This study examined the effects of elevated CO2 and its interaction with drought and nitrogen fertilizer on the growth, production and nodulation of the leguminous crop lentil (Lens culinaris Medic) cultivars ILL7979 and ILL6994 (Idlib 3). Plants were grown under ambient and elevated CO2 at full and limited irrigation conditions in both open top chambers, which were later proven to be unreliable because of CO2 leakage, and tightly sealed and ventilated chambers which were reliable. Destructive harvests at anthesis and at maturity were conducted and results from sealed chambers at maturity showed that above ground dry weight was increased by an average of 12% under elevated CO2, but this increase was not statistically significant.

Terrestrial ecosystems have continued to provide the critical service of slowing the atmospheric CO2 growth rate. Terrestrial net primary productivity (NPP) is thought to be a major contributing factor to this trend. Yet our ability to estimate NPP at the regional scale remains limited due to large uncertainties in the response of NPP to multiple interacting climate factors and uncertainties in the driver data sets needed to estimate NPP. In this study, we introduced an improved NPP algorithm that used local driver data sets and parameters in China. We found that bias decreased by 30% for gross primary production (GPP) and 17% for NPP compared with the widely used global GPP and NPP products, respectively. From 2000 to 2012, a pixel-level analysis of our improved NPP for the region of China showed an overall decreasing NPP trend of 4.65TgCa(-1). Reductions in NPP were largest for the southern forests of China (-5.38TgCa(-1)), whereas minor increases in NPP were found for North China (0.65TgCa(-1)). Surprisingly, reductions in NPP were largely due to decreases in solar radiation (82%), rather than the more commonly expected effects of drought (18%). This was because for southern China, the interannual variability of NPP was more sensitive to solar radiation (R-2 in 0.29-0.59) relative to precipitation (R-2<0.13). These findings update our previous knowledge of carbon uptake responses to climate change in terrestrial ecosystems of China and highlight the importance of shortwave radiation in driving vegetation productivity for the region, especially for tropical forests.

A juniper (Juniperus przewalskii Kom) tree-ring width chronology has been developed from the western-most forest of the Qilian Mountains. Our analyses demonstrate both temperature and precipitation have significant effects on tree growth and that both should be considered in climate reconstruction. Thus a regional drought history (A.D. 1855–2001) is reconstructed by calibrating with a linear interpolation through four Palmer Drought Severity Index (PDSI) grid values nearest the sampling site. Our reconstruction extends the drought history of this area and also reveals that the most severe drought occurred in the 1920s. In the context of the drought history of western China, this extreme drought between 1925–1931 is consistent over a large surrounding region of Northwestern China. Multi-taper spectral analysis reveals the existence of significant 40- to 46-year, 29-year, and 2.1- to 3-year periods of variability. Overall, our study provides reliable information for the research of past drought variability in the Qilian Mountains, Northwestern China.

Drought indices based on monthly precipitation and sometimes temperature are widely used due to their simple calculation with readily available climate data. The portrayal of drought through simple precipitation anomalies or water balances when accounting for temperature may not capture the potentially complex evolution of drought events due to the timing, intensity, and frequency of precipitation events at the daily scale. In this study, we present a new drought index that incorporates a deterministic soil model, HYDRUS-1D, and daily climate data to assess how representative simple drought indices are of soil moisture status in the Southwest. Specifically, we compare our drought index with two widely used drought indices: the Standardized Precipitation Index (SPI) and the Standardized Perception-Evapotranspiration Index (SPEI). Modeled soil moisture output was summed into monthly values for direct comparison between indices. SPI and SPEI proved to be representative of soil moisture status at shallow depths, correlating best at a two-month window. SPI correlated higher with our modeled drought index than SPEI in shallow settings across all study sites. Intense drought events were controlled by the magnitude and frequency of precipitation, with large events creating water surplus and then a slow decay in soil moisture until the next large event. Furthermore, heat map correlations indicate that monitoring drought at depth is dependent upon the previous years monsoon, with the best correlating window growing with distance from monsoon onset. Modeled soil moisture showed volumetric water content increased during monsoon season and remained high through the fall and into the winter months. Higher moisture content increased hydraulic conductivity, priming the soil profile for winter recharge. We believe that the addition of a soil physics based drought index greatly improves drought monitoring conditions for the southwest.

