Developing Crop-Specific Irrigation Management Strategies Considering Effects of Drought on Carbon Metabolism in Plants [chapter]

Silvia Aparecida, Arnoldo Rocha, Ricardo Enrique
2012 Water Quality, Soil and Managing Irrigation of Crops  
115 side maintain a favorable plant water status (Dry and Loveys, 1999; Dry et al., 2000 Dry et al., , 2001 Chaves and Oliveira, 2004) . This chapter will focus the changes occurred in carbon metabolism from carbohydrate production (photosynthesis) until consumption (respiration) including the partitioning (sugar metabolism), in response to drought. Such a information will be discussed as a new approach to develop crop-specific irrigation managements. Production and consumption of carbon under
more » ... ow water availability From all processes that contribute to plant development, cell growth and photosynthesis are the firstly affected by drought. When the availability of water in the soil is scarce or vapor deficit pressure of the atmosphere is high, plants firstly respond by decreasing the stomatic aperture. As the main role of stomata is to perform gas exchange, this strategy has two consequences, prevent the water loss and limit CO 2 diffusion. Consequently, this might cause the reduction of the photosynthetic rates in drought conditions. Nevertheless, the extension and the nature of the diminished carbon assimilation in water stressed leaves are due to stomatic or non-stomatic limitations, a theme still under discussion (Tzara et al., 1999; Cornic, 2000; Lawlor and Cornic, 2002; Flexas et al., 2004) . Photosynthetic rate is gradually diminished with progressive reduction in relative water content (RWC). The complexity of photosynthesis is based on the activity of Rubisco (Ribulose 1,5 Biphosphate Carboxylase/Oxygenase) per unit leaf, the rate of RuBP (Ribulose Bisphosphate) resynthesis (hence on capture of photosynthetically active radiation (PAR)) and on the CO 2 supply, given by stomatic conductance (g s ) and the ambient CO 2 concentration (C a ). The CO 2 availability to the chloroplast is intrinsically dependent of the ambient CO 2 concentration and the pathway for diffusion between air and carboxilation site, mainly stomatic conductance in gas phase (g s ) and mesophilic conductance (g m ) in liquid phase. In hydrated leaves and a saturated environment with CO 2 and light the maximum rate of photosynthesis is denominated potential photosynthesis (Apot) (Lawlor and Cornic, 2002). Moreover, when relative water content lowering, to obtain the Apot is necessary Cc to saturated Rubisco and sufficient Ca to suppress the barriers imposed by stomatic and mesophilic conductances. In the first moment, when RWC decreased and the photosynthetic rate diminished, a high CO 2 concentration ({50 a 150 mL L -1 } de 5 a 15%) can restore A to values near Apot, and consequently, the A is unaffected. Afterwards, the decreased photosynthesis cannot be restored by high CO 2 concentrations showing that Apot is impaired by metabolic factors. Parallel to this, photosynthesis fall gradually as RWC decreases showing that Apot is progressively inhibited and the effects of stomatic conductance are diminished. This response built from photosynthetic rate x internal concentration of CO 2 are termed response type I and Type II (Lawlor and Cornic, 2002) . Some species such Rhamnus alatemus, Rhamnus ludovici-salvatoris, Nicotiana sylvestris, Phaseolus vulgaris and Vitis vinifera have their photosynthetic rate mainly limited by stomatic conductance at the beginning of drought development. As drought severity evolves, activity of Rubisco is impaired and the levels of RuBP diminished, consisting as the main limiting factors for photosynthesis . In another way, plants like grasses show the carbon assimilation and quantic efficiency of photosystem II ( PSII ) decreased quickly with RWC decline (Ghannoum et al., 2003) . For these plants, the application of 2500 ⌠L L -1 of CO 2 had no effect in photosynthetic rate and  PSII , showing that biochemical factors www.intechopen.com
doi:10.5772/30419 fatcat:cnlaiudfjndhjmqljdw4faqcr4