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PBIO*3110 – Crop Physiology Lecture #22 Fall Semester 2008 Lecture Notes for Thursday 20 November Water Relations IV – Water Use Efficiency What determines the amount of water a crop needs to grow? Can this be reduced through crop breeding? Learning Objectives 1. Understand how whole crop water use efficiency is physically related to gas exchange occurring at the leaf level. 2. Know how cuticular conductance can affect calculations of water use efficiency. 3. Consider the potential for improving water use efficiency in crop species through genetic improvement. Introduction In the previous lecture, we reviewed the physiological effects of soil water deficits on crop growth and yield. One of the primary determinants of yield under water­limited conditions is the crop water use efficiency (WUE). Water use efficiency can be defined (and measured) in a variety of different ways. For instance:
· Harvested yield per unit of soil water used over the entire season.
· Total above ground dry matter produced per unit soil water used over the entire season.
· Yield (or above ground dry matter) produced per unit precipitation and / or irrigation water applied.
1 · etc.... The above definitions apply to whole­crop WUE. On a single leaf basis, instantaneous WUE is more strictly defined, as the current net CO2 assimilation rate, divided by the current transpiration rate. In this lecture, we will identify the physiological and environmental factors that determine both single leaf and whole crop WUE. Also, we will examine experimental evidence suggesting that WUE is a factor that is amenable to improvement via crop breeding programs. Instantaneous Single Leaf WUE – Theory As we will see, factors affecting whole crop WUE are many and diverse, and therefore difficult to consider in a quantitative, theoretical context. By contrast, there is a well­developed theory that describes single leaf WUE, based on the simple diffusion equations we have already considered. If we define WUE as AN / E, then it follows However, since CO2 enters the leaf via the same path by which H2O exits the leaf (i.e., the stomata and boundary layer), it is possible to remove the conductance term from the equation altogether. To a first approximation, gw = 1.6 × gc. Therefore, the equation above can be re­ written as: This last form of the equation emphasizes the importance of ci / ca in determining WUE. Any factor that tends to decrease ci will also tend to increase WUE. The most important of these factors is stomatal conductance. The figure below shows how AN and E might respond to a change in stomatal conductance. As stomata open, both AN and E increase. However, while the response of E to gs is linear, the response of AN to gs is non­linear. Why is this?
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The answer of course, is that the response of AN to ci is non­linear. As conductance increases, eventually ci becomes non­limiting to photosynthesis, and so the benefit from each additional increase in ci becomes progressively smaller. Since the leaf is using CO2, the current value of ci depends on both the stomatal conductance, and the photosynthetic capacity of the leaf. Since wi and wa are independent of stomatal conductance, there is no such curvilinear response of E. As a result, WUE (AN / E) decreases as conductance increases. Model of the responses of evapotranspiration (E) and net photosynthesis (AN) to stomatal conductance. Model of the dependence of instantaneous leaf WUE on stomatal conductance, for the case when cuticular conductance is assumed to be negligible, and the case when cuticular conductance is assumed to be 0.01 mol m ­2 s ­1 .
3 As shown in the second figure, the simplest model predicts that WUE is highest when stomatal conductance (and photosynthesis) are lowest. However, a slightly more sophisticated model shows that this is not the whole story. When stomata are completely closed, CO2 uptake by the leaf is essentially zero. However, stomatal closure can not completely prevent water loss from the leaf, since some water always moves through the epidermis and cuticle. When stomata are open, this "cuticular" water loss is a negligible fraction of total leaf transpiration. However, when stomatal conductance is very low, it becomes a significant fraction of the total water lost by the leaf. When cuticular conductance to water vapor is included in the model, the maximum WUE occurs at a higher stomatal conductance. This second model points out an interesting fact; that is, if a plant closes stomata as much as possible in order to conserve water, the net benefit from each unit of water lost will actually decline. In other words, maximum water conservation and maximum WUE are mutually exclusive. Of course, the exact stomatal conductance that maximizes WUE will depend, among other things, on the actual value of cuticular conductance to water vapour. Besides stomatal conductance, what other factors will alter leaf instantaneous WUE? This question is easily answered by carefully examining Equation 2. Clearly, WUE will increase if the ambient water vapor concentration (wa) increases, or if leaf temperature (and therefore wi) decreases. Similarly, WUE can be increased by increasing the ambient CO2 concentration (ca). As discussed above, a decrease in ci will also increase WUE. However, it is important to remember that a decrease in ci can result from two distinct mechanisms:
