In natural habitats, plants are routinely subjected to a combination of abiotic factors. Under climate change scenarios, plants will be exposed to CO2, UV-B radiation, temperature, and water stress simultaneously and their performance can be assessed only when grown under these multiple abiotic stress conditions. Many recent studies suggest that temperature and precipitation changes in future decades will modify, and often limit, the direct effect of CO2 enrichment on plants (Easterling et al., 2007). For instance, high temperature during flowering may lower positive CO2 effects by reducing reproductive traits such as grain number, size, and quality in several crops (Reddy et al., 1997b; Prasad et al., 2002; Prasad et al., 2003a,b; Baker, 2004; Prasad et al., 2006a; Caldwell et al., 2005). Prasad et al. (2006a) showed that adverse effects of high temperature on reproductive processes (seed set and harvest index) of sorghum were more severe at elevated CO2 than at ambient CO2. Similarly, for rice (Matsui et al., 1997) and red kidney bean (Prasad et al., 2002), the ceiling temperature for a seed set was 2°C cooler for plants grown at elevated CO2 than at ambient CO2. Increased temperatures may also reduce CO2 effects indirectly by increasing water demand. Rainfed wheat grown at 450 ppm CO2 demonstrated yield increases with temperature increases of up to 0.8°C, but declines with temperature increases beyond this point (Xiao et al., 2005). Future CO2 levels may favor C3 over C4 plants (Ziska, 2003; Ainsworth et al., 2004; Gifford, 2004; Long et al., 2004); however, the opposite is also expected because of coupled increases in temperature, UV-B radiation, and drought (Reddy et al., 1997b; Xiao et al., 2005; Koti et al., 2007).
Interactive studies on a combination of elevated CO2 and UV-B radiation have shown counteractive effects (i.e., responses in opposite directions). The positive effects of elevated CO2 on plant photosynthesis, growth, and yield were apparent under ambient (normal) UV-B radiation. Under enhanced UV-B radiation, the stimulated effects of elevated CO2 were decreased. In cotton, elevated CO2 significantly increased photosynthesis, growth, and dry matter production under no UV-B or near-ambient UV-B conditions. These responses to CO2 were reflected in significantly higher concentrations of carbohydrates in leaves (Zhao et al., 2003). However, the detrimental effects of high UV-B on cotton photosynthesis and growth, particularly on reproductive growth, could not be alleviated by elevated CO2. This suggests that breeding for UV-B radiation-tolerant cultivars is important in future climates with elevated CO2. Apart from plant growth and related responses like photosynthesis, the attractiveness of plant foliage to insect herbivores may vary under combinations of elevated CO2 and enhanced UV-B (Caldwell et al., 2003). Lavola et al. (1998) found that insects preferred plants grown with enhanced
UV-B, and the combination of high CO2 and enhanced UV-B increased the tendency of the insects to consume more foliage.
Sullivan and Teramura (1990) studied the combined effects of enhanced UV-B radiation and drought. Both drought and UV-B radiation altered biochemical and photochemical processes of photosynthesis and independently elicited similar reductions in growth. However, no additive affects were observed on photosynthesis, growth, or yield. Their results suggested that UV-B radiation may significantly affect soybean growth and photosynthesis primarily when water is readily available and that these effects may be obscured by drought when growth and yield are already reduced. In cowpea, for example, the combination of enhanced UV-B and drought stress elicited beneficial effects on morphological and growth characteristics (Balakumar et al., 1993).
Plant response to enhanced UV-B radiation might also be influenced by nitrogen fertilization. Hunt and McNeil (1998) reported that when plants received more nitrogen, their growth was more depressed by exposure to enhanced UV-B, whereas nitrogen-deficient plants were not responsive to UV-B. Similarly, the response of elevated CO2 is achieved only when plants are sufficiently supplied with nitrogen. Moreover, the requirement of nitrogen under elevated CO2 may be greater because of increased growth and biomass production. Interactive effects of elevated CO2 and potassium (K) supply on cotton showed that elevated CO2 significantly increased photosynthesis, leaf area, and biomass production of K sufficient plants, but did not affect K concentration (Reddy and Zhao, 2005). There were significant interactive effects of CO2 and K on leaf area, canopy photosynthesis, and biomass accumulation and partitioning. The stimulation of physiological and growth parameters observed because of elevated CO2 was lost under severe K deficiency. Interactive effects of climate change factors and soil fertility have received far less attention and need investigation. As several climate change factors can directly influence crop growth and yields, crops grown in future climate would require changes in fertilizer management practices.
