Effects of air humidity on growth, ̄owering, keeping quality and water relations of four short-day greenhouse species

The short-day plants Begonia cheimantha, Chrysanthemum morifolium (one pot and one cut cultivar), Euphorbia pulcherrima (poinsettia) and Kalanchoe blossfeldiana were grown at vapour pressure de®cits (VPDs) of 155, 420 and 660 Pa (corresponding to 93, 81 and 70% relative air humidity, respectively). The lowest VPD delayed time to ̄owering by three to four days in pot and cut chrysanthemum but did not affect the other species. When decreasing VPD from 660 to 420 Pa, plant height was increased in chrysanthemum, Kalanchoe and poinsettia. A further increase in humidity increased the height in chrysanthemum and poinsettia, but not in Begonia. Leaf size in poinsettia and chrysanthemum was increased when VPD was decreased, but was reduced in Kalanchoe at the lowest VPD. High humidity enhanced the plant dry weight of poinsettia and Kalanchoe, decreased it in Begonia and had no signi®cant effect in chrysanthemum. The highest plant quality was generally produced under the lowest humidity, with the development of more compact plants. Keeping quality as tested under indoor conditions was the same irrespective of humidity in Begonia, poinsettia and pot chrysanthemum, highest at 660 Pa VPD in Kalanchoe and at 155 Pa VPD in cut chrysanthemum. The water consumption per leaf area unit during growth was decreased by 39±58% in chrysanthemum, Kalanchoe and poinsettia when VPD was decreased from 660 to 155 Pa. Water loss from detached leaves generally increased with decreasing VPD during growth, particularly in Begonia and poinsettia. # 2000 Elsevier Science B.V. All rights reserved.