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This study investigated the effect of water potential on soil microbial C and N pools. Two soil types were treated with additions of salt solution to establish osmotic water potentials, and by ceramic plate - pressure chamber apparatus to establish matric water potentials. Soils were then subjected to short-term incubations. Soil microbial C and N contents (BC and BN) were measured mainly by the fumigation-extraction and fumigation-incubation methods. Results showed that both Microbial C and N pools were markedly affected by soil water potential. The soil microbial C content always showed an increase with increasing water stress and then a decrease beyond a threshold value of water stress, compared to the microbial C content at a control water potential of -0.03 MPa (-0.3 Bar). This response pattern to water stress was true, not only for osmotic stress, but also for matric stress, and regardless of the osmotic agent employed. The response pattern of the microbial N pool to water stress generally contrasted with that of the C pool, and depended on the osmotic strength of the extraction solution (K₂SO₄) used in the determination. Non-isotonic extraction after fumigation resulted in a decrease in microbial N content with increasing water stress, while isotonic extraction resulted in an increase with increasing water stress, beyond a threshold value of water stress. Soil microbial C/N ratio always increased with increasing water stress. Matric water stress had a more marked effect on BC and BN than osmotic stress. The possible reasons for the response patterns of BC, BN and microbial C/N ratio have been discussed in this thesis. Some suggestions on the methodology of microbial biomass measurement for water stressed soil samples have been made, and mainly relate to the biomass fumigation techniques and possible changes in the Kc, Kce and Kn values under water stress, and to the substrate induced respiration (SIR) method and suppressed respiration under water stress.

3 pp. / Determining the Amount of Irrigation Water Applied to a Field (AZ1157) / Critical to any irrigation management approach is an accurate estimate of the amount of water applied to a field. Too little water causes unnecessary water stress and can result in yield reductions. Too much water can cause water logging, leaching, and may also result in loss of yield. This publication discusses how to set the water amount and the time period, when taking the system's efficiency into consideration.

3 pp. / Critical to any irrigation management approach is an accurate estimate of the amount of water applied to a field. Too little water causes unnecessary water stress and can result in yield reductions. Too much water can cause water logging, leaching, and may also result in loss of yield. This publication discusses how to set the water amount and the time period, when taking the system's efficiency into consideration.

Revised; Originally Published: 2006 / 3 pp.

The ever-growing world population raises the concern and necessity of rational use and distribution of limited water resources. Water deficit is the single most dominant abiotic factor limiting cotton (Gossypium hirsutum L.) yield in drought-prone Texas croplands. Characterizing plant traits conferring drought tolerance to cotton genotypes and then transferring this information back to breeders and geneticists have the potential of significantly increasing and stabilizing production statewide. Although a plethora of physiological studies have been conducted and have demonstrated that drought tolerance in plants is likely to be conferred by a combination of plant traits rather than a single trait, this knowledge has not translated into improved breeding lines. Experiments were conducted in 2010 and 2011 in the Drought Tolerance Laboratory (Texas AgriLife Research and Extension Center in Corpus Christi, TX) to analyze the responses of cotton genotypes to different levels of water stress. This facility is equipped with computerized systems capable of continuously monitoring whole-plant water use as well as several environmental parameters. Sixteen cotton genotypes were provided by Monsanto Co. and the Texas AgriLife Cotton Improvement Programs at College Station and Lubbock. Seeds were pre-germinated in wet paper towels and then hand planted in large pots previously filled with fritted clay. A total of 3 and 8 (2010 and 2011, respectively) pots containing plants of each genotype were permanently placed on micro-lysimeters for continuous measurement of water use. Water regimes were imposed in 2010 (well-watered and water-stressed), and 2011 (water-stressed) when plants reached the early-flowering stage and were carried until plants reached maturity (100% open bolls). Data collected showed that genotypes have very distinct water use patterns. The water stress treatment imposed on the test plants negatively affected plant growth that was indicated by a lower plant height, total number of leaves, and main-stem nodes of stressed plants when contrasted to their well-watered counterparts. Stomatal density was remarkably different among genotypes and a higher density was found on the abaxial (lower) leaf surface for all genotypes studied. Root dry mass production had different responses depending upon the severity of the water stress. Highest root dry mass was observed when plants were exposed to a mild stress and lowest when a more severe water restriction was imposed.

Spider mites (Acari: Tetranychidae) are an important agricultural pest of many crops and landscape plants. They can reproduce rapidly and quickly develop resistance to many pesticides, making them difficult to manage. Plant water-stress and high temperatures promote spider mite infestations, while spider mite outbreaks can also result from neonicotinoid insecticide applications. Drought is predicted to increase in the Intermountain West due to increases in temperature and reduced frequency of precipitation events in the region, and neonicotinoids are currently one of the most widely used classes of insecticides in field crops. I studied the interactive effect of these two simultaneously occurring abiotic factors on spider mite outbreaks and plant biosynthesis of herbivore-related defense proteins. I also evaluated ways to alleviate spider mite outbreaks with drought-tolerant corn and the exogenous application of plant phytohormones involved in plant resistance toward biotic stressors. I found that plant water-stress increased spider mites and that neonicotinoids exacerbated the effect of water-stress. Although applications of plant hormones did not reduce the effect of water-stress and neonicotinoids, drought tolerant corn showed promise in reducing the effect of water-stress and spider mite outbreaks.

The aim of this research was to study the influence of altered water availability on stomatal patterns of leaves. A baseline study of leaves stomatal indices, (stomatal as a proportional of all epidermal cells) stomatal density, (number of stomatal per unit area of epidermis) epidermal cell density, epidermal cell length and stomatal length and width was made in wheat, and sorghum cultivars; Saudi Red, Funk, Indian White, Morgon 854, Morgon 856 and Pioneer 8ss in relation to water limitation. Seedlings of wheat and sorghum cultivars grown under water limitation showed increases in their stomatal density, epidermal cell density and trichome index in response to reduced watering frequency. Stomatal index of the two species, however, were not altered in the same way. In sorghum there was no apparent change in stomatal index in relation to water limitation treatment but stomatal density and trichome index increased. In wheat there was a marked reduction in the stomatal index in response to water limitation but there were increases in trichome indices and stomatal densities of all parts of the leaves. Wheat plants grown under water limitation imposed by polyethylene glycol (PEG) treatments to roots showed marked reductions in their growth compared with controls. Stomatal density was found to increase in response to PEG treatments while stomatal index decreased. Incubation of newly-germinated wheat seedlings in PEG resulted in an inhibition of growth of coleoptiles. However, the density of stomata decreased, unlike what was seen in leaf tissues. When wheat coleoptiles were subjected to conditions of limited water availability guard mother cell (GMC) production appeared not to be influenced by water status of the tissues. Wheat coleoptiles grown under different periods of light showed slight decreases in stomata density compared to the controls. Abscisic acid treatment was able to substitute for water limitation treatments by reducing stomatal index in both leaves and coleoptiles whilst increasing the production of trichomes in leaves. However, in coleoptile the same treatment reduced both the GMC index as well as stomata index. Salinity treatments, inhibited seed germination and were found to decrease stomatal index. Incubation of wheat seedlings in PEG resulted in a yellow pigmentation of the roots. The pigment was tentatively identified. However, it was not possible to distinguish whether the pigment was a specific metabolite of PEG in the roots or was produced by a non-biological conversion of PEG.