Stomata and long-term exposure to high relative air humidity

Long-term exposure to high relative air humidity (RH) causes poor control of water loss after transferring to low RH. This phenomenon is thought to be due to stomatal behavior. In this study we have investigated the stomatal characteristics and behavior of well-watered Tradescantia virginiana plants grown in climate rooms at moderate (55%) and high (90%) RH. The stomatal responses to treatments that normally induce stomatal closure i.e. desiccation, abscisic acid (ABA) application and exposure to darkness were studied. Stomatal size, density and index (a ratio of stomata to all epidermal cells), leaf transpiration rate, stomatal conductance and stomatal aperture were measured. Stomatal closure and heterogeneity in response to desiccation was studied using a chlorophyll fluorescence imaging system under a non-photorespiratory condition. Bigger stomata and lower stomatal density were found in plants grown at high RH. However, there was no significant difference in stomatal index. The stomata responded to desiccation, ABA and darkness in both moderate and high RH plants but to different extents. In high RH plants, transpiration rate, stomatal conductance and aperture were always higher than in moderate RH plants. During desiccation different trends of stomatal heterogeneity were found in leaves grown at moderate and high RH. Following desiccation leaves grown at high RH had both a higher heterogeneity and a higher average value of PSII efficiency compared to leaves grown at moderate RH. This confirmed non-uniform closure of stomata and the presence of some partly or completely non-functional stomata amongst normal stomata in leaves grown at high RH. These non-functional stomata were distributed mostly around the main vein.