The effects of high relative air humidity on stomatal behaviour
As a result of contemporary horticultural practices relative air humidities (RH) in greenhouses are often very high. In particular in cut flowers, this results in quality problems after harvest when flowers are transferred to low RH conditions at the consumers. The quality problems are related to excessive water loss caused by a disturbance in normal functioning of stomata. The general aim of this study was to investigate the effects of high RH during growth on the stomatal response characteristics of Tradescantia virginiana L. To reach this aim, the quantitative effects of moderate (55%) and high (90%) RH during growth on the stomatal anatomy and responses of well-watered plants in response to desiccation, abscisic acid (ABA) application and light/dark transition were studied. Higher variability of stomatal closure and the presence of some non-closing stomata in high RH grown plants were striking. The dynamics of spatial heterogeneity of stomatal closure within a leaf altered by growth at high RH were studied using a chlorophyll fluorescence imaging system under non-photorespiratory conditions, and the estimates of stomatal closure obtained by means of ΦPSII measurements was correlated with direct measurements of stomatal closure and water relation parameters in plants subjected to water stress. It was shown that following desiccation, leaves grown at high RH had both a greater heterogeneity and a higher PSII efficiency compared to leaves grown at moderate RH. The stomata of high RH grown leaves were less sensitive to decreases in leaf relative water content and water potential. Moreover, the role of ABA in the disturbed stomatal responses of plants during growth at high RH was studied. The concentration of ABA in leaves grown at moderate and high RH was compared and stomatal responses to short-term ABA application, and stomatal responses to long-term ABA application was analysed. The results reinforce the proposal that a low ABA concentration in well-watered plants during growth at high RH could be a reason for the structural or physiological changes of stomata. In addition, the consequences for stomatal responses of maintaining a high or low RH around only one leaf are described. The rate and the reversibility of adaptation of stomatal behaviour to moderate or high RH was also investigated and correlated with leaf ABA concentration and leaf hydraulic properties of the adapted plants. The main achievements and practical implications of this study are discussed, and suggestions for further research are presented.