Master of Science / Department of Agronomy / Xiaomao Lin / The high-quality climate data and high-resolution soil property data in Kansas and adjacent states were used to develop drought datasets for the monthly Palmer Drought Severity Index (PDSI), Standardized Precipitation Index (SPI), and the Standardized Precipitation-Evapotranspiration Index (SPEI) over 1900 to 2014. The statistical analysis of these multiple drought indices were conducted to assess drought occurrence, duration, severity, intensity, and return period. Results indicated that the PDSI exhibited a higher frequency for every category of drought in central and western Kansas than the SPEI by up to 10%. Severe and extreme drought frequency was the highest in southwest Kansas around the Arkansas River lowlands and lowest in the southeast. The mean total drought frequency for eastern, central, and western Kansas was 36%, 39%, and 44%, respectively. The regional mean correlations between the SPI and SPEI were greater than or equal to 0.95 for all regions, but due to statistically significant increases in potential evaporation in western Kansas, the PDSI and SPEI are recommended over the SPI for meteorological and hydrological drought analysis. Drought variability of the last 115 years was analyzed through the Empirical Orthogonal Functions (EOFs) techniques and their Varimax rotations from 1900 to 2014 in Kansas. Large-scale synoptic patterns primarily dominated the Kansas spatial drought structures, especially during long-duration events. The EOFs indicated that the first principal components of drought explained approximately 70% of the drought variability across the state and demonstrated a statistically significant wetting trend over the last 115 years, oscillating at a period of about 14 years for all drought indices. The 99° W meridian line acted as the dominant transitional line demarcating the areas of Kansas’ climate and vegetation relationship as spatial drought presented. The Multivariate El Nino Index (MEI) signal , which modulates global and regional climate variabilities, provided a low-frequency indicator to couple with Kansas drought’s leading modes by varying leads of 3 to 7 months depending on the use of drought index and time steps selected. Large-scale predictors of surface temperature and precipitation are evaluated from the monthly forecasts in Climate Forecast System version 2.0 (CFSv2) from North Dakota down through central Texas (32.6 - 47.7°N and 92.8 - 104.1°W). By using singular value decomposition (SVD), the CFSv2 monthly forecasts of precipitation and 2-m temperature were statistically downscaled using ensemble mean predictions of reforecasts from 1982-2010. Precipitation skill was considerably less than temperature, and the highest skill occurred during the wintertime for 1-month lead time. Only the central and northern plains had statistically significant correlations between observed and modeled precipitation for 1-month lead time. Beyond a 1-month lead time, prediction skill was regionally and seasonally dependent. For the 3-month lead time, only the central plains demonstrated statistically significant mean anomaly correlation. After three-month lead times, the ensemble means of forecasts have shown limited reliable predictions which could make the forecast skill too low to be useful in practice for precipitation. However, temperature forecasts at lead times greater than five months showed some skill in predicting wintertime temperatures.

Thesis submitted to the Faculty of Science, University of the Witwatersrand, in partial fulfillment of the requirements for the degree PhD Science Johannesburg, September 2014 / Drought has the potential to reset trophic interactions within natural and managed ecosystems. I tested how drought stress in maize and companion plants that are used in cereal agroecosystems of Kenya affect oviposition preference, larval feeding, and development of the spotted stemborer, Chilo partellus Swinhoe (Lepidoptera: Crambidae). Five host species were tested (all Poaceae): maize (Zea mays L.), Napier grass (Pennisetum purpureum Schumach), signal grass [Brachiaria brizantha (A. Rich) Stapf], Brachiaria cv ‘Mulato’, and molasses grass [Melinis minutiflora (Beauv.)]. Under periods of water deficit, maize was oviposited on as much as under control unstressed maize in both choice and no-choice experiments. Similarly, larval leaf damage was not significantly different in drought-stressed and unstressed maize. However in Napier and signal grasses, oviposition occurred less on drought-stressed than on unstressed corresponding individuals of the same species. Oviposition acceptance and foliar damage remained low in both drought-stressed and unstressed molasses grass and Mulato. Larval survival and development remained high in drought-stressed maize, but not in Napier, signal, and molasses grass and Mulato, where survival and development were low in both drought-stressed and unstressed plants. Drought stress resulted in increased total plant volatile emission in Napier and signal grasses. This significant change in total volatile emission of the plants upon stress was not present in molasses grass, Mulato II, Silverleaf and Greenleaf Desmodium despite variable changes in the quantities and qualities of particular constituent compounds among all the plant species during water deficit. In wind tunnel bioassays, volatile organic compounds (VOCs) from irrigated Napier and signal grasses were more attractive to C. partellus than the treatment plants. However, volatiles from molasses grass, Mulato II, Silverleaf and Greenleaf Desmodium were never preferred by the gravid moths in both irrigated and drought stressed v conditions. These changes in chemical constituents of the plant VOCs also elicited behavioural responses in parasitoids. Drought-stressed Napier grass attracted gravid Cotesia sesamiae parasitoids. Other companion plant species such as molasses grass and Mulato which constitutively emit these terpenes remained attractive to the parasitoids even under periods of drought. I attribute this differential preference of the moths to the constitutive and/or induced emission of key terpenoid compounds in plants that were tested. The induction of terpenoid compounds was coupled with a reduction in the amount of green leaf volatiles such as (Z)-3-hexenyl acetate in Napier and signal grasses. Taken together, the results suggest that control of stemborer pests using stimulo-deterrent diversion tactics remains possible under periods of environmental change.

