In recent years, concerns regarding possible links among climate, plant biology, aerobiology, and public health (Ziska et al., 2008) have increased. Atmospheric CO2, the main input needed for photosynthesis, stimulates plant growth; however, the rate of stimulation depends on the species. Plants with an indeterminate growth habit will benefit immensely because additional carbon can increase the production of branches/tillers and result in more nodes/fruiting sites on all branches, thus adding more potential for flowers to produce additional pollen (Reddy and Hodges, 2000; Ziska and Caulfield, 2000; Jablonski et al., 2002; Kimball et al., 2002; Long et al., 2005; Long et al., 2006; Easterling et al., 2007). In addition, plants grown in elevated CO2 can also stimulate earlier flowering and sustained growth due to additional carbon, which will lead to a longer flowering period and pollen production range. Higher temperatures, on the other hand, will increase the rate of development, and in most cases promote early flowering and also a greater number of flowers and pollen if the change in temperature is small. If the change in temperature is large, flower and pollen vitality (production, viability and germination) will be curtailed regardless of other factors such as elevated CO2. Other climate change factors, such as elevated UV-B, normally suppress flower and pollen production. However, the effects vary depending on the species and cultivars within a species (Teramura, 1983; Caldwell et al., 1989; Teramura and Sullivan, 1994; Rozema et al., 1997; Krupa, 1998; Searles et al., 2001; Kakani et al., 2003a).
The influence of multiple climatic variables (e.g., UV-B radiation, CO2, and temperature) is limited, and a definite conclusion regarding the combined effects of these variables on plant pollen is harder to assess at this time. Studies conducted across natural gradients in climatic factors from rural to urban areas indicate that increasing levels and temporal shifts in aeroallergen production and allergenicity are linked to rising temperatures and/or CO2 (Ziska et al., 2003; Ziska and George, 2004; Mohan et al., 2006; Rogers et al., 2006; Ziska et al., 2008). However, quantity and seasonality of pollen production depend on the plant response to environmental conditions. As previously indicated, several climate change factors influence pollen production of not only crop plants, but also several weed species (Ziska and Caulfield, 2000; Ziska et al., 2003) and allergenic pollen-producing tree species (Emberlin et al., 2002; Wan et al., 2002). Allergenic tree pollen from birch showed earlier spring floral initiation and pollen release in response to spring warming (Emberlin et al., 2002). Similarly, a simulated increase of summer temperature (+4°C) increased growth and re-growth following cutting, with an 85% increase in overall pollen production (Wan et al., 2002). Research on loblolly pine (Pinus taeda L.) also showed that elevated CO2 resulted in early pollen production from younger trees and greater seasonal pollen production (LaDeau and Clark, 2006). Recent research also suggested that algernicity associated with poison ivy will increase with rising CO2 (Mohan et al., 2006). As described in the previous section, climate change factors can also influence fungal spore production, which can influence allergen production. Increased exposure to allergic fungal spores can also influence human diseases, such as asthma (Dales et al., 2004). The linkage between aeroallergen production and alergenicity and the incidence of asthma intensity and incidence needs further investigation.
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