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Environmental stresses such as drought, salinity and low temperature have been major selective forces throughout plant evolution and are important factors which limit crop plant distribution and agricultural productivity. An understanding of how crops adapt to adverse conditions is not only of theoretical interest, but also has considerable practical value . Low-temperature stress subtraction libraries were constructed in a pBluescript vector with the two-step-PCR amplified cDNAs using subtractive hybridization. One insert cs18 was obtained and the sequence analysis of insert cs18 revealed that the insert cDNA had a 76% homology with the sequences of the corresponding portion of glucose dehydrogenase from Thermoplasma acidophilum and 62.0% homology with a genomic DNA of Arabidopsis. Four clones, cs18-13, cs18-14, cs18-15, and cs18-16 from low-temperature stress soybean root conventional cDNA library have been confirmed to have inserts that could hybridize to the cs18 insert. One cDNA with a Xba I and Xho I fragment of approximately 3,500 bp in length corresponded to the insert cs18 , which probably encodes for glucose dehydrogenase, was obtained. Northern blot analysis indicated that cs18 mRNA was highly expressed in soybean root but moderately expressed in leaves under low temperature. Changes in the nuclei of meristematic root cells in response to severe salinity were studied. Roots are in direct contact with the surrounding solution . Thus, they are the first to encounter the saline medium and are potentially the first site of damage or line of defence under salt stress. Nuclear deformation or degradation in the soybean root meristem with 150 mM or higher NaCI led to sequential cell degradation, cell death and cessation of plant growth . However, this study indicates that an increase in CaCI[2] concentration up to 5 mM could partially prevent salt injury to the cells. Tissue culture is an excellent tool for elucidat ing the correlation between plant organizational levels and salt tolerance because of the possibility it offers for studying the physiology of intact plantlets together with that of organs and single cells using homogenous plant material under uniform environmental conditions. One NaCI-tolerant cell line (R100) was isolated during this study. The R100 callus cell line was significantly more tolerant to salt than the salt-sensitive line (S100) during exposure to salt stress. Salt tolerance in this culture was characterized by an altered growth behaviour, reduced cell volume and relative water content, and accumulation of Na+, Cl ¯, K+, proline and sugars when grown under salt stress and with its subsequent relief. The selection of this salt tolerant cell line has potential for contributing new genetic variability to plant breeders. Sugars are not only important energy sources and structural components in plants , they are also central regulatory molecules controlling physiology, metabolism, cell cycle , development, and gene expression in plants. The concentrations of glucose and fructose increased during salt stress and after relieving salt stress, at a rate closely corresponding to the increase in relative water content. Their accumulation was the earliest response detected during the removing of salt stress indicating that glucose and fructose may play important roles during salt-stress. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2000.

