Impact of Drought and Temperature on Growth and Leaf Gas Exchange of Six Bedding Plant Species Under Greenhouse Conditions

The effect of drought on the growth and gas exchange of six bedding plant species--agastache [Agastache urticifolia (Benth.) O. Kuntze 'Honeybee Blue'], dusty miller (Cineraria maritima L. 'Silverdusty'), petunia (Petunia xhybrida 'Wave Purple'), plumbago (Plumbago auriculata Lam. 'Escapade'), ornamental pepper (Capsicum annuum L. 'Black Pearl'), and vinca [Catharanthus roseus (L.) G. Don 'Titan']--was quantified under greenhouse conditions. Seeds were sown in January and seedlings were grown in the greenhouse until 18 Apr., when two irrigation treatments--drought (D, approximately equal to 18% volumetric moisture content at reirrigation) and control (C, approximately equal to 25% volumetric moisture content at reirrigation)--were initiated. Leaf net photosynthetic rate (P(n)), stomatal conductance (g(s)), and transpiration (E) were determined in response to a range of substrate moisture content (from approximately equal to 5% to 30% by volume) and temperature (from 20 °C to 40 °C). Dry weight of agastache, ornamental pepper, and vinca was unaffected by drought, whereas that of other species was reduced. Leaf area of plumbago and height of plumbago and vinca were reduced by drought. As substrate moisture content decreased from 25% to 10%, P(n), E, and g(s) decreased linearly in all species except petunia and plumbago. Leaf net photosynthetic rate of all species declined as leaf temperature increased from 20 °C to 40 °C. In contrast, E of all species, except petunia, increased as temperature increased. Transpiration rate of petunia increased as temperature increased from 20 °C to 30 °C, and then decreased between 30 °C and 40 °C. Although petunia had the highest P(n) among the tested species, its P(n) and g(s) declined more rapidly compared with the other species as temperature increased from 20 °C to 40 °C or as substrate moisture content decreased, indicating that petunia was most sensitive to high temperature and drought.