Info

Commiphora spp.

Africa

Cochlospermaceae

Cochlospermum vitifolium

Costa Rica

Moringaceae

Moringa ovalifolia

Namibia, Africa

Vitaceae

Cyphostemma currori

Namibia, Africa

gregorii in Australia were found to be S 13C = - 29.06%c and SD = - 90.10%e (H. Ziegler, unpubl.).

These succulent stems are water-stores and below ground lignotubers or xy-lopodia (see Sect. 10.3.3) may also serve water storage. A vast quantity of wa-

Fig. 10.8A-G "Stem-succulent" trees. A Caatinga in Brazil (Martius' Flora Brasiliensis, 18401906). B Savanna in Queensland, Australia. C Pseudobombax pilosus, Paraguana Peninsula, Venezuela. D, E Adansonia digitata (baobab), Okawango Delta, March 1982, Botswana, Africa. F Commiphora sp., Namibia, Africa. G Moringa ovalifolia, Namibia, Africa. (D, E courtesy Helga and Bodo Lüttge, Munich)

Fig. 10.8A-G "Stem-succulent" trees. A Caatinga in Brazil (Martius' Flora Brasiliensis, 18401906). B Savanna in Queensland, Australia. C Pseudobombax pilosus, Paraguana Peninsula, Venezuela. D, E Adansonia digitata (baobab), Okawango Delta, March 1982, Botswana, Africa. F Commiphora sp., Namibia, Africa. G Moringa ovalifolia, Namibia, Africa. (D, E courtesy Helga and Bodo Lüttge, Munich)

Fig. 10.8 (Continued)

ter is present in the succulent stems. This water is mainly stored in a proliferate parenchyma and water storage is highly correlated with wood density (Meinzer 2003; Meinzer et al. 2003), so that the wood density is low (< 0.5gcm-3) and the stem water potential in the dry season may be high (> - 0.5 MPa) (Borchert and Rivera 2001). The water storing parenchyma is separated from the transpiration stream by a high-resistance pathway, which applies to both transport directions, i.e. filling and emptying. Thus, this water storage does not function as a buffer against

daily water deficits in the rainy season because refilling of stem water via the roots is slow (Goldstein et al. 1998) and rather is a long term commodity (Chapotin et al. 2006a). It is the most important resource for phenological leaf flushing before the onset of the rainy season (Sect. 10.1.2.1). In baobab trees stem water content declined by up to 12% during this period and the water was almost exclusively used for leaf growth. Stomatal opening and transpiration only started with the rainy season after considerable rainfall and was associated with the onset of sap flow from the soil at the base of the trunks. High transpiratory water flow can not be supported by the water stored in the tree trunk parenchyma but must come from uptake via the roots (Chapotin et al. 2006b).

10.1.2.3 Hydraulic Architecture, Water Use and Photosynthesis

Some extraordinarily high maximum photosynthetic rates of the mature leaves of savanna trees have been reported, i.e. around 40|mol CO2 m-2s-1 for dominant trees, Curatella americana (Fig. 10.5) and Byrsonima crassifolia, of the Llanos of Venezuela (Medina 1982). However, in trees of the cerrados of Brazil rates near light saturation ranged between 4 and 18 |mol CO2 m-2s-1 (Moraes and Prado 1998; Franco and Lüttge 2002) comparable to average rates of 10-25|mol m-2s-1 for C3-photosynthesis (see Box 10.3). Savanna trees are always C3-plants. Water use efficiency may vary considerably among tropical trees, e.g. between 1.6 mmol carbon mol-1 H2O in Tectona grandis and 4.0 mmol carbon mol-1 H2O in Platymiscium pinnatum which is not directly correlated with relative growth rate (Cernusak et al. 2006).

Water relations and a very high irradiance load are conspicuous stressors for photosynthesis in savannas. Life long acclimation to drought basically can involve three contrasting changes in the water transport capacity per unit leaf area for given plant size, namely a decrease or constancy or an increase (Maseda and Fernández 2006). It is interesting to note therefore, that savanna trees are isohydric with respect to their minimum leaf water potential which is highly regulated at similar levels between dry and wet seasons. It is also noted for the cerrados that in relation to the water factor diurnal limitations are more important than seasonal ones (Gottsberger and Silberbauer-Gottsberger 2006). Among other adaptations a reduction in total transpiring leaf surface area in the dry season is contributing to this, but a major physiological mechanism involved is strong stomatal control of evaporative water loss (Bucci et al. 2003, 2005). A midday depression (see Sect. 5.2.2.1) may assist in regulating the water economy on hot days both during the rainy and the dry season. It is expressed in:

• daily courses of root and leaf petiole hydraulic conductivity,

• photosynthetic gas exchange, and

• photoinhibition.

Often there is root limitation of water movement in the soil-leaf continuum, and in cerrado trees a linear correlation is given between stomatal conductance and loss of water conductivity of the roots in the afternoon (Domec et al. 2006). Daily courses of cavitation and embolism in the afternoon and refilling during the night were observed in both roots (Domec et al. 2006) and petioles of leaves of cerrado trees

Fig. 10.10 Daily courses of specific hydraulic conductivity of leaf petioles of two cerrado trees, Cary-ocar brasiliense (circles) and Schefflera macrocarpa (triangles) in the wet season (January), in the dry season (August) and at the end of the dry season (September). (After data of Bucci et al. 2003)

Was this article helpful?

0 0
Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


Post a comment