Seasonality in the Amazon Rainforest is predicted to become more extreme, with dry seasons increasing in length and severity, while severe episodic droughts are expected to occur with greater frequency. Drought stress can reduce the capacity of the rainforest to sequester carbon, and severe drought events can switch the region from being a net sink to a temporary source of carbon to the atmosphere. A key component in the drought-induced carbon flux is tree mortality, and there is evidence of strong feedbacks globally and regionally in the Amazon with climate change. Although the exact cause of drought-induced mortality in trees is difficult to ascertain, recent data suggests that reduced functionality of the water transport pathway (hydraulic failure) is an important factor. Hydraulic vulnerability in trees is often assessed using measurements of the capacity of stems and branches to cope with the strongly negative internal water pressures associated with drought. However, leaves play a vital role in protecting the integrity of the ‘upstream’ hydraulic pathway by influencing the rate of transpiration and thus the tension in the water column. Therefore, the physiology of leaves can be informative of, and influence, tree species’ sensitivity to drought. This thesis uses a long-term large-scale rainfall exclusion experiment in the Eastern Amazon to examine the possible link between leaf physiology and drought sensitivity (or tolerance) by different taxa, and the capacity of mature, upper canopy Amazonian trees to respond to drought via plastic changes in leaf physiology. The plasticity in response to experimental drought and the differences between taxa classed as drought-sensitive and drought-resistant based on drought induced mortality records were tested by the study of leaf water relations (Chapter 2), leaf anatomy (Chapter 3) and foliar water uptake (Chapter 4). No consistent differences were found between drought-resistant and drought-sensitive species suggesting that the sensitivity of these species to drought may be due to other aspects of plant physiology. However, a limited response to the imposed drought conditions was detected across all taxa and included reductions of osmotic potential at full turgor and turgor loss point (Chapter 2), and increases in the thickness of the upper epidermis and the leaf internal cavity volume (Chapter 3). Interestingly, drought-sensitive taxa showed more seasonal osmotic adjustment than drought-resistant taxa, indicating that short-term responses to drought (e.g. season) are not representative of the capacity for adjustment in response to long-term water deficits. No significant changes occurred in leaf size, thickness, stomatal and vein density, the quantity of the inner leaf tissues (i.e. the palisade and spongy mesophyll) and mesophyll cell size, in response to the experimental drought. The experiments on foliar water uptake in Chapter 4 revealed that this rarely-considered process occurs in all taxa, but the response to the drought treatment differed among taxa. Using a simple model, foliar water uptake was scaled up to canopy level. Under normal conditions (i.e. no drought) canopy foliar uptake was calculated to be 29.9 ± 2.3 mm year-1 from rainfall alone, but this increased to a maximum of 51.9 ± 2.3 mm year-1 when including the input of dew in the dry season. However, lower water potential in the drought plot causing increased rates of foliar water uptake, led to estimates of 38.7 ± 3.0 mm year-1 (rainfall only) and 68.9 ± 2.9 mm year-1 (including dry season dew). Taken together, these results demonstrate that Amazonian trees show some limited capacity for acclimation to drought through the changes in leaf physiology measured in this thesis. Low turgor loss point is associated with dry climate-adapted plants, so the finding that this parameter reduced in response to the drought reveals some potential for Amazonian trees to acclimate with the predicted changes in moisture availability. However, the limited response of leaf anatomy to long-term drought might suggest that acclimation may only occur within a narrow range. The finding that six common Amazonian tree genera can take water up through their leaves has considerable implications for understanding the Amazon water budget, in terms of the contribution of dew and light rainfall to canopy water status, but also the implications it has for the hydraulic vulnerability of trees in rainforests right across the Amazon basin.

