Storage is a characteristic feature of most plants, particularly perennials, and the subject has been thoroughly reviewed according to its chemistry and physiology (9, 40, 59, 80, 133, 139). However, in ecology much of the information on storage is based on observation rather than experimentation, and experiments often fail to confirm common perceptions of the nature and dynamics of stored reserves. For example, clipping studies show that not all carbohydrates are available to the plant, even though they are considered to be stored reserves. In this review we suggest criteria for defining storage in ecological and eonomic contexts in order to examine the costs and benefits of storage. We then evaluate the evidence for, and ecological importance of, different types of storage. We discuss storage in relation to vegetative growth

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The Ecology and Economics of Storage in Plants

Publisher Summary This chapter evaluates traits for improving crop yield in water-limited environments. The chapter describes the components of yield and the determinants of survival against which the proposed and demonstrated contributions by traits are critically assessed. Both modern and subsistence agriculture are differentiated mainly by the degree of risk that can be tolerated. Many traits have been proposed for improving the performance of drought-affected crops. Putative traits are more effective in enhancing drought resistance and contributing to grain yield in water-limited environments. The potential value of a trait depends upon the crop and the moisture environment in which it is grown. The chapter presents simulation models, which are very powerful tools for critically assessing the value of putative traits. However, more work is needed in the development and testing of suitable models, and in their application for this purpose. Use of near-isogenic populations as opposed to isogenic lines also appears to hold great promise.

A critical evaluation of traits for improving crop yields in water-limited environments.

Although they represent only 2% of terrestrial plant species, halophytes are present in about half the higher plant families and represent a wide diversity of plant forms. Despite their polyphyletic origins, halophytes appear to have evolved the same basic method of osmotic adjustment: accumulation of inorganic salts, mainly NaCl, in the vacuole and accumulation of organic solutes in the cytoplasm. Differences between halophyte and gly-cophyte ion transport systems are becoming apparent. The pathways by which Na+ and Cl− enters halophyte cells are not well understood but may involve ion channels and pinocytosis, in addition to Na+ and Cl− transporters. Na+ uptake into vacuoles requires Na+/H+ antiporters in the tonoplast and H+ ATPases and perhaps PPi ases to provide the proton motive force. Tonoplast antiporters are constitutive in halophytes, whereas they must be activated by NaCl in salt-tolerant glycophytes, and they may be absent from salt-sensitive glycophytes. Halophyte vacuoles may have a modified...

Salt Tolerance and Crop Potential of Halophytes

The McCree-de Wit-Penning de Vries-Thornley Respiration Paradigms: 30 Years Later

Continuous increase in the yield of maize (Zea mays L.) in the U.S. Corn Belt has involved an interaction with plant density. A number of contributing traits and mechanisms have been suggested. In this study we used a modeling approach to examine whether changes in canopy and/or root system architecture might explain the observed trends. A maize crop model was generalized so that changes in canopy and root system architecture could be examined. A layered, diurnal canopy photosynthesis model was introduced to predict consequences of change in canopy architecture. A two-dimensional root exploration model was introduced to predict consequences of change in root system architecture. Field experiments were conducted to derive model parameters for the base hybrid (Pioneer 3394). Simulation studies for various canopy and root system architectures were undertaken for a range of sites, soils, and densities. Simulated responses to density compared well with those found in field experiments. The analysis indicated that (i) change in root system architecture and water capture had a direct effect on biomass accumulation and historical yield trends; and (ii) change in canopy architecture had little direct effect but likely had important indirect effects via leaf area retention and partitioning of carbohydrate to the ear. The study provided plausible explanations and identified testable hypotheses for future research and crop improvement effort.

Can Changes in Canopy and/or Root System Architecture Explain Historical Maize Yield Trends in the U.S. Corn Belt?

The daily (24 hour) changes in carbon balance, water loss, and leaf area of whole sorghum plants ( Sorghum bicolor L Moench, cv BTX616) were measured under controlled environment conditions typical of warm, humid, sunny days Plants were either (a) irrigated frequently with nutrient solution (osmotic potential −008 kilojoules per kilogram = −08 bar), (b) not irrigated for 15 days, (c) irrigated frequently with moderately saline nutrient (80 millimoles NaCl + 20 millimoles CaCl 2 ·2H 2 O per kilogram water, osmotic potential −056 kilojoules per kilogram), or (d) preirrigated with saline nutrient and then not irrigated for 22 days Under frequent irrigation, salt reduced leaf expansion and carbon gain, but water use efficiency was increased since the water loss rate was reduced more than the carbon gain Water stress developed more slowly in the salinized plants and they were able to adjust osmotically by a greater amount Leaf expansion and carbon gain continued down to lower leaf water potentials Some additional metabolic cost associated with salt stress was detected, but under water stress this was balanced by the reduced cost of storing photosynthate rather than converting it to new biomass Reirrigation produced a burst of respiration associated with renewed synthesis of biomass from stored photosynthate It is concluded that although irrigation of sorghum with moderately saline water inhibits plant growth in comparison with irrigation with nonsaline water, it also inhibits water loss and allows a greater degree of osmotic adjustment, so that the plants are able to continue growing longer and reach lower leaf water potentials between irrigations

