Low air humidity during cultivation promotes stomatal closure ability in rose

In greenhouse horticulture, evaporative demand varies between seasons. For instance, plants are typically exposed to low relative air humidity (RH) during summer, whereas elevated RH prevails in winter. Since high RH during cultivation impairs stomatal functioning, some opposite changes might be expected, when plants are subjected to long-term low RH. To investigate this, Rosa hybrida ‘Pasadena’ was cultivated at 40, 60 or 90% RH. Plant performance, transpiration, stomatal closing ability and anatomy were recorded. As RH increased from 40 to 60% as well as from 60 to 90%, plants showed larger leaf area and thinner leaves. Plant water loss was mainly determined by ambient RH in the growing environment, with stomatal conductance (gs) being of secondary importance. With increasing RH, plant transpiration declined at growth environment. Larger stomata were found at 90% RH, as compared to 40 or 60%. Stomatal physiology was considerably affected by 90% RH, including reduced gs oscillations within the photoperiod, attenuated opening response following dark/light transition, as well as reduced closing response upon darkening. The plants cultivated at 90% RH had a reduced ability to control water loss upon water deprivation, compared to those grown at 60%. In contrast, cultivation at 40% RH resulted in stomata, which were much more responsive to water stress, compared to 60% RH-grown plants. This superiority was dual: lower transpiration rate combined with less severe leaf drying to induce stomatal closure. In conclusion, low RH during cultivation, which is typical during summer, leads to thicker leaves with very responsive stomata.