Differential growth and photosynthetic response of selected cultivars of English ivy to irradiance

Effects of irradiance on growth and net photosynthesis were studied in four cultivars of Hedera helix L. grown in controlled environments at a mean daily temperature of 24degreesC. Shoot dry weights of the variegated cultivars Ingelise and Mini Adam were greatest at around 2.9 MJ m(-2) d(-1). For the two green cultivars, with increasing daily light integral from 1.2 to 7.2 MJ m(-2) d(-1), shoot dry weight increased in 'Dark Pittsburg', but decreased in 'Evergreen'. Shoot or internode lengths, and leaf area decreased linearly while leaf thickness increased as light integral increased from 1.7 to 7.2 MJ m(-2) d(-1) in cultivars Ingelise, Dark Pittsburg and Evergreen. Leaf variegation increased in 'Ingelise' under 4.3 or 7.2 MJ m(-2) d(-1), while leaf variegation in 'Mini Adam' was affected little by irradiance. Plants of 'Dark Pittsburg' had both higher light-saturated net photosynthesis rate and light saturation point than other cultivars. The variegated cultivars Ingelise and Mini Adam had higher light compensation points than the green cultivars Dark Pittsburg and Evergreen.