High CO2

Stomatal movements and, in turn, plant transpiration are regulated by CO2 levels in the atmosphere. These elevated atmospheric CO2 concentrations could induce stomatal closure and thereby reduce plant transpiration. Implications of low transpiration rates in K+ uptake were studied in short-term experiments (hours) and it was found that the accumulation of K+ by the cells of the root was unaffected by water flux, whereas the passage through the root to the shoot via the vessels was linearly related to it (Bowling and Weatherley 1964). Similarly, K+ transport to the shoot was shown to be increasingly reduced at low transpiration rates (high relative humidity) in comparison to normal conditions, whereas K+ absorption by the root was less affected by such changes in transpiration (Hooymans 1969).

A long-term study concluded that, in response to sustained exposure to elevated CO2 concentration, biomass is enhanced by 47% in C3 plants, 21% in CAM plants, and 11% in C4 plants (Poorter and Navas 2003). Generally, elevated CO2 alters root architecture and fine-root turnover (Tingey et al. 2000) and increases the proportions of fine roots and secondary roots, implying an expansion of the rhizosphere (Curtis et al. 1994; Norby 1994; Pregitzer et al. 1995). Several studies have demonstrated that elevated CO2 increases the root to shoot ratio (Norby 1994; Rogers et al. 1996; Rogers et al. 1994; Stulen and den Hertog 1993), thereby improving the capacity of the root system to acquire nutrients from the soil. In fact, it was described that after 2 years of exposure to elevated concentrations of CO2 (around 700 ppm), an increase in the K+, Pi, and N accumulation was observed in every organ in Larix kaempferi; together with an increase in dry matter (Shinano et al. 2007). The enhanced

Fig. 4.4 Predicted effects of global warming and conse- may cause secondary salinization. Drought and flooding quences over K+ nutrition. An increase in CO2, O3, and periods may also alter soil moisture triggering nutrient temperature is expected, enhancing stomatal closure, and imbalance reducing transpiration. Irrigation with low-quality water

Fig. 4.4 Predicted effects of global warming and conse- may cause secondary salinization. Drought and flooding quences over K+ nutrition. An increase in CO2, O3, and periods may also alter soil moisture triggering nutrient temperature is expected, enhancing stomatal closure, and imbalance reducing transpiration. Irrigation with low-quality water uptake of nutrients in plants exposed to elevated CO2 resulted from increased root biomass rather than increased root activity.

Although, as stated above, elevated CO2 may have a positive effect on biomass production, important adverse effects also take place. Some plant species could respond to high CO2 by a downregulation of net photosynthesis rate, which may be attributed to an end-product inhibition by elevated carbohydrate concentration in the leaf, combined N and P deficiency. In addition, photoinhibition, possibly due to the damage of PS core complexes, may also occur (Reddy 2010).

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