Tridestromia Oblonguifolia Definicion

| Fig. 2.4.11. A Temperature dependence of C02 assimilation of the C4 plant Atriplex rosea and the C3 plant Atriplex pa-tula showing clearly differences in maximum rate of C02 uptake and in the temperature optimum. B The same experimental conditions as in A but 1.5% 02 and not 21% 02 in the surrounding air. C02 uptake of C3 plants changes so that it is like the C4 plants. Investigation at low partial pressure of 02 demonstrates experimentally the existence and importance of light-induced respiration on C02 assimilation. (Bjorkman 1971)

Tridestromia oblongifolia (C4)

10 20 30 40

Leaf temperature (°C)

10 20 30 40

Leaf temperature (°C)

| Fig. 2.4.12. A Temperature dependence of C02 assimilation of Tidestromia oblongifolia, Amacarnthaceae, in Death Valley, California. This plant reaches a maximum rate of C02 assimilation at 48 °C. B Tidestromia oblongifolia grows in Death Valley as a summer annual in erosion gullies in which rainwater gathers and then percolates into the soil. (Photo E.-D. Schulze)

Tridestromia oblongifolia (C4)

C fixation. After several frosts net photosynthesis decreases and then recovers slowly, but does not reach the values as during the summer.

• High temperatures affect opening of stomata and result in a disproportional increase in transpiration (Lange 1959); thus a leaf is more likely to dry out completely than to be damaged by heat.

C4 plants have a higher temperature optimum of photosynthesis than C3 plants, because the PEP-carboxylase of C4 plants has its optimum at high temperatures and is thus able to reassimi-late respiratory C02. This effect may also be achieved by exposing C3 plants to an atmosphere with little 02 and thus reduce the oxygenase function of RuBP-carboxylase (Fig. 2.4.11). The highest temperatures at which higher plants reach optimum rates of photosynthesis were observed on Tidestromia oblongifolia at 46 °C in Death Valley, California (Fig. 2.4.12).

2.43.3 Humidity

Net photosynthesis decreases with stomatal closure induced by low humidity; however, it has often been observed that C02 assimilation does not decrease in proportion to stomatal conductance. It is difficult to explain this, because direct effects of humidity on photosynthesis have also been described (Mott and Parkhurst 1991). Obviously, stomata do not close synchronously, so that some areas between the veins still have open stomata whilst others are closed. Thus, there is a higher C02 influx to some small areas, but a lower influx into other areas of the leaf; consequently, the calculated average C02 concentration in the mesophyll remains constant, even though the total conductance of the leaf is decreased. The temperature distribution (Jones 1999) shows that a leaf is not a homogeneous area and so the heterogeneity, so-called patchi-ness of the leaf, contributes to a partial decoupling of the flux of C02 and water vapour for the whole leaf (Mott et al. 1993). Nutrition

C02 assimilation and stomatal conductance change in proportion to the nutrition of the leaf. This not only applies to changes in N nutrition (see Chap. 2.4.5), but also to phosphorus (P). C4 plants, despite a generally lower N concentration, have a potentially higher rate of assimilation than C3 plants, as in C4 plants RuBP-car-boxylase is concentrated in the bundle sheath. Under natural conditions the higher photosyn-thetic capacity is not used, but rather a lower stomatal conductance is reached resulting in lower evaporation but similar assimilation as in C3 plants. Therefore, saving water appears more important than maximising photosynthesis. This leads to a decrease of RuBP-carboxylase and of the N concentration compared with C3 plants (see Penning de Vries and Djiteye 1982) with consequences for assimilate production and for distribution of plants exhibiting the two types of photosynthesis (see Chap. 2.4.4).

The response of C02 assimilation to water stress is easier to study on plants without an epidermis, such as lichens, as here stomatal effects do not occur. When dry lichens are moistened, only respiration becomes measurable at a water content (per dry weight) of 20% (Fig. 2.4.13). With increasing water content respiration and photosynthesis increase. The maximum C02 assimilation is reached at a water content of 80%. The equilibrium humidity for activation of gas exchange thus corresponds to an air humidity of 80% and a water potential of -30 MPa. This water potential is much lower than ever measured for higher plants (-11 MPa; Kappen et al. 1972).

Obviously, photosynthesis starts at a range which is normally not reached in intact leaves. However, photosynthesis responds to changes in turgor (Kaiser 1982). As the changes in cell volume and thus in turgor depend on the rigidity of the cell wall (s = AP/AV), plants with a large leaf area per dry weight, the so-called meso-phytes, are more sensitive towards drying out of the soil than plants with smaller leaf area per dry weight (so-called sclerophytes; Fig. 2.4.5).

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