Modification of leaf morphology and anatomy as a consequence of columnar architecture in domestic apple (Malus × domestica Borkh.) trees

• Leaves directly subtending fruits are morphologically modified in columnar apples.
• Leaf modifications affect water relations and carbon fixation.
• Columnar cultivars have increased palisade mesophyll compared to standard ones.
• Columnar architecture significantly alters the environment for leaf development.

A quantitative study has been made of the modifications to leaf morphology and anatomy evident in columnar apples trees when compared to standard ones, using the original cultivar and the first columnar mutant derived from it, as well as other closely and more distantly related cultivars. Significant increases in leaf number, area, weight per unit area, thickness and midrib angle, together with altered shape, have been recorded consistently for the leaves subtending the developing fruits of the columnar cultivars. Additionally, significant increases in leaf rolling, epicuticular wax, stomatal size and density and leaf chlorophyll content were recorded. Scanning electron microscopy revealed that columnar cultivars develop an additional layer of well-developed palisade mesophyll and a greater depth of spongy mesophyll when compared to their standard counterparts. This large-scale modification of leaf characteristics is considered in terms of the very open architectural phenotype of columnar trees and the impact this may have on the canopy microclimate that influences leaf development.