Publisher Summary This chapter discusses the dilution effect in plant nutrition studies. “Environmental conditions” include changes in the soil environment because of the addition of inorganic and organic materials, and water to soil, as well as temperature and light, the application of living organisms such as rhizobia and mycorrhizal fungi, and the inclusion of toxic materials such as heavy metals. A summary is presented of the physical, chemical, and biological reasons that accounts for the observed changes in the rate of nutrient uptake, and the rate of dry matter accumulation as functions of time. The chapter proposes that data on total uptake and total dry matter yield be considered wherever possible, and that consideration of these factors be coupled with consideration of concentrations. In instances where total nutrient uptake is difficult to calculate, it is suggested that this be estimated by the product of concentration and yield. The effect of a chemical or environmental treatment on the concentration of a nutrient in the plant will be considered in two categories—noninteractive and interactive. Of these two, the interactive effects have been most carefully studied in soil-plant nutrition work.

The Dilution Effect in Plant Nutrition Studies

On a global basis, water is a paramount factor in determining the distribution of species, and the responses and adaptations of a species to water stress are critical for its success in any environmental niche. Numerous studies have reported a myriad of changes elicited by water stress. The changes observed are dependent on the species under study and on the severity, duration, and time course of the stress. Before reviewing the changes in detail, we will first present an overview of stress and responses using the stress-strain concept of physics. Next, we will discuss specific water-related parameters for quantifying plant water status and briefly consider how changes in the parameters may affect plant functions. This is followed by the main body which first reviews and analyzes selected responses to water stress and then examines the integrated adaptive behavior of whole plants.

Physiological Responses to Moderate Water Stress

Publisher Summary This chapter emphasizes the measurement of plant water potential following the realization of the importance of atmospheric demand in determining plant water deficits. The total water potential concept determines the movement of water through the soil-plant-atmosphere continuum in response to gradients in water potential and of the soil, plant, and atmospheric factors that influence the development of water deficits in plants. A water deficit occurs whenever water loss exceeds absorption. The use of total water potential as the best single indicator of plant water status has its limitations while attempting to understand the effect of water deficits on the various physiological processes involved in plant growth. The chapter discusses the effects of water deficits on crop growth, crop development, and crop yield. The differences in response of plants grown under controlled conditions and in the field are discussed. Some difficulties in extrapolating from controlled environments to the field are presented in the chapter.

Crop Water Deficits

SUMMARY
Current views on membrane construction are presented as a basis for discussing certain modifications of structure that may render membranes leaky, allowing solutes to diffuse from them. Biophysical experiments on dehydrated membranes indicate that, when their water content is reduced below 20%, they no longer adopt the lipid bilayer conformation. When dry seeds, spores or lichens are placed in water, soluble cell constituents leak from them for a few seconds or minutes until membrane integrity is re-established; the same may apply also to the liberation of solutes from pollen grains. Moreover, there are indications that ‘dry’ in this context means having less than about 20% water.

Some plants are sensitive to chilling, their tissues becoming leaky when exposed to temperatures between 0 and 10°C. This chilling-induced leakage, attributed to a phase change in all the phospholipids of which the membrane is composed occurs at a slightly lower temperature than the change in the activation energy of membrane-bound enzymes.

At senescence, leaf tissues become leaky and may eventually dry out. So much phospholipid disappears from senescing cotyledons of cucumber that there is no longer sufficient to fabricate complete and intact membranes around the cells. Treating cucumber cotyledons with iodoacetate also causes a loss of phospholipids, allowing solutes to leak out of the cells.

It is possible that exposing seeds or mature plant tissues to oxygen at pressures of 1 or more atm renders them leaky because of lipid peroxidation.

Phospholipids and plant membrane permeability

Nutrient Uptake by Plants Under Stress Conditions

Measurements were made of phosphorus uptake by intact tomato plants from solutions labelled with 32P. The plants were exposed to low water potentials by the addition of mannitol to culture solutions. The amounts of labelled phosphorus in the roots and in the shoots wore determined after a one- or two-hour period. Down to -5.4 atmospheres, the amount of labelled phosphorus in the roots remained constant, hut the amount transported to the shoots was reduced. However, potentials of -10.4 atm reduced the amount of labelled phosphorus in both the root and the shoot. Similar results were obtained when plants were tested immediately after water stress was imposed and when tested after water potentials had been lowered gradually.



Plants were treated for one hour at low water potentials and then returned to control solutions (−0.4 atm). For a considerable time, these plants had a much lower phosphorus uptake than plants which had remained continuously at −0.4 atm. These data support the idea that a disturbance in mineral nutrition is partly responsible for reduced growth in plants which experience a moderate water deficit.

Effects of Water Deficit on Phosphorus Nutrition of Tomato Plants

In excised roots of barley and tomato plants, lowering the water potential of nutrient solutions to-10.4 and-20.4 atm decreased the uptake of bromide and phosphorus while increasing the loss of these ions to the external solutions.

Effects of low water potential on ion uptake and loss for excised roots

Relation between Anion Transport and Water Flow in Tomato Plants

The effects of water flow on phosphorus uptake by roots and on its subsequent translocation to shoots were separated by giving short-term pulses of 32P-labelled nutrient to intact tomato plants. At the end of a 5 min pulse, all the 32P taken up by the plants was confined to the roots. Only about half of this 32P was later translocated to shoots; there was very little translocation after 4 hours.

Phosphorus transport to the xylem and its regulation by water flow.

Tomato plants were treated for one hour in nutrient solutions at-10.4 atm. Roots were excised, transferred to solutions at-0.4 atm and put into a pressure chamber to induce rates of water flow similar to those in transpiring plants.

H. Greenway

Papers

Effects of Water Deficit on Phosphorus Nutrition of Tomato Plants

Effects of low water potential on ion uptake and loss for excised roots

Relation between Anion Transport and Water Flow in Tomato Plants

Phosphorus transport to the xylem and its regulation by water flow.

Effects of water stress on phosphorus transport to the xylem