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0 100 200 300 ¿100 500 GOO Stomatal conductance (mmol nr2 s1)

Figure 28. The effect of stomatal conductance (gs) on the transpiration rate (E, mmol m-2 s-1), rate of CO2 assimilation (A, mmol m-2 s-1), intercellular CO2 concentration (Ci, mmol mol-1) and photosynthetic water-use efficiency (WUE, mmol CO2 (mol H2O) -1 s-1) as a function of stomatal conductance. Calculations were made assuming a constant leaf temperature of 25°C and a negligible boundary layer resistance. The arrow indicates gs at the co-limitation point of carboxylation and electron transport. For the calculations, Equations as described in Box 2A.1 and Sect. 2.2.2 have been used.

Table 5. Intrinsic water-use efficiency (WUE, A/gs) and nitrogen-use efficiency of photosynthesis (PNUE, A/Nla) of leaves of Helianthus annuus (sunflower), growing in a field in the middle of a hot, dry summer day in California.*

Nla A gs Ci WUE PNUE

Mmol m-2 mmol m-2 s-1 mol m-2 s-1 mmol mol-1 mmol mol-1 mmol mol-1 s-1

Low W 180 25 0.4 200 63 139

LowN 130 27 1.0 260 27 208

Source: Fredeen et al. (1991).

Plants were irrigated and fertilized (high N + W), only irrigated but not fertilized (low N), or only fertilized but not irrigated (low W). Since transpiration is approximately linearly related to gs, A/gs is used as an approximation of WUE.

expense of WUE (Table 5). This trade-off between efficient use of water or N explains why perennial species from lower-rainfall sites in eastern Australia have higher leaf N concentration, lower light-saturated photosynthetic rates at a given leaf N concentration, and lower stomatal conductance at a given rate of photosynthesis (implying lower Ci) when compared with similar species from higher-rainfall sites. By investing heavily in photosynthetic enzymes, a larger draw-down of Ci is achieved, and a given photosynthetic rate is possible at a lower stomatal conductance. The benefit of the strategy is that dry-site species reduce water loss at a given rate of photosynthesis, down to levels similar to wet-site species, despite occurring in lower-humidity environments. The cost of high leaf N is higher costs incurred by N acquisition and possibly increased herbivory risk (Wright et al. 2001).

When a plant is subjected to water stress, stomata tend to close. This response is regulated initially by abscisic acid (ABA), a phytohormone that is produced by roots in contact with dry soil and is transported to the leaves (Sect. 5.4.1 of Chapter 3 on plant water relations; Box 7.2). There are also effects that are not triggered by ABA arriving from the roots, mediated via ABA produced in the leaf (Holbrook et al. 2002, Christmann et al. 2005). In addition, both electrical and hydraulic signals control stomatal conductance in response to soil moisture availability (Grams et al. 2007). Stomatal conductance may also decline in response to increasing vapor pressure deficit (VPD) of the air (Sect. 5.4.3 of Chapter 3 on plant water relations). The result of these regulatory mechanisms is that, in many cases, transpiration is fairly constant over a range of VPDs, and leaf water potential is constant over a range of soil water potentials. Water loss is therefore restricted when dry air likely imposes water stress (a feedforward response) or when the plant experiences incipient water stress (a feedback response). In dry environments these two regulatory mechanisms often cause midday stomatal closure and therefore a decline in photosynthesis (Fig. 34 in Chapter 3 on plant water relations).

It was long assumed that stomata respond homogeneously over the entire leaf; however, leaves of water-stressed plants exposed to 14CO2 show often a heterogeneous distribution of fixed 14C. This shows that some stomata close completely (there is no radioactivity close to these stomata), whereas others hardly change their aperture (label is located near these stomata) (Downton et al. 1988, Terashima et al. 1988). This patchy stomatal closure can also be visualized dynamically and nondestructively with thermal and chlorophyll fluorescence imaging techniques (Mott & Buckley 2000); patches with closed stomata are identified by their high temperature and low quantum yield. Patchiness of stomatal opening complicates the calculation of Ci (Sect. 2.2.2), because the calculation assumes a homogeneous distribution of gas exchange parameters across the leaf lamina.

Leaves of plants that reduce stomatal conductance during the middle of the day may only close some of their stomata, while others remain open. This nonuniform reaction of stomata may occur only when plants are rapidly exposed to water stress, whereas stomata may respond in a more uniform manner when the stress is imposed more slowly (Gunasekera & Berkowitz 1992). Stomatal patchiness can also occur in dark-adjusted leaves upon exposure to bright light (Eckstein et al. 1996, Mott & Buckley 2000).

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