An infusion technique was used with an aqueous radiocalcium ( 45CaCl2) solution during the seed-filling stage of well-watered and moisture-stressed soybean in a greenhouse. The kinetics of infusion volume showed a quadratic reduction in absorption which approached zero on the sixth day for non-irrigated plants. The concentration of 45Ca increased quadratically from the point of injection towards the apex independent of both water status and plant parts. The difference in concentration of 45Ca between irrigated and non-irrigated plants was significant (P < 0.05) and concentrations attained the maximum values at the sixth node from the plant base. Seeds contained considerably less 45Ca than either stem or leaves. / The effects of a long (LD, 16h) and a short (SD, 12h) photoperiod with two water stress levels (SL) of stress (ST) and no stress (NS) on the distribution of 45Ca in plant organs (PO) of leaves, petioles, and stem at different node number (NN) of soybean were studied during the seed-filling stage. The univariate and Manova analyses showed the main effects of photoperiod (PP), SL, and PO to be highly significant (P < 0.001) on Ca distribution. The long PP increased Ca concentration in top leaves compared with the short PP regardless of SL. Water stress significantly (P < 0.001) modified the Ca distribution and reduced its concentration in PO within NN irrespective of the photoperiod (a measure of light stress). A possible mechanism for the regulation of Ca distribution is discussed in terms of nitrate reduction. / Ca uptake was also studied by immersing the central tip of a trifoliate leaf in various concentrations of 45CaCl2 solutions and drought conditions during the seed-filling period of soybean. The beta-ray gauging and the diurnal leaf temperature variation showed similar characteristics for leaf water status. The activities of 45Ca were significantly higher (P < 0.0001) at 5, 10, 20, and 30 mM concentrations for water-stressed and non-stressed leaves compared with the control. 45Ca activities at 5, 10, and 20 mM Ca concentrations between stressed and non-stressed leaves were not significant, but the difference in their mean values at 30 mM Ca concentration was significant (P = 0.0159). The relationship between 45Ca uptake and Ca concentration was parabolic for both stressed (R 2 = 0.77) and non-stressed (R2 = 0.81) leaves. Autoradiograms indicated Ca movement through the mid-rib and veins of the tip-immersed trifoliate leaf but showed no activity in other plant parts. An activity gradient developed between seeds when a pod-tip was immersed in the radioactive solution. Solutions of ruthenium red (RR, 0.01 mM), Ethylene Glycol-bis-(beta-aminoethyl ether)- N,N,N,N-Tetraacetic Acid (EGTA, 0.1 mM), calcium (Ca, 1 mM), and double distilled water (control) were fed through a bottom branch of soybean with (ST) and without (NS) water stress. The volume absorptions and transpiration rates were significantly higher for NS than ST plants and decreased almost linearly with time for all treatments. The transpiration rates of Ca-feeding ST plants and the control overlapped while the NS plants approached the same rate of transpiration by the third week. Ca was implicated in stomatal closure for the reduction in the transpiration rates. The relative amounts of chlorophyll decreased with time but chlorophyll was least affected for Ca-absorbing plants for both ST and NS plants. The use of RR (Ca transport blocker), and EGTA (Ca chelator) indicated the role of intracellular Ca conce

This study was conducted to improve knowledge of the impact of chronic soil water deficit and to test a novel technique of water management consisting of Lipo-chitooligosaccharides (LCO) spray application. It also aimed at evaluating changes in canopy reflectance due to water stress and LCO spray. Water stress during reproductive development resulted in an important decrease of plant physiological activity, vegetative growth, and productivity, and accelerated plant senescence at both water stress levels. Water deficit increased leaf reflectance in the visible and decreased it in the infrared ranges of the spectrum at both imposed stress levels. Foliar application of LCO affected overall plant physiological activity, increased flower and pod numbers. LCO treatment had the largest positive effect on the growth pattern of soybean at the medium stress level, which is the stress level most commonly observed in standard farm-field conditions. LCO treatment constitutes a potential technology for reducing water deficit effects. (Abstract shortened by UMI.)

Plants possess highly sensitive perception systems by which they recognize signal compounds originating from microbes. These molecular cues play an important role in both symbiotic and pathogenic relationships. Establishment of the soybean (Glycine max)-Bradyrhizobium symbiosis is orchestrated by specific signal molecules exchanged between appropriate plant and microbe partners: flavonoids as plant-to-bacteria signals, and Nod factor as bacteria-to-plant signals. How this signaling process interacts with stress conditions (abiotic and biotic) is the subject of this thesis. The abiotic stresses were suboptimal growth temperature, low pH, and salinity. Suboptimal growth temperatures affected the ability of the microsymbiont, Bradyrhizobim japonicum, to perceive nod gene inducers (genistein) and produce Nod factor. Nod Bj-V (C18:1, MeFuc) production by B. japonicum strains 523C and USDA110 was strongly affected by suboptimal growth temperature. Nod factor production declined with temperature, from 28 to 15°C. Strain USDA110 was more affected by decreased temperature than strain 532C. Decreased Nod factor production at low temperature was due to both decreased bacterial growth and lower production efficiency (Nod factor per cell). When a 1:1 mixture of Nod factor Nod Bj-V (C18:1, MeFuc) and Nod Bj-V (Ac, C16:0, MeFuc) was applied to soybean roots, root hair deformation increased as Nod factor concentration increased under stressfully low temperature and low pH conditions. High salinity stress strongly reduced the root hair deformation caused by Nod factor, and increasing the concentrations of added Nod factor did not over come this. Exogenous application of Nod Bj-V (C18:1, MeFuc), from strain 532C, to soybean root systems under two root zone temperatures (RZTs---17 and 25°C) reduced the progression of disease (powdery mildew---Microsphaera difussa) development on soybean leaves; this effect increased with Nod factor concentration and was gr

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