· a decrease in stomatal conductance, or
· an increase in leaf photosynthetic response to internal CO2. Whole Crop WUE As we have seen, crop physiologists have a strong theoretical understanding of the factors that determine WUE at the single leaf level. However, it is quite difficult to apply these concepts in a quantitative manner at the whole­canopy level. First, there is the obvious complexity of trying to model the activity of all leaves in the canopy simultaneously, when each leaf has its own distinct light environment, and its own distinct photosynthetic response to light and CO2. Additional complications arise from the fact that whole crop WUE is measured in terms of dry matter accumulation, over relatively long periods of time. This means that WUE, measured on a whole­crop basis, will be affected by any factor that affects final dry matter. For example, all other things being equal, a higher rate of maintenance respiration will decrease crop WUE. Similarly, crops that produce large amounts of energy­rich compounds, such as lipids, will
4 experience more carbon loss through growth respiration, and will therefore tend to have lower WUE. Whereas it is sometimes difficult to pinpoint the physiological basis of differences in whole­crop WUE, there is no doubt that such differences exist. For instance, it has been known for over 70 years that common crop species differ in WUE (see the table below). Based on the theory described above, can you speculate as to why C4 plants would tend to have higher WUE than C3 plants? Also, why do you think sugar beet has such a high WUE? Values of WUE recorded for some potted plants. (Shantz and Piemeisel, 1927) Species Classification millet sorghum WUE (mg DM g ­1 water) 2.72 C4 cereals 2.63 maize 2.67 wheat 1.93 oats C3 cereals 1.66 rice 1.47 alfalfa 1.09 pulses Other C3 crops sugar beet 1.33 2.65 Varietal Differences in WUE Crop breeders interested in developing cultivars that can perform well under water­limited conditions would like to increase whole­crop WUE. Over the last few decades, evidence has accumulated that there is substantial variability for WUE within species, suggesting that WUE is a factor that can be improved through selection. However, recall that maximum single leaf WUE tends to occur at very low stomatal conductance, where photosynthetic CO2 assimilation is also very low; obviously, this is not an ideal characteristic for a commercial cultivar. Thus, selecting for maximum WUE per se is not a promising strategy for breeders. Instead, they must select for both high WUE and high crop growth rates. Such a combination would arise from:
5 · Appropriate stomatal regulation (no "luxury" water consumption, but enough conductance to support high rates of photosynthesis)
· High leaf photosynthetic response to CO2 (thus maintaining low ci)
· Low rates of maintenance respiration The next figure shows the differences in WUE recorded in Israel for a number of upland cotton (Gossypium hirsutum), and Pima cotton (Gossypium barbadense) lines, as well as a hirsutum x barbadense interspecific cross. In this field experiment, differences in WUE between the lines were similar, whether WUE was measured on the basis of total above ground DM, or yield of seed cotton. 1.4 Seed Cotton WUE (g kg ­1 ) 2.5 2 1.5 1 0.5 0.6 0.4 0.2 F
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­1 Total DM WUE (g kg ) 3 WUE measured on an above ground DM basis (left), or on a seed­cotton yield basis (right), for three G. hirsutum lines, two G. barbadense lines, and an interspecific cross (Saranga et al., 1998). The final figure shows the frequency distribution for WUE among F4­derived lines from a cross between the soybean variety "Young" (high WUE) and an exotic plant introduction (low WUE). The approximately normal distribution of WUE among these lines suggests that WUE is a quantitatively inherited (multi­gene) trait.
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Frequency distribution of WUE for a group of F4­derived lines, from a cross between the soybean variety Young and a soybean plant introduction. (Mian et al., 1996) A subsequent study investigated the physiological basis of the difference in WUE between Young and PI416937. Those analyses indicated that the two lines do indeed differ in instantaneous (i.e., leaf level) WUE, but that this difference is due entirely to differences in stomatal regulation. Leaf gas exchange of soybean lines ‘Young’ and ‘PI416937’. PPFD = 1200 µmol m ­2 s ­1 . Young PI416937 stomatal conductance 0.333 0.402 (mol m ­2 s ­1 ) leaf internal CO2 250 263 (ppm) CO2 assimilation rate 17.9 19.1 (µmol m ­2 s ­1 ) leaf WUE 3.78 3.46
(µmol CO2 mmol ­1 H2O) 7