Wayne R. Jordan

Breeding crop varieties for stress environments

Plant stress is a major limitation to crop yield. The amelioration of crop environments is either impossible or costly. The breeding of crop varieties resistant to environmental stress is the most effective economical means to improve and stabilize yield under conditions of stress. Crop resistance to various environmental stresses is being improved by traditional and costly breeding methods that involve the stability of yield performance over different environments as a major criterion. Recent advances in stress physiology allow embarkation upon breeding programs that employ distinct physiological selection indices for stress tolerance. Using physiological selection indices in breeding work requires the definition of the importance and effectiveness of given physiological attributes under stress conditions, the design of a proper selection scheme within the logistic framework of the breeding program and the development of rapid and effective selection techniques. These requirements, the progress made and ...

Ethylene Biosynthesis during Aerenchyma Formation in Roots of Maize Subjected to Mechanical Impedance and Hypoxia

Germinated maize (Zea mays L.) seedlings were enclosed in modified triaxial cells in an artificial substrate and exposed to oxygen deficiency stress (4% oxygen, hypoxia) or to mechanical resistance to elongation growth (mechanical impedance) achieved by external pressure on the artificial substrate, or to both hypoxia and impedance simultaneously. Compared with controls, seedlings that received either hypoxia or mechanical impedance exhibited increased rates of ethylene evolution, greater activities of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, ACC oxidase, and cellulase, and more cell death and aerenchyma formation in the root cortex. Effects of hypoxia plus mechanical impedance were strongly synergistic on ethylene evolution and ACC synthase activity; cellulase activity, ACC oxidase activity, or aerenchyma formation did not exhibit this synergism. In addition, the lag between the onset of stress and increases in both ACC synthase activity and ethylene production was shortened by 2 to 3 h when mechanical impedance or impedance plus hypoxia was applied compared with hypoxia alone. The synergistic effects of hypoxia and mechanical impedance and the earlier responses to mechanical impedance than to hypoxia suggest that different mechanisms are involved in the promotive effects of these stresses on maize root ethylene biosynthesis.

Growth, Water Relations, and Accumulation of Organic and Inorganic Solutes in Roots of Maize Seedlings during Salt Stress

Seedlings of maize (Zea mays L. cv Pioneer 3906), hydroponically grown in the dark, were exposed to NaCl either gradually (salt acclimation) or in one step (salt shock). In the salt-acclimation treatment, root extension was indistinguishable from that of unsalinized controls for at least 6 d at concentrations up to 100 mM NaCl. By contrast, salt shock rapidly inhibited extension, followed by a gradual recovery, so that by 24 h extension rates were the same as for controls, even at 150 mM NaCl. Salt shock caused a rapid decrease in root water and solute potentials for the apical zones, and the estimated turgor potential showed only a small decline; similar but more gradual changes occurred with salt acclimation. The 5-bar decrease in root solute potential with salt shock (150 mM NaCl) during the initial 10 min of exposure could not be accounted for by dehydration, indicating that substantial osmotic adjustment occurred rapidly. Changes in concentration of inorganic solutes (Na+, K+, and Cl-) and organic solutes (proline, sucrose, fructose, and glucose) were measured during salt shock. The contribution of these solutes to changes in root solute potential with salinization was estimated.

Seasonal Plant Water Relationships in Acacia berlandieri

In order to assess how plant water potential is related to soil water availability and evaporative demand, determinations of diurnal and seasonal plant water potentials between 23 April 1997, and 22 September 1998, were studied in Acacia berlandieri Benth., a native shrub of the northeastern region of Mexico. Average plant water potentials during the wettest period ranged from-0.37 MPa (predawn, PD) to -2.79 MPa (midday, MD )whereas, during the long, hot, and severe drought period PD and MD water potentials measurements diminished down to -5.9 and -6.8 MPa, respectively. Average diurnal depression of plant water potential (calculated as the difference between midday and predawn water potentials) during the wettest and driest season varied from -2.28 - 0.50 MPa to -0.86 - 0.46 MPa, respectively. An exponential function was found between PD water potential with average soil water content and vapor pressure deficit (VPD). Both variables supported about 85% and 66%, respectively, of the variability in PD water potential. PD water potential values were high and relatively constant above soil water content of 0.17 kg kg-¹; below this threshold value the PD water potential declined. Highest positive (r = 0.870) and negative (r = -0.706) correlation coefficients of PD water potential data were observed with soil moisture content at the 20-30cm soil layer and VPD, respectively. MD water potential, VPD, and air temperature explained about 83% of the variation in PD water potential. Soil water content at the 20-30cm soil layer, PD water potential, andrelative humidity described about 82% of the variation in MD water potential. It was concluded A. berlandieri can endure substantial drydown periods and its water relations are strongly associated with soil water content and atmospheric evaporative demand components.

Gene expression induced by physical impedance in maize roots

Two cDNA clones, pIIG1 and pIIG2, corresponding to mRNAs that accumulate in maize root tips subjected to 10 min of physical impedance, were isolated by differential screening of a cDNA library. The deduced proteins, based on DNA sequence analysis, have molecular masses of 13 and 23 kDa for pIIG1 and pIIG2, respectively. pIIG1 showed 97% similarity at the nucleic acid level to a maize root cortical cell delineating protein (pZRP3) and was also similar to some bimodular proteins that are developmentally or stress regulated in other plant species. In situ localization of pIIG1 showed some expression in cortical cells of control maize roots; however, after a 10 min physical impedance treatment, pIIG1 accumulation increased greatly in cortical cells and extended to include the procambial region. pIIG2 did not show sequence similarity with any identified gene of known function, but a bipartite nuclear targeting sequence occurs in its deduced amino acid sequence which indicates it may function in the nucleus. Thus, rapid accumulation of specific mRNAs occurs in maize roots in response to impedance stress, and these mRNAs may be responsible for some responses of the roots to physical impedance.

Regulation Of Ethylene Synthesis In Maize Root Responses To Stress

The role and regulation of ethylene synthesis was investigated in maize roots subjected to stresses that inhibit elongation, promote radial swelling and promote formation of aerenchyma. Physical impedance was imposed by compression of the growing medium around unemerged seedlings at controlled pressures, and ethylene production rates by intact seedlings were assayed with the aid of a continuous flow system. Ethylene production increased before effects on growth were observed, and AVG plus STS restored root extension to 90% of control values. One hour after application of 100 kPa pressure to the medium, ACC and conjugated ACC levels and ACC synthase and ACC oxygenase activities all had increased sharply. Transient -N treatment increased sensitivity to ethylene and initiated formation of aerenchyma. Effects of both -N and hypoxia treatments were blocked with Ag+. Both treatments induced synthesis of cellulase which was prevented by AVG. Perception of impedance stress and effects of -N on ethylene binding capacity and affinity are being studied.

Ethylene Signal Transduction Pathway in Cell Death During Aerenchyma Formation in Maize Root Cells: Role of Phospholipases

Ethylene biosynthesis is accelerated in roots of maize when they become hypoxic (partially O2 deficient) following soil flooding. The increased concentration of ethylene to which cells close to the root tip are exposed subsequently induces premature death and degeneration of cells in the root cortex, leaving air-filled cavities (aerenchyma). The interconnected cavities improve oxygenation of the root cells, and contribute to plant tolerance of flooding.