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Drought is a major physiological stress that influences the abundance and distribution of species, as well as the growth, performance and fitness of individual plants (Stebbins, 1952; Bohnert et al., 1995; Bray, 1997; Gurevitch et al., 2006; Taiz & Zeiger, 2006). Climatic data show an increase in the frequency of drought over the past century, and models predict further impacts of drought, as well as substantial variation in their global occurrence (IPCC, 2007; Portmanna et al., 2009). To anticipate how plants will respond to changes in climatic conditions, such as drought, it is important to understand dehydration physiology, as well as how drought responses are shaped by natural selection (Heschel et al., 2004; Ludwig et al., 2004; Sherrard & Maherali, 2006; Agrawal et al., 2008; Wu et al., 2010).
Plants can cope with drought either through escape or avoidance (Ludlow, 1989). Escaping drought entails the completion of the life cycle in advance of the effects of drought (Bazzaz, 1979; Heschel & Riginos, 2005; Wu et al., 2010). By contrast, plants avoid drought by maintaining high fitness despite drought conditions. This strategy, sometimes referred to as drought tolerance (Heschel & Riginos, 2005), is generally accomplished through a high water use efficiency (WUE), measured as photosynthetic carbon gain (A) over transpirational water loss (stomatal conductance, g) (Schulze, 1986; Arntz & Delph, 2001). Although plants could potentially escape and avoid drought, theory and previous findings suggest that there are likely to be trade-offs between these strategies (Bazzaz, 1979; McKay et al., 2003; Heschel & Riginos, 2005). The reason for this is that a trait that allows for greater drought avoidance, such as high WUE, may reduce the rate of growth and development and constrain or prevent drought escape.
Drought can also potentially cause either plastic or evolutionary changes in avoidance or escape. With plasticity, the expression of the phenotype is shaped by environmental conditions (Via et al., 1995; Schlichting & Pigliucci, 1998). A plastic response to drought would mean that the plants alter their phenotype by increasing avoidance or escape traits in drought relative to nondrought conditions (Mal & Lovett-Doust, 2005; Caruso, 2006). By contrast, drought could also act as an agent of selection and cause genetically based evolutionary changes in avoidance or escape (Fox, 1990; Ludwig et al., 2004). To what extent adaptive plasticity and local adaptation enable organisms to deal with spatial and temporal environmental variation remains one of the central questions of ecological genetics (Conner & Hartl, 2004; Kawecki & Ebert, 2004; Ghalambor et al., 2007). However, rarely have both plasticity and evolution been examined in a single study.
Seasonal drought is a regular feature of environments with a Mediterranean climate, such as southern California, which has a wet season in the winter and spring and a dry season in the summer and autumn (Minnich, 2007). In these habitats, the length of the growing season is determined by the duration of rains in the spring, which varies greatly between years (Franks et al., 2007). In such environments with high variation in the seasonal pattern of precipitation, plants could potentially respond via an avoidance or escape strategy, and could show either plastic adjustments or evolution following changes in precipitation patterns (Sherrard & Maherali, 2006).
In this study, a set of populations of the annual plant Brassica rapa was used that had been shown previously to have evolved earlier flowering following a natural drought comprising five consecutive years of low precipitation and short growing seasons (Franks et al., 2007). This previous work was the first study to demonstrate a rapid evolutionary change in natural plant populations in response to a change in climate (Franks & Weis, 2008). However, the following questions remain. Did avoidance as well as escape contribute to the drought response? Is there plasticity in drought response strategies? What are the physiological and developmental mechanisms of the drought response? Did the drought cause evolutionary changes in plasticity?