Salinity is a major abiotic stress in plant agriculture which reduces seed germination, vegetative growth, and flowering, and limits crop productivity world-wide. Salinity causes water deficit, ion toxicity, and nutrient deficiency in plants, which can result in cellular damage, growth reduction, and even death. Kentucky bluegrass (Poa pratensis L.) is the most widely used cool-season species in cool-arid climates; however it has relatively poor salt-tolerance. Thus the development of Kentucky bluegrass genotypes with increased salt tolerance is of interest to turf breeders. One impediment to selection towards this goal is finding an efficient and accurate method to evaluate the salt tolerance. The objective of this study was to examine physiological responses to salt stress and to evaluate the genetic diversity among the accessions used in the research. Salt-tolerant accessions PI371768 (768) and PI440603 (603) and salt-sensitive varieties Midnight and Baron were exposed to four levels of salinity imposed by irrigating with salt solutions of 0 dS m-1 (control), 6 dS m-1, 12 dS m-1, and 18 dS m-1 or 24 dS m-1. Soil salinity was measured using Acclima Digital TDT sensors and grass response to the stress was measured using turf quality ratings, stomatal conductance, leaf water potential and electrolyte leakage. In general, turfgrass quality, stomatal conductance, and leaf water potential decreased while electrolyte leakage increased under salinity stress. Midnight and Baron exhibited greater changes in these measurements, indicating more sensitivity compared to 768 and 603. The 6 dS m-1 treatment had little effect on the salt-tolerant accessions. Salt tolerance of 603 and 768 was confirmed and likewise, salt sensitivity of Baron and Midnight was confirmed. The genetic similarity of all cultivars used in this study was very high. All of the evaluation measurements were highly correlated, with water potential and electrolyte leakage being the most reliable and accurate methods due to the low standard deviations. Due to more repeatable methods and less user error, electrolyte leakage and turfgrass quality are recommended methods for screening salt tolerance of turfgrasses.

Cultivated chickpea (Cicer arietinum) has a narrow genetic base making it difficult for breeders to produce new elite cultivars with durable resistance to major biotic and abiotic stresses. As an alternative to genome mapping, microarrays have recently been applied in crop species to identify and assess the function of putative genes thought to be involved in plant abiotic stress and defence responses. In the present study, a cDNA microarray approach was taken in order to determine if the transcription of genes, from a set of previously identified putative stress-responsive genes from chickpea and its close relative Lathyrus sativus, were altered in chickpea by the three abiotic stresses; drought, cold and high-salinity. For this, chickpea genotypes known to be tolerant and susceptible to each abiotic stress were challenged and gene expression in the leaf, root and/or flower tissues was studied. The transcripts that were differentially expressed among stressed an d unstressed plants in response to the particular stress were analysed in the context of tolerant/susceptible genotypes. The transcriptional change of more than two fold was observed for 109, 210 and 386 genes after drought, cold and high-salinity treatments, respectively. Among these, two, 15 and 30 genes were consensually differentially expressed (DE) between tolerant and susceptible genotypes studied for drought, cold and high-salinity, respectively. The genes that were DE in tolerant and susceptible genotypes under abiotic stresses code for various functional and regulatory proteins. Significant differences in stress responses were observed within and between tolerant and susceptible genotypes highlighting the multiple gene control and complexity of abiotic stress response mechanism in chickpea. The annotation of these genes suggests that they may have a role in abiotic stress response and are potential candidates for tolerance/susceptibility.

Thesis (M.A. (Psychology))--North-West University, Potchefstroom Campus, 2008.