The [CO2] in greenhouse cultivation during daylight hours is often lower than the atmospheric level, possibly as low as 200 µmol·mol−1 when ventilation is limited (Kläring et al., 2007; Sánchez-Guerrero et al., 2005). In such cases, CO2 fertilization can avoid the inhibition of photosynthesis and effects on plant growth (Mortensen, 1987; Nilsen et al., 1983). Low [CO2] might also affect plant characteristics such as resistance to biotic stresses, because when photosynthesis is limited, plants might not allocate sufficient resources to defense responses (Bazzaz et al., 1987; Bryant et al., 1983). Resistance to biotic stresses might have been lower under the lower ambient [CO2] levels of the past (Sage and Cowling, 1999), but this possibility has not been well studied experimentally.
The effects of [CO2] on the development of foliar diseases has been investigated at [CO2] from atmospheric (≈380 µmol·mol−1) to higher (650–800 µmol·mol−1) levels, covering the possible range of [CO2] expected in the future. Several studies reported that higher [CO2] suppresses the development of foliar diseases (e.g., powdery mildew, blight, leaf spot) in some plant species through changes in leaf properties that defend against infection and the development of pathogens (Manning and Tiedemann, 1995; Mathur et al., 2013; McElrone et al., 2005; Strengbom and Reich, 2006), although conflicting results were also reported (Kobayashi et al., 2006; Lake and Wade, 2009). It had been suggested that a greater leaf mass per area and lower N content and the consequent increase in C:N ratio of host plant at higher [CO2] suppress the development of foliar diseases. Physiological responses opposite to those observed under higher [CO2] are likely to occur under lower [CO2] because of the reduction in C uptake due to photosynthetic reduction (Gerhart and Ward, 2010; Sage, 1995).
The present study evaluated the development of powdery mildew, a foliar disease caused by the fungus P. xanthii, on leaves of cucumber (C. sativus L.) acclimatized to [CO2] levels both lower and higher than atmospheric, and investigated the relationships between susceptibility and leaf properties. The goal was to evaluate plant–pathogen interactions under the low to high [CO2], which can occur in greenhouse cultivation.
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