CHARACTERIZING ENVIRONMENTAL EFFECTS ON THE WATER STATUS OF CUTTINGS ACCLIMATED INDOORS

Vertical indoor propagation (VIP) systems that use sole-source lighting and temperature, relative humidity (RH), and carbon dioxide (CO₂) control are increasingly being used by young-plant growers to start unrooted cuttings (URC) indoors for greenhouse finishing. However, optimal environmental setpoints for VIP systems are unknown. Providing an environment that limits water loss by URC prior to root initiation is particularly critical for VIP systems. Thus, understanding the isolated and combined effects of different environmental factors on the water status of URC will help ensure the rooting success and growth of high-quality liners. In chapter 1, we characterized the effect of blue light and CO₂ concentration on the water status of Chrysanthemum and Begonia cuttings, as both factors are known to affect stomatal behavior of plants. The first experiment evaluated short-term effects of blue light (15% to 60% blue light) on water status and physiological responses by URC. This was followed by a second experiment that evaluated short-term effects on water use (water loss, water uptake) and long-term effects on evapotranspiration, physiological responses, and growth of cuttings under two blue-light treatments (21% or 45% blue light) and two CO₂ concentrations (ambient or high at ~500 or 1200 μmol·mol^–1, respectively). In the first experiment, increasing blue light increased short-term water use but did not affect stomatal conductance (g_s) and transpiration (E), likely due to limitations in stomatal control by URC. Results from the second experiment showed there were few differences in shoot growth and root development in response to blue light at the two CO₂ concentrations applied during indoor acclimation and subsequent greenhouse finishing phases, suggesting that growth of cuttings is more responsive to environmental stimuli after root initiation. When significant, growth responses were species-specific, likely attributed to morphological and anatomical differences. The only effect in long-term evapotranspiration was measured in begonia under high CO₂, which indicated that cuttings under 45% blue light had the highest water loss. This result corresponds with the general findings for g_s and E. In chapter 2, we evaluated the combined effect of photosynthetic photon flux density (PPFD) (0 to 210 µmol·m^–2·s^–1) and vapor pressure deficit of the air (VPD_air) (0.00 to 0.76 kPa) on various environmental factors and on the short-term water status of cuttings, as are both major drivers of water loss. Results showed that PPFD was weakly correlated with both VPD_air and VPDl_eaf, indicating that PPFD had a minimal heating effect on the air and leaves. Furthermore, results evaluating the relationship of both VPD_air and VPD_leaf for predicting the different water status variables showed similar responses, suggesting that measurements of leaf temperature would not be critical for irrigation control in VIP systems, where PPFD is typically relatively low and environmental conditions tend to be constant. In general, the water status of begonia was minimally affected by PPFD and VPD_air, but water status of chrysanthemum was responsive to both variables. For chrysanthemum URC, water loss and water uptake tended to increase in response to increasing PPFD, but there was no response to PPFD in g_s, E, evapotranspiration, and relative water content (RWC). For rooted cuttings (RC), however, g_s and E followed a linear increasing response to increasing PPFD, suggesting they were able to regulate transpirational losses through water uptake from the substrate. Increasing VPD_air linearly increased the rate of water loss, water uptake, and evapotranspiration by chrysanthemum URC and RC, which in turn reduced RWC, whereas the leaf-air temperature difference linearly decreased in response to increasing VPD_air, likely attributed to an increase in evaporative cooling under less saturated conditions. Overall, results from our studies provide baseline information on how different environmental conditions in VIP systems affect water status by cuttings from two ornamental herbaceous species.