Bibliography: leaves 134-145. The improvement of drought tolerance of commercial soybean varieties via indirect selection for transpiration efficiency (TE) in breeding programs was investigated. The extent and nature of variation for TE among soybean genotypes were established through glasshouse experiments under well watered conditions, and confirmed in the field under contrasting water stress conditions. The results suggest that increasing TE will be a beneficial strategy to improve soybean grain yield at the crop level, and a protocol developed suited to indirect selection for high TE soybean genotypes under a range of environments. This will have immediate application in the development of soybean varieties specifically adapted to the dryland production areas of the Australian sub-tropics.

Plants have evolved numerous strategies for the adaptation to drought. Although many investigations reported on the potential value of proline accumulation during environmental stress, it is still unknown whether or not a constitutive higher level of proline accumulation enhances plant tolerance. Thus, it was investigated if underproduction and overproduction of proline will influence the susceptibility to drought stress in soybean plants. This was made possible with the transformation of soybean plants with an L-Δ¹-pyrroline-5-carboxylate reductase (P5CR) gene. First, an Agrobacterium-mediated vacuum infiltration transformation system, using partially germinating Carnia 2233 soybean seed, was established through the assessment of several conditions that can affect transformation efficiency with the use of β-glucuronidase reporter genes. Transformation was confirmed with PCR and Southern blot analysis and results indicated that stable transgenic soybean plants were obtained within one generation with a transformation rate of± 30%. This technique was used in the transformation of Carnia 2233 soybean seed with the P5CR gene in the antisense orientation under the control of an inducible heat shock gene promoter (IHSP). It was confirmed that the P5CR-IHSP gene construct was integrated into the soybean cells and was conserved over three generations. Physiological screening of the antisense P5CR transgenic plants in the greenhouse proved that, with activation of the promoter, an under-expression of the P5CR gene and subsequent inhibition of the accumulation of proline were experienced during drought and osmotic stress. The decline of the viability of the transgenics with prolonged drought stress, as monitored with a woodenbox screening test, is an indication that proline is needed for survival of soybean plants under drought stress conditions. The transgenic plants demonstrated a sensitive reaction in contrast to the control plants that displayed a tolerant reaction to osmotic stress in a TTC assay. The underexpression of the P5CR gene resulted in a decline protein synthesis due to proline shortage as was observed with the evaluation of the efficiency of protein synthesis. All these results suggest that a decrease in the proline level due to the antisense P5CR gene, yielded plants that are more osmotic and drought stress sensitive. Subsequently, the soybean cultivar Ibis was successfully transformed with the P5CR-IHSP construct in the sense and antisense directions in order to test the reproducibility of the transformation process and to assessed the link between the biochemical traits involved in the drought stress mechanism. Three different experiments were conducted: a mild heat and drought stress on "To" transgenic plants exploring changes in chlorophyll fluorescence transients, a mild heat stress on "T1" transgenic plants comparing proline accumulation and chlorophyll fluorescence transients and a severe drought and heat stress on the "T1" transgenic plants comparing proline accumulation NADP⁺synthesis and chlorophyll fluorescence transients. Chlorophyll fluorescence transients were successfully used as a screening method for transgenic soybean plants during this study. The sense transgenics responded to the mild stresses with a significant decrease in their electron transport, trapping and absorption compared to the antisense plants that displayed significant increases in electron transport and trapping. During the severe stress, the antisense transgenics experienced total photoinhibition indicated by the enormous loss of electron transport but the sense plants had the ability to overcome the stress as is revealed in the increase in the electron transport. It was demonstrated that although proline accumulation yielded no significant differences during the mild heat stress, the sense plants accumulated substantially more proline than the control and antisense plants during the severe heat and drought stress. It was demonstrated that proline plays an important role in the plant's response to a drought stress as well as in the recovery phase after drought, as the sense plants also had the ability to reduce the accumulated proline during the recovery period in contrast to the antisense transgenics that experienced protein degradation. The transgenics responded to a period of heat and drought stress with a reduction in NADP⁺ levels in the antisense plants and increasing levels in the sense plants. The sense plants were able to fully recover after the stress period, thus adaptation to drought may depend on different mechanisms, including the capacity to maintain high levels of proline and to regenerate them through the "reduction" of NADP⁺. It was possible to alter the drought tolerance of Ibis by transformation with antisense and sense P5CR gene constructs, which resulted in respectively more sensitive and more tolerant Ibis plants. It can be concluded that over-expression of P5CR during a drought stress resulted in higher proline levels, better photosynthetic efficiency, higher NADP⁺ production and thus a more drought tolerant plant. This study gave additional proof that a constitutively higher level of proline accumulation enhances drought tolerance in soybean. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2000.

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.

With the increasing scarcity of water resources, soil moisture stress is the single most threat to global soybean production causing extensive yield losses. The objectives of this study were to investigate soil moisture stress effects on all aspects of soybean growth and development processes and to develop functional algorithms that could be used for field management decisions and in soybean crop modeling. To fulfill these objectives, six experiments were conducted; one in vitro osmotic stress study on seed germination, four studies by imposing five soil moisture treatments, 100, 80, 60, 40, and 20% of daily evapotranspiration of the control at different growth stages using sunlit plant growth chambers, and one transgenerational study on seed germination at different osmotic levels and offspring growth at three irrigation treatments (100, 66, and 33% based on field capacity) for plants grown at different soil moisture levels. Two cultivars from maturity group V, Asgrow AG5332 and Progeny P5333RY, with different growth habits were used in all these studies. Midday leaf water potential, plant height, mainstem nodes, gas-exchange traits, canopy reflectance, and several yield components including pod weight, seed yield, and seed quality were measured. Soil moisture stress decreased biomass, net photosynthesis, yield, individual seed weight, maximum seed germination, protein, fatty acids, sucrose, N, and P and increased oil, stachyose, Fe, Mg, Zn, Cu, and B contents. Overall, Asgrow AG5332 was more tolerant to drought stress than Progeny P5333RY. Soil moisture stress induced changes in seed quality that were correlated with seed germination and seedling vigor in the F1 generation. These data can be used to build a model-based decision support system capable of predicting yield under field conditions.