Effects of elevated atmospheric CO2 have been examined on a number of crops (Kimball 1983; Cure and Acock 1986; Kimball et al. 2002a) in which the responses were related to other environmental factors (Fig. 1.1), including light, temperature, water, salinity and nutrients (Bowes 1993), especially nitrate and phosphate (McKee and Woodward 1994).
Plant physiological and biochemical responses (Bowes 1993) to elevated CO2, known as the CO2-fertilization effect (Dhakhwa et al. 1997), have been studied in plants with different photosynthetic pathways, mostly in C3 species, but also in C4
Fig. 1.1 Crop responses to CO2, temperature and other environmental factors
Fig. 1.1 Crop responses to CO2, temperature and other environmental factors
and CAM plants (Bowes 1993). At present, the ambient CO2 concentration is a limiting factor for plants with the C3 photosynthetic pathway, and doubling of the atmospheric CO2 will be beneficial to this group of plants, because the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) can fix more CO2 due to increased CO2:O2 ratio and that results in reduced photorespiration (Ziska and Bunce 2006). It has been indicated that plants can detect CO2 concentration, but the mechanisms of such CO2 signalling are poorly understood (Woodward 2002).
Carbon dioxide enrichment affects plant structure (Pritchard et al. 1999), transiently enhances the relative growth rate (RGR) of plants (Lambers et al. 1998) and increases biomass and yield (Kimball 1983). It alters the timing of developmental stages of plants (Bowes 1993), but accelerates growth and as a result, induces earlier leaf senescence (Heineke et al. 1999). Growing plants at elevated CO2 concentration leads to increased leaf area, leaf area index (LAI), leaf area duration and leaf thickness as indicated by decreased specific leaf area (SLA) (Bowes 1993; Bray and Reid 2002), which is partly related to the accumulation of non-structural carbohydrates (Lambers et al. 1998).
Elevated CO2 causes plants to produce more number of mesophyll cells and chloroplasts as well as longer stems and extended large roots with altered branching patterns (Rogers et al. 1992; Bowes 1993). High carbon gain might increase root length, diameter and number (Lee-Ho et al. 2007), and stimulate lateral root production in plants grown under elevated CO2 (Pritchard and Rogers 2000). A shift in biomass allocation from leaves to roots can occur under CO2 enrichment (Stulen and Den Hertog 1993). On the basis of FACE experiments, Kimball et al. (2002b) have reported that in some agricultural crops, elevated CO2 stimulated growth of roots more than that of shoots.
Elevated CO2 can increase the number of flowers, fruits and seeds (Bowes 1993; Jablonski et al. 2002), which results in greater individual seed mass and total seed mass, but lower seed nitrogen concentration (Jablonski et al. 2002). It also increases seed yield but decreases grain and flower protein, as shown in various wheat culti-vars (Ziska et al. 2004). Seed quality of some species, grown under elevated CO2, can be affected through changes in lipid metabolism. For example, elevated CO2 altered wheat grain lipids (Williams et al. 1994) and doubled the number of mitochondria in wheat leaves, compared to the ambient CO2 level (Williams et al. 1998).
Elevated CO2 affects growth through changes in chemical composition of plants, as shown in twenty-seven C3 species, including nine crops (Poorter et al. 1997). They reported that elevated CO2 caused an accumulation of non-structural carbohydrates, decreased organic nitrogen compounds and minerals and increased concentration of soluble phenolic compounds in leaves. Also, CO2 enrichment can affect nitrogen and phosphorus, which are required for the photo-oxidative and photore-ductive C cycles (Rogers et al. 1999). Plants grown under elevated CO2 have higher nitrogen use efficiency (NUE) and photosynthetic N use efficiency (PNUE) (Tuba et al. 2003).
Elevated CO2 stimulates photosynthesis in various intensities during different phenological phases (Mitchell et al. 1999), and its direct consequence is increased dry matter production (Lawlor and Mitchell 2000; Ziska et al. 2004). There are many studies indicating that the initial stimulation of photosynthetic rates declines with exposure to elevated CO2 (Bowes 1993; Moore et al. 1999; Stitt and Krapp 1999). Acclimation of plants to elevated CO2 has thoroughly been described (Heineke et al. 1999; Moore et al. 1999). A decline in photosynthesis can be accompanied by a reduction in Rubisco content (Moore et al. 1999) and an adjustment in leaf carbohydrate signalling (Heineke et al. 1999; Moore et al. 1999), such as increased starch content and a decrease in nitrogen concentration (Stitt and Krapp 1999). Some of these alterations that lead to decreased photosynthesis might be caused by restriction of root growth due to limited physical rooting space in pots, although results from different studies have been inconsistent (Berntson et al. 1993). In such situations, two important regulating factors are: (1) reduced nutrient availability (McConnaughay et al. 1993), particularly nitrogen and phosphorus, and (2) sugar sensing and signalling (Rolland et al. 2002). In the latter case, under elevated CO2, high sucrose levels can act as signals that modify the activities of sources and sinks (Taiz and Zeiger 2002) and downregulate biosynthetic activity. Reduced photosynthesis may also be related to the utilizing capacity of plants for the extra photosyn-thate, which is produced under CO2 enrichment (Arp 1991; Reekie et al. 1998).
