduling methods that use canopy temperature measurements

Abstract Canopy temperature (Tc) provides an easy-to-acquire indication of crop water deficit that has been used in irrigation scheduling systems, but interpretation of this measurement has proven difficult. We compared the timing of irrigation application of the Stress Time (ST) method of irrigation scheduling with the Stress Degree Hours (SDH) method on deficit irrigated cotton (Gossypium hirsutum L.) where each irrigation event delivered 5 mm of water through subsurface drip tape. A well-watered (WW) control and a dry land (DL) treatment were also part of the experimental design. We used data collected from the WW and DL treatments to develop upper and lower baselines for the Crop Water Stress Index (CWSI) appropriate for cotton grown at our location. The ST method detected drought stress earlier in the growing season when both the SDH and CWSI indicated very little drought stress. The SDH method resulted in the application of irrigations relatively later in the growing season when the CWSI also detected higher levels of drought stress. These results suggest that the adding certain micrometeorological variables to simple Tc methods of deficit irrigation scheduling may improve the ability to detect and quantify the degree of crop drought stress.

Effects of Changes in Irrigation and Land Use on Streamflow in the Revuelto Creek Watershed, a Tributary of the Canadian River in New Mexico, USA

Extensive development in the Canadian River watershed in New Mexico and Texas occurred in the 20 th century to supply water for irrigation, and municipal and industrial uses. In recent years (2000-2009), these infrastructures have not been able to supply sufficient water to meet demands. The objective of this study was to assess the effects of changes in irrigation allotments and land use to the Arch Hurley Conservancy District on streamflow out of Revuelto Creek, a tributary of the Canadian River in eastern New Mexico, USA. Outflow from Revuelto Creek varied considerably from 1961 to 2009. The best predictor of annual outflow was a two-component linear model with terms for annual precipitation and the annual irrigation allotment to the Arch Hurley Conservancy District. ArcSWAT (GIS extension of the Soil Water Assessment Tool) simulated outflow from Revuelto Creek best when historical values for irrigation allotments were used in the management operations for cropland. These results indicate that annual irrigation allotments of approximately 123,000,000 m 3 increased outflow from the creek by approximately 25,000,000 m 3 . Years in which the irrigation allot- ments to the district exceeded 100,000,000 m 3 corresponded to years in which water was released from Ute Lake. These results suggest that managers of Lake Meredith need to take into account water availability upstream of Ute Dam when devising plans for Lake Meredith.

Assessment of DNA Damage as a Tool to Measure UV-B Tolerance in Soybean Lines Differing in Foliar Flavonoid Composition

Damage to DNA, in the form of cyclobutane pyrimidine dimers (CPD), may occur in soybean (Glycine max (L.) Merr) plants when they are exposed to increasing levels of ultraviolet-B (UV-B) radiation. Flavonoids and other phenolics accumulate in the epidermal layer of leaves and may provide protection for sensitive tissues including DNA molecules. We evaluated the steady state levels of accumulated damage and the protection afforded by flavonoids in two soybean isolines: Clark producing high levels of flavonoids, and Clark-magenta producing extremely low flavonoid levels. Both cultivars were grown in the field under ambient and supplemental UV-B radiation. Leaf tissue was harvested in a diurnal sequence, and the samples were analyzed. Two methods of analysis were used in order to develop a common reference point between the two. In one method, DNA was isolated and treated with UV endonuclease, and the DNA fragments were separated using unidirectional pulsed field electrophoresis and quantified through electronic imaging. In the alternate method, a western blotting procedure, immobilized DNA was reacted with monoclonal antibodies specific to CPD DNA damage. Results were similar in both techniques and show lesion frequency to be low in both isolines. However significant differences were found between cultivars, UV treatments, time of day collected, and levels of PAR. The average level of dimers per megabase for the isoline Clark was ∼4 (with or without supplemental UV), and for Clark-magenta, ∼4 for samples with no supplemental UV and ∼6 for those exposed to supplemental UV radiation. Diurnally, dimer levels were frequently higher in the Clark-magenta isoline, especially when exposed to supplemental UV-B. Both isolines appear to be either well-protected from DNA damage, or repair is efficient enough to minimize biologically significant accumulation of DNA damage. This suggests that protection mechanisms, other than flavonoids alone, contribute to maintenance of DNA integrity in soybean.

Short-term responses of barley to changes in ambient levels of UV-B radiation and their role in UV protection

While many studies have evaluated the chronic effects of exposure to enhanced levels of UV-B radiation on plants very few studies have evaluated the implications of plant development within a background of fluctuating levels of UV-B radiation. Much interest and concern surround the issue of stratospheric ozone depletion and concurrent increases in UV-B radiation and this remains a concern. However, variation in UV-B levels on a daily basis is largely due to cloud cover and tropospheric air quality as well as possible effects of fluctuations in the total ozone column. Therefore the importance of the effects of short-term changes in UV-B radiation is not predicated on the assumption of continued ozone destruction. In this study we evaluated to change in foliar phenolic composition in barley and the consequences of changes in these putative protection compounds on subsequent sensitivity to UV-V radiation. The UV-B exposure levels ranges from less than 1 to nearly 8 kJ m-2 of biologically weighted UV-B radiation. Barley plants that developed under height ambient levels of UV-B radiation had higher levels of phenolics than control plants grown under the same conditions except with UV-B excluded. Those plants with higher phenolic content show some degree of increased protection from subsequent levels of UV-B as evidenced by less damage to DNA. However, it was also found that other environmental factors contributed to the induction of foliar screening compounds.

Characterization of trends in reservoir storage, streamflow, and precipitation in the Canadian River watershed in New Mexico and Texas

Abstract Dams and reservoirs were created on the Canadian River in New Mexico and Texas between 1918 and 1984 to supply water for irrigation and industrial and municipal uses; however, there are indications that the storage in the 4 major reservoirs is insufficient for current demand. This study was conducted to document changes in reservoir storage and streamflow throughout the watershed and to investigate the hypothesis that decreases in reservoir storage since 1990 were associated with changes in rainfall. Time-series analyses indicated that the reservoir storage values for all 4 major impoundments (Eagle Nest Lake, Conchas Lake, Ute Lake, and Lake Meredith) on the Canadian River in New Mexico and Texas during the decade of 2000–2009 were less than the historical means. Streamflows at all 4 USGS gauges on the Canadian River in New Mexico and Texas from 2000 to 2009 were less than the long-term mean flows. Mean annual precipitation from 36 weather stations either in or adjoining the watershed tended to be lower from 2001 to 2010 than the period from 1971 to 2000. The frequency of rainfall events in excess of 50 mm from a weighing rain gauge at Bushland, Texas, from 2000 to 2009 was different from that reported from 1960 to 1979. ArcSWAT, a hydrological model, was able to simulate the observed declines in storage in Lake Meredith from 1990 to 2009 using historical rainfall data. These results support the hypothesis that the decreases in storage in Lake Meredith were associated with changes in rainfall.