A fundamental aspect of climate change is the potential shifts in flowering phenology and pollen initiation associated with milder winters and warmer seasonal air temperature. Earlier floral anthesis has been suggested, in turn, to have a role in human disease by increasing time of exposure to pollen that causes allergic rhinitis and related asthma. However, earlier floral initiation does not necessarily alter the temporal duration of the pollen season, and, to date, no consistent continental trend in pollen season length has been demonstrated. Here we report that duration of the ragweed (Ambrosia spp.) pollen season has been increasing in recent decades as a function of latitude in North America. Latitudinal effects on increasing season length were associated primarily with a delay in first frost of the fall season and lengthening of the frost free period. Overall, these data indicate a significant increase in the length of the ragweed pollen season by as much as 13-27 d at latitudes above ∼44°N since 1995. This is consistent with recent Intergovernmental Panel on Climate Change projections regarding enhanced warming as a function of latitude. If similar warming trends accompany long-term climate change, greater exposure times to seasonal allergens may occur with subsequent effects on public health. aerobiology | allergies | global warming
Despite mounting evidence showing that C 4 plants can accumulate more biomass at elevated CO 2 partial pressure (p(CO 2 )), the underlying mechanisms of this response are still largely unclear. In this paper, we review the current state of knowledge regarding the response of C 4 plants to elevated p(CO 2 ) and discuss the likely mechanisms. We identify two main routes through which elevated p(CO 2 ) can stimulate the growth of both well-watered and waterstressed C 4 plants. First, through enhanced leaf CO 2 assimilation rates due to increased intercellular p(CO 2 ). Second, through reduced stomatal conductance and subsequently leaf transpiration rates. Reduced transpiration rates can stimulate leaf CO 2 assimilation and growth rates by conserving soil water, improving shoot water relations and increasing leaf temperature. We argue that bundle sheath leakiness, direct CO 2 fixation in the bundle sheath or the presence of C 3 -like photosynthesis in young C 4 leaves are unlikely explanations for the high CO 2 -responsiveness of C 4 photosynthesis. The interactions between elevated p(CO 2 ), leaf temperature and shoot water relations on the growth and photosynthesis of C 4 plants are identified as key areas needing urgent research.
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