(Bucci et al. 2003). Figure 10.10 shows such midday depressions of specific hydraulic conductivity, ks, of petioles in the dry and wet seasons. The degree of embolism in petioles is a function of tension in the xylem water stream and the rate of refilling is determined by internal pressure imbalances, where effects of starch remobilization may be involved (Bucci et al. 2003). Nevertheless, transpiration during the night may also be high and can amount to 15 - 25% of total daily water loss (Bucci et al. 2005).
Midday depressions of net CO2-exchange, JCO2, are frequently found in cerrado trees (Moraes and Prado 1998; de Mattos 1998; Franco and Lüttge 2002; de Mattos et al. 2002) where JCO2 considerably drops at the times of highest irradiance (Fig. 10.11). In the examples of Fig. 10.11 there was no recovery of JCO2 in the afternoon. Figure 10.12 shows measurements revealing rapid responses to changing weather conditions in the rainy season. After a dry spell amidst the rainy season there was a pronounced midday depression of net CO2 uptake, JCO2, and transpiration, Jh20, which was reversible in the afternoon, and immediately after a rainfall on the following day no midday depression was expressed any more (de Mattos et al. 2002).
The midday depression of gas exchange is a reflection of the strong stomatal control involved in the isohydric performance over the seasons as noted above (Franco and Lüttge 2002), but also creates the danger of photoinhibition as discussed in Sects. 4.1.6 and 126.96.36.199 (Fig. 5.3). Protective mechanisms, such as photorespiration (Sect. 4.1.3) and harmless thermal energy dissipation via the xanthophyll cycle
Fig. 10.11 Daily courses of net CO2-exchange (/co2> closed circles) and photosyn-thetically active photon flux density (PPFD, open triangles) of three cerrado tree species. (From Franco and Luttge 2002)
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