To address these questions, seeds from two populations were used (a wet site and a dry site), collected before and after the drought; 140 maternal families were created, which were grown under low-water (drought) and high-water (nondrought) conditions to examine plasticity in drought escape. For all of these individuals, the time of first flowering was recorded and the number of leaf nodes at the time of first flowering was measured, which has been shown previously to relate to the rate of development and to drought escape in this species (Franks & Weis, 2008). For a subset of individuals, data were collected on instantaneous WUE using an infra-red gas analyzer. Data were also collected on carbon stable isotope ratios (δ13C, hereafter abbreviated as δ), which have been shown to provide a long-term average estimate of WUE because of the slower rate of diffusion and enzymatic discrimination against the heavier 13C isotope in CO2 (Farquhar et al., 1989; McKay et al., 2003). The following hypotheses were tested: that drought caused escape through either a plastic adjustment or an evolutionary shift to earlier flowering, and that there is a trade-off between drought avoidance and escape.
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The results of this study show that a natural drought caused an evolutionary shift to earlier flowering in natural populations of B. rapa, and demonstrate that the mechanism of this drought escape response is rapid development facilitated by high stomatal conductance, which maximizes carbon gain at the expense of transpirational water loss. The findings indicate that there is a trade-off between drought avoidance and escape, and that these plants escape drought through early flowering rather than avoiding or tolerating drought or through plasticity in drought response. This study shows that the approach of comparing ancestors with descendants under common conditions (Franks et al., 2008) is highly useful for testing hypotheses about plasticity and evolution in natural populations.
For two B. rapa populations grown in common environments under both wet and dry conditions, the post-drought (2004) plants flowered earlier than the pre-drought (1997) plants, indicating an evolutionary shift to earlier flowering. This is in keeping with the results of a previous study (Franks et al., 2007), which is an important validation considering that the previous study was the first to show an evolutionary response in a plant population to a natural change in climate. The study reported here shows that the difference in flowering time was maintained through a second generation of crosses in the glasshouse. This finding indicates that rapid evolution can occur in natural plant populations following changes in environmental conditions, as has also been demonstrated in several studies on animals (Thomas et al., 2001; Berteaux et al., 2004; Umina et al., 2005; Balanyáet al., 2006; Bradshaw & Holzapfel, 2008).
The results of this study provide the physiological and developmental mechanism of a rapid evolutionary change in a natural population. According to this study, the mechanism of the evolutionary shift to earlier flowering in B. rapa following drought was rapid development and reproductive maturation, facilitated by inefficient water use in early flowering plants. Evidence to support this mechanism includes the fact that plants that flowered earlier had fewer leaf nodes, indicating that plants that flower early do so at an earlier stage (Franks & Weis, 2008), plants that flowered early had lower integrated WUE (δ), and plants already in flower had lower instantaneous WUE and higher stomatal conductance than plants that had not yet flowered. This indicates that early flowering plants maintain high stomatal conductance, which allows them to rapidly gain carbon as they grow and develop at a cost of heavy water use. This has been called the ‘live fast, die young’ approach (Wu et al., 2010) and appears to fit the description of the favored escape strategy in this system following drought. The results are consistent with the hypothesis of a trade-off between drought avoidance and escape (McKay et al., 2003; Heschel & Riginos, 2005; Wu et al., 2010).
The reason that the drought favored escape rather than avoidance is probably a result of the interaction between environmental conditions and plant life history in this system. It is well known that conservative water use can be adaptive, particularly in environments that are consistently water limited (Ludlow, 1989; Bray, 1997; Taiz & Zeiger, 2006). However, in seasonally dry and variable environments, such as the Mediterranean climate of southern California where this study was conducted, the strategy of heavy and inefficient water use during periods of abundance in order to rapidly develop, reproduce and escape drought can also be a successful strategy (McKay et al., 2003). In a previous study, Heschel & Riginos (2005) found selection for lower WUE and early flowering under early season drought in Impatiens capensis. By contrast, Heschel et al. (2002) found positive selection for greater WUE in I. capensis under late season drought. The pattern from these two previous studies is in contrast with the results of this study, where late season drought selected for earlier flowering and lower WUE. The reason for this difference is probably a result of differences between the Mediterranean environment of this study and the temperate environment of the previous studies. In the environment of this study, where there is generally at least some precipitation early in the growing season and droughts are characterized by shorter growing seasons and potentially a complete lack of precipitation late in the season, it makes sense that an annual plant, such as B. rapa, would show selection for drought escape. Using Avena barbata, another plant occurring in the Mediterranean climate of California, Sherrard & Maherali (2006) also found selection for earlier flowering under drought conditions. Interestingly, drought did not select for lower WUE in their study.