Ultraviolet radiation also interacts with temperature stress (both low and high temperatures). Temperature above and below optimum can negatively influence crop growth and yield, and interaction among factors can alter limits of temperature tolerance. Beerling et al. (2001) reported that frost sensitivity of subarctic plant species was enhanced under elevated UV-B. They also showed that elevated CO2 led to an increase in frost sensitivity of these species and if both elevated CO2 and enhanced UV-B were imposed, there was a further increase in frost sensitivity. At high temperatures, some synergistic effects of enhanced UV-B and the elevated temperature were observed (Caldwell et al., 2003). In some tropical legumes, enhanced UV-B reduced growth of the plants at moderate temperatures (20 °C - 30 °C); but at 40 °C, chloroplasts in leaves were modified and thus masked the UV-B depressions of growth (Kulandaivelu and Nedunchezhian, 1993; Nedunchezhian and Kulandaivelu, 1996). Recent studies on cotton showed that of various growth and developmental processes, square and boll retention were most sensitive to high temperature and UV-B radiation (Reddy et al., 2004). Positive interactions were found on the number of main stem nodes, total leaf area, and total fruiting sites. Leaf photosynthesis increased with higher temperatures, but was reduced only by extreme UV-B radiation at high temperatures. The interaction between temperature and UV-B was additive on boll retention causing severe boll loss. Reddy et al. (2004) concluded that in current and future climates, severe yield losses would occur in the presence of high temperatures and UV-B radiation. Both these environmental factors stress plants by reducing reproductive mechanisms as well as vegetative growth.
Drought and high-temperature stress often occur simultaneously, but they can have very different effects on various physiological, growth, developmental, and yield processes. Few studies that examined the impact of combined effects of drought and high-temperature stress suggested that this combination produced a significantly higher detrimental effect on crop growth and productivity compared with each stress applied individually (Craufurd and Peacock, 1993; Savin and Nicolas, 1996). In addition, combinations of drought and heat stress were found to alter physiological processes, such as photosynthesis, accumulation of lipids, and transcript expression (Jagtap et al., 1998; Jian and Huang, 2001; Rizhsky et al., 2004). The interactive or combined effects of drought and high-temperature stress on reproductive processes of crop plants have not been well defined or quantified for any crop species and require further investigation. There might be differences in the response of reproductive function to these stresses. For example, in corn, both drought and heat stress have a direct influence on seed set or seed formation (Westgate, 1994). However, the cause is a result of effects on different processes. Heat stress decreases pollen viability, whereas drought stress (as measured for low leaf water potential) inhibits pistillate flower development and function.
Experiments designed to explore the interaction among these factors are useful for determining the potential effect of these abiotic stresses on crop plants (Caldwell et al., 2007). In a modeling approach, Runeckles and Krupa (1994) suggested that there may be no interaction between these stress factors as a whole, or to certain plant processes, and that the major variable will override the plant response. Otherwise, there may be an additive effect or greater-than-additive effect when the plant response is greater than the sum of responses to the individual factors. Additionally, there is a possibility of a less-than-additive interaction; for example, if CO2 and/or temperature would stimulate more plant dry matter production and repair processes in UV-B sensitive plants (as shown in sunflower (Helianthus annuus L.) and maize seedlings in one of the earliest interactive studies involving CO2, temperature), and UV-B radiation (Mark and Tevini, 1997).
In a recent study, Tegelberg et al. (2008) reported no significant interaction between elevated CO2, temperature, and UV-B for the activity of defensive enzymes, growth-regulating polyamines, photosynthetic pigments, and soluble protein in silver birch (Betula pendula L. Roth). In contrast, there were significant interactions between these abiotic stresses for most of the vegetative and reproductive parameters in soybean (Koti et al., 2005; Koti et al., 2007). However, studies that simultaneously evaluate both vegetative growth and yield attributes under multiple stress conditions are limited. In a recent study, Singh (2008) demonstrated that vegetative and reproductive processes in cowpea respond differently under multiple abiotic stress conditions, including UV-B radiation. Therefore, given the changing climate, it will be useful to study the relative response of vegetative and reproductive plant attributes for important grain and legume crops.
The interaction between abiotic stresses can drastically alter the response mechanisms in plants. This interaction may cause either a positive or negative effect, or can even counteract (neutralize) the effects of individual stresses depending upon the species. Elevated temperatures alleviated the damaging effects of UV-B radiation on various growth parameters in sunflower and corn (Mark and Tevini, 1997), whereas high temperatures in combination with UV-B resulted in an increased reduction in the growth of soybean (Koti et al., 2007). The response of plants to multiple abiotic stresses is unique and should be treated as a new state of abiotic stress rather than a combination of two or more stress factors (Mittler, 2006). One abiotic stress factor evokes a chain of complex metabolic processes in plants in the presence of other stress factors. Developing new crop genotypes of a species with enhanced tolerance to a given stress factor may fail to withstand in the presence of another abiotic stress. Therefore, plant breeders must consider the variable effects of possible climate change when developing breeding programs or transgenic plants for abiotic stress tolerance (Hall and Ziska, 2000; Mittler, 2006).
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