Vesicular-arbuscular mycorrhizal fungi can affect the water balance of both amply watered and droughted host plants. This review summarizes these effects and possible causal mechanisms. Also discussed are host drought resistance and the influence of soil drying on the fungi.

Water relations, drought and vesicular-arbuscular mycorrhizal symbiosis

There is now strong evidence that the plant hormone abscisic acid (ABA) plays an important role in the regulation of stomatal behaviour and gas exchange of droughted plants. This regulation involves both long-distance transport and modulation of ABA concentration at the guard cells, as well as differential responses of the guard cells to a given dose of the hormone. We will describe how a plant can use the ABA signalling mechanism and other chemical signals to adjust the amount of water that it loses through its stomata in response to changes in both the rhizospheric and the aerial environment. The following components of the signalling process can play an important part in regulation: (a) ABA sequestration in the root; (b) ABA synthesis versus catabolism in the root; (c) the efficiency of ABA transfer across the root and into the xylem; (d) the exchange of ABA between the xylem lumen and the xylem parenchyma in the shoot; (e) the amount of ABA in the leaf symplastic reservoir and the efficiency of ABA sequestration and release from this compartment as regulated by factors such as root and leaf-sourced changes in pH; (f) cleavage of ABA from ABA conjugates in the leaf apoplast; (g) transfer of ABA from the leaf into the phloem; (h) the sensitivity of the guard cells to the [ABA] that finally reaches them; and lastly (i) the possible interaction between nitrate stress and the ABA signal.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.0016-8025.2001.00824.x

ABA-based chemical signalling: the co-ordination of responses to stress in plants

Publisher Summary This chapter discusses that the knowledge of phytic acid had its beginning in the discovery by Hartig, who isolated small particles or grains (which were not starch grains) from the seeds of various plants. Phytic acid has been generally regarded as the primary storage form of both phosphate and inositol in almost all seeds. The amount of phytic acid varies from 0.50% to 1.89% in cereals (except polished rice), from 0.40% to 2.06% in legumes, from 2.00% to 5.20% in oil seeds except soybeans and peanuts (grouped under legumes), and from 0.40% to 7.50% in protein products. Many foods and seeds contain myo-inositol hexaphosphate as an important source of phosphorus, and accurate methods for its determination are needed. Phytase (meso-inositol hexaphosphate phosphohydrolase, EC 3.1.3.8) is widely distributed in plants, animals, and fungi. In mature cereal grains, legumes, and oil seeds, the major portion of the total phosphorus is present in the form of phytic acid (phytate). Experiments with animals have suggested that phytic acid in plant foods complexes with dietary essential minerals such as calcium, zinc, iron, and magnesium and makes them biologically unavailable for absorption. The correlation of phytate with the cooking quality of peas was first suggested by Mattson. Phytic acid, myo-inositol 1,2,3,4,5,6-hexakis dihydrogen phosphate serves as the main phosphorus store in mature seeds and grains. In cereals and legumes, phytic acid content ranges from 0.14% to 2.05%, which accounts for 18 to 88% of the total phosphorus.

Phytates in Legumes and Cereals

Traits that enable plants to exploit low-resource environments (e.g., slow tissue turnover, low transpiration rate, high root: shoot ratio, and high concentrations of plant defenses against pathogens and herbivores) are physiologically linked to key growth-related traits (low rates of photosynthesis, nutrient uptake, and growth). Similar suites of traits occur as both phenotypically plastic and evolutionary responses to stress. We suggest that a genetic change in a switch or underlying trait that turns on this stress resistance syndrome (SRS), which causes it to be expressed over a wider range of environmental circumstances, would effectively convert a high-resource genotype into one that is more stress-tolerant. Because of physiological linkages between growth-related traits and the SRS, any heritable change in a key growth-related trait will pleiotropically affect the SRS. Therefore, heritable changes in these key growth-related traits could be accompanied by evolution of the entire SRS. Evidence for this hypothesis comes from single-gene mutants that differ in many stress-related traits, rapid evolution of metal- tolerant populations that are broadly stress-resistant, and consistent patterns of traits in species along gradients in resource availability. Similar evolutionary patterns occur in many animal taxa, which suggests that it is a general evolutionary phenomenon. We suggest that rapid evolution in response to changing environmental stress may allow many short-lived species to respond to human-induced environmental change and provide opportunities to develop stress-resistant crops. However, the time lag between generations of long-lived species that dominate most natural vegetation may not allow mature individuals of these species to keep pace with rapid global change.

https://www.jstor.org/stable/info/2462710

Evolution of suites of traits in response to environmental stress

In both natural and agricultural communities, the environment is seldom optimal for plant growth. Environmental stress limits the overall productivity of US agriculture to 25% of its potential (Boyer 1982). All mesic environments experience large seasonal fluctuations in light, moisture, temperature, and nutrients, often to levels that are suboptimal for plant growth, so the plant is continuously encountering new combinations of environmental stresses. Moreover, most natural environments are continuously suboptimal with respect to one or more environmental parameters, such as water or nutrient availability. The nature of controls over plant growth in suboptimal environments is of particular interest, because these are the only habitats into which agriculture can expand in most developing countries, and impending global climate change will alter the suitability of most terrestrial habitats for plant growth. Consequently, we need to understand the physiological mechanisms that enable plants to survive and reproduce under suboptimal conditions. To date, most research on the physiological responses of plants to environmental stress has focused on the responses of plants to specific stresses (Osmond et al. 1987). For example,

