Genotypic Variation in Growth, Gas Exchange, and Water Use Efficiency of Coleus under Well-watered and Mild Substrate Drought Conditions

Coleus ( Coleus scutellarioides ) is a widely cultivated ornamental plant with high water demand; therefore, improved water use efficiency (WUE) is an important breeding objective. This study evaluated the drought responses of C. formosanus (CF), ‘Wizard Jade’ (JA), and their interspecific hybrid (JA × CF) under two substrate volumetric water content (VWC) regimes (40% well-watered and 20% mild drought) during winter and summer. Growth, biomass accumulation, leaf gas exchange [net photosynthesis ( Pn ), μmol CO 2 ·m −2 ·s −1 ; stomatal conductance ( gs ), mol H 2 O·m −2 ·s −1 ; transpiration rate ( E ), mmol H 2 O·m −2 ·s −1 ; and intercellular CO 2 concentration ( Ci ), μmol·mol −1 ], and WUE-related indices [intrinsic WUE (iWUE); instantaneous WUE (WUEinst); transpiration coefficient; and intercellular-to-ambient CO 2 ratio ( Ci / Ca )] were assessed. Mild drought significantly reduced branch number, leaf number, leaf area, and total dry weight per plant in all genotypes during summer. Under both VWC regimes, JA × CF consistently produced the greatest biomass. In winter, CF and JA × CF exhibited similarly high iWUE and WUEinst; however, in summer, CF showed a distinct advantage. Across both seasons, CF consistently exhibited a lower transpiration coefficient and Ci / Ca ratio than those of JA, as did JA × CF at 40% VWC in summer, when the two genotypes did not differ significantly. In contrast, JA showed the lowest Pn and WUE and the highest Ci and Ci / Ca ratio. Effective use of water (total dry weight per plant per cumulative irrigation volume) was greater under 20% VWC than under 40% VWC in all genotype × season combinations, except the difference was not statistically significant for CF in summer. The hybrid JA × CF combined the variegated foliage of JA with vigorous growth and improved water use performance, suggesting partial inheritance of drought-adaptive morphophysiological traits from CF.