Virtual Special Issue (VSI) on mechanisms of plant response to global atmospheric change

In 1991, Plant, Cell & Environment was the first journal to publish a Special Issue on mechanisms of plant responses to global atmospheric change. All of those articles have become citation classics underlying the journal's leadership in this key field of contemporary research critical to understanding the future of both our natural ecosystems and crops, and critical to understanding crop adaptation strategies. This latest Special Issue in this area, ‘Mechanisms of Plant Response to Global Atmospheric Change’, consolidates 11 of our most ground-breaking recent articles in this research area at a single easy-to-access point (http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1365-3040/homepage/mechanisms_of_plant_response_to_global_atmospheric_change.htm) (Betzelberger et al. 2010; Cai, Flanagan & Syed 2010; Dieleman et al. 2010; Dillaway & Kruger 2010; Drake et al. 2010; Fu et al. 2010; Ghannoum et al. 2010; Lehmeier et al. 2010; Yamori, Evans & Von Caemmerer 2010; Tazoe et al. 2011; Ainsworth et al. 2012). Over the first 50 years of this century, it is anticipated that global mean temperature will rise by 2–5 °C, droughts will intensify, the carbon dioxide concentration [CO2] will rise to about 500 ppm and tropospheric ozone concentration [O3] will rise by 20–25% (http://www.ipcc.ch/). The following outlines some highlights and lessons in relation to these changes revealed by these articles. Tropospheric ozone is the overlooked aspect of global atmospheric change when it comes to plants. While carbon dioxide is the most important greenhouse gas with a direct positive effect on plant productivity, tropospheric ozone is the third most important greenhouse gas with a strong negative effect of plant productivity and function. Pre-industrial concentrations of ozone in the troposphere ([O3]) were less than 10 ppb, but have risen to 40–60 ppb today in the northern hemisphere, with higher levels in eastern and southern Asia, and expected to increase globally by a further 20–25% by mid-century. Tropospheric O3 is a secondary pollutant formed by the interaction of sunlight and nitrogen oxides, catalyzed by gaseous hydrocarbons. While the primary pollutant may be formed in cities, O3 is often formed hundreds and thousands of miles from the primary source, elevating levels in rural areas and so affecting natural communities, production forests and crops. In the United States alone, it is estimated to cause crop losses of $1–3 billion annually (Betzelberger et al. 2010). Under open-air conditions, the effects of a doubling of [O3] on soybean cultivars, from sowing to harvest, were examined using free air concentration enrichment (FACE). It might be expected that because [O3] has been rising since the 1950s across the US Midwest, plant breeders working under these conditions will have selected cultivars with increased tolerance. However, this study shows that while there is significant variation between cultivars, there is no relationship to cultivar release dates, which ranged from 1952 to 2003 (Betzelberger et al. 2010). This Issue also covers new approaches to analysing plant response to rising [CO2]. In C3 leaves, the mesophyll conductance to CO2 diffusion, gm, determines the drawdown in [CO2] between the intercellular airspace to the chloroplast stroma. It has long been established that stomatal conductance (gs) declines in response to increased [CO2] so providing a negative feedback on the stimulation of C3 photosynthesis by rising [CO2], but the response of gm has been uncertain. Using carbon isotope discrimination measured by laser absorption technology, it is shown that as [CO2] is increased from 200 to 1000 ppm, gm declines significantly by 26–40% in the three C3 species tested, proving a second negative feedback that is independent of the effect on gs (Tazoe et al. 2011). Examining gm in four major tree species, grown in a common garden experiment across seven degrees of latitude in the US Midwest, little evidence of temperature acclimation was found. The same was true for biochemical/biophysical measures of photosynthesis. The prevailing view from earlier studies was that there is significant potential for photosynthesis to acclimate to rising temperatures; however, this study of major tree species, along with other recent studies of photosynthetic acclimation, suggests that this is not the case (Dillaway & Kruger 2010). However, account needs to be taken of the fact that temperature will rise in concert with atmospheric [CO2]. When grown at high temperature, both a fast- and a slow-growing Eucalypt were found to increase their temperature optimum of photosynthesis by 2–7 °C with increase in growth [CO2] (Ghannoum et al. 2010). Early expectations that global atmospheric change might drive a decrease in respiration have given way to recognition of increased respiration. This is largely a result of the application of improved technologies of measurement and scale of measurement. While rising temperatures are expected to decrease production in tropical and temperate climates, increases are expected in cold climates. However, application of automated community gas exchange chambers coupled with parallel eddy-covariance measurements of a northern Alberta fen community showed that while warmer conditions boosted photosynthesis, respiration was increased by more than twice the increase in photosynthesis (Cai et al. 2010). In the same vein, a meta-analysis of 131 of elevated [CO2] tree studies found that while increased photosynthesis drove an increase in carbon input to the soil, this was nullified by an equal increase in heterotrophic respiration, such that soil C did not increase, although the result was strongly affected by nitrogen availability (Dieleman et al. 2010). Nitrogen dilution, in combination with hydraulic limitation, has been widely considered to drive the progressive decline in tree photosynthesis and productivity with age. Examining stands of loblolly pine from 14 to 115 years age in the Duke forest using carbon isotope discrimination, gas exchange, sap flow and nitrogen dynamics, it was found that only increasing hydraulic resistance, and not nitrogen dilution, accounted for declining production (Drake et al. 2010). This finding has important implications for global change, as nitrogen dilution has been expected to drive a long-term decrease in the ability of tree photosynthesis to respond to rising [CO2]. One factor that could contribute to continued production under nitrogen dilution is the finding of a much lower specific respiration rate, despite increased allocation of carbon to roots, in plants grown with a low versus high nitrogen supply rate (Lehmeier et al. 2010). A further complication in predicting the future is how altered metabolism of plants under elevated [CO2] will affect interactions with pests and diseases. Tobacco plants grown under elevated [CO2] were able to produce more defensive secondary metabolites when challenged with a mosaic virus and when challenged with an aphid. However, when challenged with both the aphid and virus, elevated [CO2] shifted defence from the virus to the aphid (Fu et al. 2010). The finding shows how difficult it may be to make generalized predictions of the responses of vegetation to rising [CO2]. Rising temperature and ozone, coupled with increased incidence of drought, will generally drive down production of natural communities and crop yields, offset in part and to an uncertain degree by rising [CO2]. These pressures come at a time when traditional plant breeding may be failing to deliver the year-on-year yield increases to which the world became accustomed over the last half of the 20th century. A review of soybean, the world's most important leguminous crop in terms of global production, reasons a number of specific biotechnological opportunities for delivering the yield jump under global change that we may need (Ainsworth et al. 2012).