Although there was a general pattern of escape following drought, it is important to recognize that there are a range of physiological responses among individuals within the populations in this study. This is illustrated by the significant G × E in flowering time across watering treatments, with some genetic lines flowering earlier under low-water conditions and other lines flowering earlier under high-water conditions. There also seems to be some capacity for physiological adjustment to drought conditions, given that tissue samples collected from the last leaves produced at the time of flowering in the drought treatment tended to have higher δ values than leaves in the well-watered treatment, indicating that these plants can respond to drought conditions by increasing WUE. This finding is in keeping with a previous study showing lower WUE in alpine Ipomopsis plants in flower than those still in a vegetative state (Campbell et al., 2005). However, in desert shrubs, juveniles have been shown to be less water use efficient than adults (Donovan & Ehleringer, 1992), with this increased WUE leading to greater photosynthesis but not, in this case, an increase in growth rate (Donovan & Ehleringer, 1994). These studies indicate that major physiological shifts may occur during development and, in particular, at the transition to a reproductive state.
Plants can respond to drought through evolution or plasticity. Although the main response to drought in this system seemed to be evolution, there was some evidence for a limited degree of plasticity in flowering time. Although average flowering time did not differ between the low- and high-water treatments, there was genetic variation in flowering time plasticity (G × E). However, there was not an evolutionary change in plasticity following drought, as determined by comparison of the plasticity in pre-drought (1997) and post-drought (2004) families. This indicates that drought did not act to increase or decrease flowering time plasticity in these populations. There was also no difference in plasticity between the wet site and dry site populations. This was somewhat surprising as the Arb site shows considerably more variation than the BB site in both soil moisture and flowering time within the site and over time (Franke et al., 2006). Theory predicts that more heterogeneous sites should produce greater plasticity (Baker, 1965; Schlichting & Pigliucci, 1998). However, few studies have compared plasticity in populations from areas that differ in the degree of environmental heterogeneity. In a notable exception, Sultan (2001) found greater plasticity in Polyganum species with a wider distribution and greater ecological breadth than in species more narrowly distributed. The lack of plasticity in flowering time in B. rapa in this study may be a result of the fact that drought occurs late in the growing season, making it difficult for plants to adjust the timing of flowering in response to drought. By contrast, traits, such as stomatal conductance, which is constantly regulated and adjusted by most plants (Taiz & Zeiger, 2006), are perhaps more likely to be adjusted by drought.
Despite widespread interest in plasticity, this is the first study to use the ancestor–descendant comparison approach (Franks et al., 2008) to investigate evolutionary changes in plasticity in a natural plant population, although similar approaches have been taken with studies of bacteria (Bennet et al., 1992). In addition, relatively few studies have measured plasticity in drought responses. Previous studies have shown significant plasticity and G × E to soil moisture variation for morphological and reproductive traits in Lobelia siphilitica (Caruso, 2006) and Lythrum salicaria (Mal & Lovett-Doust, 2005). In experiments with swamp milkweed (Asclepias incarnata), Agrawal et al. (2008) found selection for reduced WUE in the field (under wet conditions), and plasticity for traits, such as specific leaf area, but not WUE, when wet and dry treatments were compared. Heschel et al. (2004) found positive selection for WUE under drought conditions and plasticity, but no G × E to soil moisture variation, in populations of Polygonum persicaria from wet and dry sources. Even from this small number of studies, it is evident that plasticity, genetic variation and selection on drought response traits are quite variable depending on the environmental conditions and species. Whether or not there are any general patterns, such as increased plasticity with increasing spatial or temporal variation in soil moisture, remains to be determined (Sultan, 2001).