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Thesis (M.Sc. (Botany))-- University of Limpopo, 2013 / The aim of the study was to identify drought tolerant South African soybean cultivars for cultivation where water is a limited resource. Soybean [Glycine max. (L.) Merr] is one of the most important legumes in the world. A lot of attention has been focused on soybean cultivation in South Africa recently. Soybean production is mainly affected by several biotic and abiotic factors which reduce the yield and quality of the crop. Six South African soybean cultivars (LS 677, LS 678, Mopanie, Sonop, Knap and Pan 1564) and two American cultivars (R01 416 and R01 581) were carefully studied for morphological and physiological markers which contribute to drought tolerance. The study was conducted at the University of Limpopo (Turfloop campus). Soybean plants were grown in a glasshouse in a randomised block design given same amounts of nutrients and differing amounts of water (limited and overwatering). Data was collected at R3 growth stage by measuring several morphological (stem length, leaf surface area, flowers and seeds counts) and physiological (percentage chlorophyll, moisture content, total phenolics, total flavonoids, ureide content and antioxidant activity) parameters. An anatomical study was also carried out on the transverse sections of leaves, roots, leaf stalk and nodules. The different cultivars reacted differently to the three water treatments. LS 678 produced the tallest plants whereas those of Pan 1564 were the shortest. Water stress affected plants by reducing the number of flowers produced, the leaf surface area as well as the relative leaf water content. The moisture content of the growth medium was reduced faster as the plants matured and it was also lowered by the limited water availability. Percentage chlorophyll is another trait which was affected by water limitation. Cultivars with high phenolic and flavonoids content were associated with high antioxidant activity and slightly yielded higher than the others. The anatomical transverse sections of the roots and petioles have shown some secondary growth. The anatomy of the nodules of Mopani has shown some interesting differences in response to the three treatments. Limited water decreased xii the size of the vascular tissue and sclerenchyma as a result altering the functionality of the nodule. The anatomy of Sonop’s petiole had a thickened sclerenchymatous bundle sheath covering the phloem tissue. The sclerenchyma tissue is thought to guard against loss of water. The cross section of the leaf had a double layer of palisade mesophyll (upper surface) and only a single layer of spongy mesophyll (lower surface). In addition, the mesophyll and the epidermal cells of Mopani appeared much thicker. In terms of yield, there was no cultivar which yielded the highest but Mopani yielded the lowest. Since Mopani was low yielding, the main focus of the discussion was on the features (morphological, physiological and anatomical) of Mopani which can be associated with drought susceptibility. Some of these features include reduced stem length, large leaf surface area, low relative leaf water content, low growth medium moisture content and low antioxidant activity.

Drought is a major limiting abiotic stress influencing wheat production in many parts of the world. The erratic nature of water deficits makes breeding and selection for drought resistance deficient. In environments with late season drought stress, yield losses are usually associated with kernel abortion or reduction in kernel growth. Remobilization of soluble carbohydrates from the stem has been associated with drought resistance. The objectives of this investigation were i) to asses the role of soluble carbohydrates in the determination of drought resistance, ii) their association with productivity, and iii) to evaluate a rapid technique to identify genotypes with higher capacity of soluble carbohydrate remobilization. Nine winter wheat cultivars differing in their response to drought stress were grown under irrigated and nonirrigated conditions during the grain filling period at the Sherman Branch Experiment Station, Moro. The cultivars were also grown at the Hyslop Crop Science Laboratory, where plots were chemically desiccated with Sodium Chlorate or left untreated. All control and treated plots were evaluated for soluble carbohydrates in two vegetative fractions, stem plus sheath and leaf blades. These values were correlated with the relative reductions in kernel weight and yield observed on the treated plots. Differences among cultivars were observed for the concentration of soluble carbohydrates in the stem and leaf fractions. Time elapsed from anthesis was a major determinant of the variation in carbohydrates concentration observed during grain filling. Stem soluble carbohydrates accumulated to a much greater extent than leaf soluble carbohydrates. The concentration of stem carbohydrates was not related with the reductions caused by chemical desiccation or drought stress. However, potential contributions from stem reserves (ratio between potential spike weight and stem reserves) were marginally associated with drought resistance. Stem soluble carbohydrates were positively associated with productivity, suggesting that stems are not competitive sinks, but temporary storage organs of excess of assimilates. Under chemical desiccation most of the soluble carbohydrates were lost in respiration, and the reductions in kernel weight and yield observed were not correlated with those observed under drought. It was concluded that the technique did not reasonably simulated drought in terms of yield reductions nor carbohydrate remobilization. / Graduation date: 1998

Since Texas normally receives most of its precipitation in the warm season, precipitation deficits in summertime may bring serious agricultural and hydrological disasters. While the underlying physical processes of summer precipitation deficit and drought are unclear, they can be understood in terms of convective instability. This research is designed to investigate how convective instability influences monthly mean precipitation in Texas in the summertime and to examine the modulation of convective instability and precipitation by upper-level circulations, soil moisture, vertical motion, and low-tropospheric warm air transport using NCEP/NCAR reanalysis data. Statistical approaches including correlation analysis, multiple linear regression analysis and back trajectory analysis were used to reveal the underlying dynamics of their linkage and causality. The results show that warming at 700 mb and surface dryness result in excessive convective inhibition (CIN), leading to precipitation deficits on a monthly time-scale. Temperature at 700 mb (Tlt) and surface dewpoint have little correlation suggesting different processes contribute to warming at 700 mb and surface dryness, respectively. Correlation analysis among the surface variables emphasizes the role of soil moisture on the dewpoint and thermodynamics at the surface. Back trajectory analysis indicates that a significant contributor to warming at 700 mb is the inversion caused by warm air transport from the Rocky Mountains and the Mexican Plateau where the surface potential temperature is greater than 307.5K rather than by subsidence. It was found that downward motion and warm air transport are enhanced in Texas when upper-level anticyclonic circulation develops in the southern US. Upper-level anticyclonic circulations in the southern US strongly affect Texas summertime precipitation by modulating the principal processes as follows. They increase CIN not only by enhancing warm air transport from the high terrain but also by suppressing occurrence of disturbances. The resulting reduced precipitation and dry soil significantly modulate surface conditions, which elevates CIN and decreases precipitation. The aforementioned chain-reaction of upper-level anticyclone influences can be understood in the context of CIN.