Growth of pot roses and post-harvest rate of water loss as affected by air humidity and temperature variations during growth under continuous light

The effect of diurnal variations in air humidity and temperature under continuous lighting period (LP) on growth, flowering and water loss were studied in two pot-rose cultivars. Experiment one consisted of a constant temperature of 21 degrees C and two constant relative air humidities (RH; low, 60%; high, 80%), corresponding to vapour pressure deficits (VPD) of 0.5 and 1.0 kPa, respectively) and two diurnal variations in humidity between 60 and 80% RH (RH reduced from 80 to 60% for 6 and 12 h, respectively). Experiment two consisted of one constant temperature (21 degrees C and three combinations of diurnal temperature variations between 21 and 27 degrees C (temperature increased from 21 to 27 degrees C for 6, 12 and 18 h) under continuous lighting. VPD at 21 and 27 degrees C was 0.5 and 0.7 kPa. In both experiments plants grown at 18 h photoperiod, 80% RH and 21 degrees C were used as comparisons. Parameters measured were growth, stomatal conductance and leaf-water loss under post-harvest conditions. Air humidity had no effect on the number of flowers and days to flowering of plants grown under continuous lighting. Increasing the lighting period from 18 to 24 h day(--1) increased the number of flowers by 34% and decreased the number of days to flowering by 12%. Increasing average daily temperature decreased days to flowering and increased mean growth rate. Stomatal conductance of plants grown at constant 80% RH showed that the stomata failed to close under post-production conditions and darkness. The rate of leaf-water loss during the first hour after detachment from plants grown at constant high RH and continuous lighting was much greater than of plants grown at constant low RH or diurnal variation between low and high RH. It was found that temperature variation and air humidity variation were equally effective for avoiding stomata malfunction of roses grown under continuous lighting. It is concluded that high constant RH under continuous illumination causes malfunctioning of the stomata, and that a daily variation in VPD is required to ensure the development of normal functional stomata. (C) 2007 Published by Elsevier B.V.