On the other hand, Garcia et al. (1998) have found little evidence of a decline in photosynthetic capacity of spring wheat under field conditions, using free-air CO2 enrichment. In their study, photosynthesis increased significantly and substantially for the entire life of the crop. Despite the acclamatory loss of photosynthesis per unit leaf area, changes in morphological characteristics, such as greater leaf area, can increase plant biomass and yield (Bowes 1993).
Elevated CO2 reduces transpiration by partially closing the stomata and decreasing stomatal conductance (Morison and Gifford 1983; Bunce 2000), which decreases the ability of plants to dissipate heat load through nonphotochemical mechanisms under extreme temperature events (Shaw et al. 2005). On the other hand, sub-ambient CO2 level stimulates stomatal opening or inhibits stomatal closure (Assmann 1999). Reduced stomatal opening leads to improved water use efficiency (Guy and Reid 1986; Clifford et al. 2000) and as a result, lowers water stress in plants (Kimball 1983). Wilson and Bunce (1997) have reported that both leaf temperature and leaf-to-air vapour pressure difference played a role in the reduction of stomatal conductance in soybean. Improved water status of plants, due to partial closure of stomata, causes a higher turgor pressure, which stimulates leaf expansion (Lenssen and Rozema 1990). Also, plant water use efficiency is strongly affected by stomatal density (Woodward and Kelly 1995). Both stomatal density and stomatal index of leaves, which are negatively correlated with elevated CO2, have decreased over the past 100 years (Woodward 1987).
Decreased transpiration at elevated CO2 with no increase in leaf water potential in wet soil indicates decreased hydraulic conductance. In some crops (e.g., corn and soybean), grown at elevated CO2, both reversible and irreversible decreases occurred in hydraulic conductance, which could have been related to decreased transpiration (Bunce and Ziska 1998). Robredo et al. (2007) have also shown that hydraulic conductance decreased markedly in barley plants grown under elevated CO2 than those grown under the ambient CO2 level.
In response to elevated CO2, the rate of respiration is increased in some species, but decreased in others (Poorter et al. 1997). As shown previously, elevated atmospheric CO2 leads to a reduction in mitochondrial respiration and doubling of the current CO2 level will reduce the respiration rate by 15-18% per unit dry weight (Drake etal. 1999).
Elevated CO2 can affect plant hormones and its promoting effect on ethylene production through increasing the amount of the enzyme 1-amino-cyclopropane-1-carboxylate (ACC) oxidase has been shown in different plant species (Sisler and Wood 1988; Smith and John 1993), including sunflower, Helianthus annuus L. (Dhawan et al. 1981; Finlayson and Reid 1994). Woodrow and Grodzinski (1993) have reported that prolonged growth under elevated CO2 increased endogenous ethylene production in the leaves of tomato (Lycopersicon esculentum L.) plants, compared to the leaves of those plants grown under lower CO2 levels.
Ethylene can affect many aspects of plant growth and development (Khan 2006), including leaf orientation and carbon partitioning, as shown in tomato (Woodrow et al. 1988). It also influences root growth of sunflower, which is affected by CO2 concentration (Finlayson and Reid 1996). From their study on rice (Oryza sativa L.), grown under elevated CO2, Seneweera et al. (2003) concluded that increased ethy-lene production is central in promoting accelerated development, which coincides with enhanced rates of tiller appearance and the release of auxiliary buds leading to increased grain yield under CO2 enrichment.
Overall, crops will benefit from doubling of the atmospheric CO2, which affects various aspects of plant metabolism by acting as activator and substrate for Rubisco, promoting stomatal closure, and influencing respiration and hormone levels (Bowes 1993). All of these alterations can lead to increased crop growth and yield. However, one must not forget the confounding effects of higher temperatures and other factors, such as water and mineral nutrient supply (Fig. 1.1).
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