Effects of blue-light percentage and carbon dioxide concentration on the water status and growth of chrysanthemum and begonia cuttings in vertical indoor propagation systems

Managing environmental conditions to limit water loss of cuttings during the initial propagation phase (“acclimation”) is critical. Using chrysanthemum (Chrysanthemum indicum) and begonia (Begonia × hiemalis) cuttings, our objectives were to characterize 1) short-term effects of blue light [15 % to 60 % at a photosynthetic photon flux density (PPFD) of 70 µmol·m^–2·s^–1] on water use (Experiment 1); and 2) short- and long-term effects on water use and growth of cuttings acclimated under two blue-light treatments [21 % (21B) or 45 % (45B)] at two carbon dioxide (CO₂) concentrations (ambient or high at ∼500 or 1200 μmol·mol^–1, respectively) (Experiment 2). Results from Experiment 1 generally showed that increasing blue light increased water loss and water uptake by unrooted cuttings. However, no differences were measured in stomatal conductance (g_s) or transpiration (E), suggesting that stomata may be unresponsive to blue light during acclimation. In contrast, increasing blue light linearly increased g_s and E of rooted begonia cuttings, but no differences were measured in water use. There were few growth differences in response to blue light at the two CO₂ concentrations evaluated in Experiment 2, suggesting that cuttings may be more responsive to some environmental stimuli after initial root development. Various responses were species-specific. For example, begonia cuttings were shorter and produced smaller leaves under 45B than 21B when acclimated under high CO₂. In contrast, chrysanthemum cuttings were slightly taller under 45B and high CO₂ but were otherwise generally unresponsive to blue light. Evapotranspiration only increased in response to blue light when begonia cuttings were acclimated under high CO₂. Overall, our findings show that blue-light percentage and CO₂ concentration may not have large effects on rooting and growth of ornamental cuttings during acclimation, but some physiological and water-use differences can be expected, particularly under different blue-light percentages.