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

There has been considerable recent interest in the assessment of the en­ ergy allocation of plants (38, 50, 52). It is likely that through a quantitative understanding of how different plants gain and allocate their resources it will be possible to make predictions as to their success in any given physical envi­ ronment in combination with any competitor and predator. We are still far from this reality. However, as a contribution toward this goal, this review seeks to gather the available. information on the various evolutionary strate­ gies which plants have evolved to gain energy and to account for the numer­ ous ways in which this energy is utilized to meet the demands of the environ­ ment, as well as to successfully produce progeny. To date, studies which have considered allocation have generally been at a rather gross level-e.g., reproductive versus nonreproductive (38), roots versus shoots (117), and photosynthetic versus nonphotosynthetic tissue (86). A study by Harper & Ogden (52) has attempted, however, to account in somewhat more detail for the various sources of energy allocation. To account in more detail for energy gain in allocation and for its even­ tual loss through a variety of routes, the approach that will be used here is the capture of carbon by plants and its SUbsequent diversion into various products which perform a multiplicity of functions. Carbon is the vehicle by which organisms store and transfer energy by chemical binding. This device enables the utilization of a large reservoir of physiological and ecological data into a single coherent body. However, it becomes rather clear that the total information required for any given plant simply does not exist. Thus, this review unfortunately can represent only a composite picture of the gen­ realized routes of carbon movement in plants. It does, however, call attention to the continual partitioning of resources and the multiplicity of carbon path­ ways. Emphasis is placed on the modes of capture of carbon. Hopefully, this analysis will stimulate studies which will provide, for a given series of bio­ types, the total information on carbon gain, use, and loss, so that quantitative models can be derived relating these to shortand long-term environmental influences.

The Carbon Balance of Plants

Plants metabolize carbon dioxide photosynthetically either through a 3-carbon (Calvin) or 4-carbon pathway. Most plants are of the C-3 type; C-4 plants are primarily grasses adapted to hot, arid environments. Since C-4 plants have a higher 13C/12C ratio than C-3 plants, animals and humans with a significant C-4 plant food-intake will have higher 13C/12C ratios as well. Maize is a C-4 plant, hence maize cultivators living in predominantly C-3 plant environments should show significant isotopic differences from local hunter-gatherers in their skeletal remains; the importance of maize in their diet should also be measurable. The practicability of this method is demonstrated for New York State archaeological materials and wider implications are mentioned.

Isotopic Evidence of Early Maize Cultivation in New York State

Publisher Summary This chapter describes the various aspects of crop water deficits. The water status of a crop plant is usually defined in terms of its water content, water potential, or the components of water potential. The simplicity of measuring water content led to its early adoption, but the diurnal and seasonal changes in dry weight make comparisons of water content at different times of day or during the season unsatisfactory. Water deficits develop inevitably as a consequence of water loss from the leaf as the stomata open to allow the uptake of carbon dioxide from the atmosphere for photosynthesis. There have been developments in both direct and indirect methods of measuring water deficits that have resulted in considerable progress in the field. The use of infrared thermometry for the measurement of crop water deficits and the use of in situ psychrometers for the measurement of water potential respectively is elaborated. It is found that the identification of the root as the site of sensing soil water deficits does not eliminate the role of turgor pressure as the transducer of water deficits, but moves the emphasis from leaf to the root. It is observed that at the whole crop level, the water use efficiency will depend not only on the transpiration efficiency of the leaves, but also on the water loss from the soil and the optimization of yield per unit of water used.

Crop Water Deficits: A Decade of Progress

Plant responses to water deficits, water-use efficiency, and drought resistance

Assimilation of carbon dioxide in the dark and on inverted stomatal rhythm are important adaptations of xerophytes.

Physiological Adaptation to Drought in the Carbon Assimilation and Water Loss of Xerophytes

FLAX plants (Linum usitatissimum var. Ventnor) were grown in nutrient cultures supplied with increasing concentrations of boron. The methods were such that an addition of 0.001 p.p.m. boron to the basal medium produced a large growth response. After four weeks the plants were harvested and weighed (Fig. 1). The 0 p.p.m. boron plants grew very little throughout the four weeks; both these and the 0.001 p.p.m. boron plants showed typical deficiency symptoms in stems and roots.

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Effect of Boron Supply on the Sugars, Soluble in 80 per cent Ethanol, in Flax Seedlings

Effect of Ambient Carbon Dioxide Concentration on the Rate of Transpiration of Agave americana in the Dark

Effect of ambient carbon dioxide concentration on the rate of transpiration of Agave americana in the dark.

SURFACE monomolecular films of cetyl alcohol (CH3(CH2)14CH2OH) have been shown to reduce losses by evaporation from open water surfaces1, and this application has had some successful use in Australia2. There recently appeared a report3 that the mixing of cetyl alcohol with the soil in which corn (Zea mays) plants were growing reduced their loss of water by transpiration by 40 per cent without reducing yield. If it was confirmed that some substance could be taken up by plants which reduced their water needs without impairing growth, such a finding would have great potential agricultural importance. It was therefore decided to repeat Roberts's experiment on a pot scale, measuring the effects of cetyl alcohol on growth, transpiration rate and photosynthetic capacity.

T. F. Neales

Papers

Physiological Adaptation to Drought in the Carbon Assimilation and Water Loss of Xerophytes

Effect of Boron Supply on the Sugars, Soluble in 80 per cent Ethanol, in Flax Seedlings

Effect of ambient carbon dioxide concentration on the rate of transpiration of Agave americana in the dark.

Reduction of Plant Transpiration by Cetyl Alcohol