Influences of LED Light Quality and Intensity on Stomatal Behavior of Three Petunia Cultivars Grown in a Semi-closed System

Petunia (Petunia hybrida Vilm.) is one of the most popular ornamental potted and bedding plants with an annual wholesale value exceeding 100 million dollars in United States (Anderson, 2005). However, conventional petunia seed and plant production is often limited by climatic condition, especially in a tropical region. An alternative petunia cultivation method is needed in order to supply enough petunia seeds and plants to meet the rising market demand. Recently, plant cultivation under controlled environmental conditions has been receiving more attention in many countries. Controlled environmental systems can produce higher quality plants that require lower pesticide use compared to conventional, open field plant production. In the case of plant factories with artificial light (PFAL), in which the system is airtight and all of the major environmental factors are tightly regulated, plant production can be done regardless of location and climatic condition (Kozai and Niu, 2016). The typical PFAL consists of an artificial light source, air conditioning, fertigation system, CO2 enrichment system and an environmental control unit. These components are used to control all the major environmental factors to match plant demands and maximize productivity. Hence, knowledge of the physiological behaviors of certain plant species is pivotal for optimizing environmental parameters inside these controlled systems. Stomatal behavior strongly influences photosynthesis in plants. Opening and closing of stomata directly affects stomatal conductance which is correlated with CO2 assimilation in most plants (Hogewoning et al., 2010; Kim et al., 2012). Stomatal movement occurs partially in response to a change in quantity and quality of external factors such as light (Wheeler et al., 1999; Mott et al., 2008; Araújo et al., 2011). For instance, daily changes in stomatal conductance are positively correlated with sunlight intensities in glasshouse grown grapes (Sabir and Yazar, 2015). In cucumber, both CO2 assimilation rate and stomatal conductance increased in response to an increase in the proportion of blue light (Hogewoning et al., 2010). Similarly, opening of stomatal aperture was found to be stimulated by blue light in various other plants (Lu et al., 1993; Assmann and Shimazaki, 1999; Talbott et al., 2002). Moreover, it has been reported that stomatal response to red light is photosynthesis-dependent, whereas the response to blue light is both photosynthesis-dependent and independent (Wang et al., 2011). Apart from light, other environmental factors, such as vapor pressure deficit (VPD), CO2 concentration and temperature, have also been shown to affect stomatal behavior (Miller and Davis, 1981; Wheeler et al., 1999; McAdam and Brodribb, 2015). In a controlled environmental cultivation system where light cycle, temperature, relative humidity and CO2 concentration are fairly constant, daily oscillation of stomata related parameters have been observed (Kerr et al.,