Integrated Responses of Plants to Stress

Suboptimal N nutrition increased the water potential for stomatal closure in water stressed cotton ( Gossypium hirsutum L.) leaves. This increased sensitivity to water stress had two components, increased accumulation of abscisic acid (ABA) and increased apparent stomatal sensitivity to ABA. Low N increased the threshold water potentials for stomatal closure and ABA accumulation by about 4 bars and 2 bars, respectively. Low N also greatly increased stomatal response to low concentrations of exogenous ABA applied to excised leaves through the transpiration stream. In low N leaves, kinetin decreased stomatal response to ABA to the level observed with high N leaves. Kinetin by itself had little effect on stomata, nor did it alter stomatal response to ABA in high N leaves. The results suggest a cytokinin-ABA balance which is altered by suboptimal N nutrition to favor stomatal closure during stress. Ambient temperature and N nutrition interacted to alter stomatal response to water stress. Stress-induced ABA accumulation and apparent stomatal sensitivity to ABA were independently affected. The effects of each treatment, and their interaction, could be explained as the net result of changes in both accumulation and apparent sensitivity. Although the results document environmental control of stomatal response to ABA, either altered partitioning of ABA between active and inactive pools, or altered sensitivity of the guard cells, could account for the data.

Water Relations of Cotton Plants under Nitrogen Deficiency V. Environmental Control of Abscisic Acid Accumulation and Stomatal Sensitivity to Abscisic Acid

Young Gossypium hirsutum L. seedlings chilled at 5 degrees showed a continual decrease in ATP concentration with time of chilling. Chilled plants returned to optimum conditions were able to restore the initial ATP concentration when chilled only 1 day, but not when chilled 2 days. The decrease in ATP with chilling was prevented by hardening the seedlings at 15 degrees for 2 days (14-hr-day-length) immediately before chilling. The ATP level of hardened plants was higher than of unhardened plants. When hardened plants were chilled at 5 degrees , the ATP level increased in the leaves but decreased in the roots.A mechanism of chilling injury is discussed in relation to the decrease in ATP concentration at low temperature.

/pdf/chilling-injury-and-changes-in-adenosine-triphosphate-of-1y2e53qqk0.pdf

Chilling injury and changes in adenosine triphosphate of cotton seedlings.

Abscission of Cotton Floral Buds and Bolls as Influenced by Factors Affecting Photosynthesis and Respiration 1

The relationships of fruit age, abscisic acid (ABA) concentration, ethylene evolution, and abscission rates were studied in an effort to determine why cotton (Gossypium hirsutum L., cv. Deltapine 16) fruits rarely abscise more than 15 days after anthesis. Because abscission of cotton fruits is increased by conditions that limit photosynthesis, greenhouse-grown plants with fruits of various ages were placed in dim light for 3 days to induce high rates of fruit abscission. Abscission rates, ABA concentrations, and ethylene evolution rates were determined for fruits of various ages. Almost all of the young fruits abscised, but abscission rate declined with age until almost no abscission was observed in fruits that were 15 or more days past anthesis.Dim light increased the ABA concentrations of fruits that were 6 to 11 days old but did not increase ABA concentrations in fruits that were younger or older. The concentration of ABA declined with fruit age from peak values at 4 and 6 days after anthesis. Dim light also increased ethylene evolution from fruits up to 10 days old but had little effect on ethylene production or abscission of fruits more than 11 days old. Ethylene evolution declined with fruit age from peak values at 4 and 6 days after anthesis. Fruits of various ages (from plants not exposed to dim light) were sliced to induce high rates of wound ethylene production. The results indicated that the capacity for ethylene production declined with fruit age, parallel with a decline in abscission rate. Decreases in ABA concentration and ethylene evolution with fruit age indicate that change in the capacity to synthesize these hormones, especially in response to stress, is one cause of the decline in abscission rates as cotton fruits become older.

/pdf/fruit-age-and-changes-in-abscisic-acid-content-ethylene-mo2gyp1ath.pdf

Fruit Age and Changes in Abscisic Acid Content, Ethylene Production, and Abscission Rate of Cotton Fruits

A procedure was selected for the simultaneous extraction and purification of abscisic acid (ABA) and indoleacetic acid (IAA). Unnecessary steps were eliminated and an accumulation of aqueous phase was avoided. The superior performance of diethyl ether (compared to ethyl acetate) for bulk purification and the superior resolution provided by 250 millimeter columns packed with 5-micrometer spherical particles of strong anion exchanger and octadecylsilane (C18) greatly facilitated the purification of samples. A fixed-wavelength (254 nanometer) ultraviolet detector and a fluorescence detector connected in series on a high performance liquid chromatograph permitted nondestructive monitoring and measurement of ABA and IAA. Derivatization was not necessary for chromatography or for detection. Isocratic elution with simple mobile phases gave sharp peaks. A few simple precautions minimized losses. Recoveries through the entire procedure averaged about 75% for ABA and about 50% for IAA. Purified ABA and IAA fractions were usually free of interfering contaminants. Identities were confirmed by gas chromatography-mass spectrometry.

Gene Guinn

Papers

Water Relations of Cotton Plants under Nitrogen Deficiency V. Environmental Control of Abscisic Acid Accumulation and Stomatal Sensitivity to Abscisic Acid

Chilling injury and changes in adenosine triphosphate of cotton seedlings.

Abscission of Cotton Floral Buds and Bolls as Influenced by Factors Affecting Photosynthesis and Respiration 1

Fruit Age and Changes in Abscisic Acid Content, Ethylene Production, and Abscission Rate of Cotton Fruits

Purification and Measurement of Abscisic Acid and Indoleacetic Acid by High Performance Liquid Chromatography