Carbon balance and water relations of sorghum exposed to salt and water stress

(…) Simulations with the model demonstrated how the assumption of a hyperbolic dependence of photosynthetic rate on internal CO 2 concentration could lead to an increase in water use efficiency as stomates close. The model confirmed published data showing that stomatal closure induced by salinization increases the efficiency under water stress and leads to a greater C gain per irrigation cycle. Other simulations demonstrated how an increase in the volume of soil explored by unit mass of new roots could lead to greater amounts of water uptake and C gain per cycle. Interactions among these and other factors can be studied in a way that would not otherwise be possible

Simulation Model for Studying Physiological Water Stress Responses of Whole Plants

The daily (24-hour) carbon balances of whole sorghum plants ( Sorghum bicolor L. Moench cv BTX616) were continuously measured throughout 15 days of water stress, followed by rewatering and 4 more days of measurements. The plants were grown under controlled environment conditions typical of warm, humid, sunny days. During the first 12 days, osmotic potentials decreased in parallel with decreased water potentials to maintain pressure potentials near 0.5 kilojoules per kilogram (5 bars). Immediately before rewatering on day 15, the water potential was −3.0 kilojoules per kilogram. Osmotic adjustment at this point was 1.0 kilojoules per kilogram, as measured by the decrease in the water potential at zero turgor from its initial value of −1.4 kilojoules per kilogram. Gross input of carbon was less but the fraction retained was greater because a smaller fraction was lost through respiration in stressed plants than in unstressed plants. This was attributed to a lower rate of biomass synthesis, and conversely a higher rate of storage of photosynthate, due to inhibition of leaf expansion. The reduction in the cost associated with biomass synthesis more than balanced any metabolic cost of osmotic adjustment. The net daily gain of carbon was always positive in the stressed plants. There was a large burst of respiration on rewatering, due to renewed synthesis of biomass from stored photosynthate. Over the next 3 days, osmotic adjustment was lost and the daily carbon balance returned to that typical of nonstressed plants. Thus, osmotic adjustment allowed the stressed plants to accumulate biomass carbon throughout the cycle, with little additional metabolic cost. Carbon stored during stress was immediately available for biomass synthesis on rewatering.

Carbon Balance of Sorghum Plants during Osmotic Adjustment to Water Stress.

Calculations of the errors involved in measuring "photo-synthetically active radiation" in different ways are based on the assumption that the photosynthetic rate of a leaf in "white" light is equal to the sum of the products of (a) the photo-synthetic rate per unit of incident energy flux (action spectrum) by (b) the spectral energy flux distribution of the white light, the products being summed over all wavelengths at which the action spectrum is greater than zero. The calculations are valid only if the effects of different wavelengths are independent and additive. Although interactions are well documented in photo-synthesis ("enhancement"), tests showed that the photosynthetic rates of leaves of six species, in four different types of white light, were within +/-7% of the rates calculated in this way.

http://www.plantphysiol.org/content/plantphysiol/49/5/704.full.pdf

Significance of Enhancement for Calculations Based on the Action Spectrum for Photosynthesis

Irrigated crops experience some water stress between irrigations. If poor quality water is used, the crops are also likely to be salinized. Daily C gains and water losses of salinized and nonsalinized sugarbeet (L.), and cowpea (L. Walp.) plants were measured continuously, in whole-plant assimilation chambers, throughout 2 to 3 weeks of water stress under controlled environment conditions. Equivalent data for sorghum plants [ (L.) Moench.] were published previously. Even though the three species showed quite different patterns of response to water stress, mild salinization always had the same effect. It reduced the water loss rate per plant, which allowed the length of the irrigation cycle to be increased, which in turn increased the C gain per cycle and the water use efficiency (C gain per unit of water lost).

Keith J. McCree

Papers

Carbon balance and water relations of sorghum exposed to salt and water stress

Simulation Model for Studying Physiological Water Stress Responses of Whole Plants

Carbon Balance of Sorghum Plants during Osmotic Adjustment to Water Stress.

Significance of Enhancement for Calculations Based on the Action Spectrum for Photosynthesis

Salt Increses the Water Use Effeciency in Water Stressed Plants 1