The influence of orchard row canopy discontinuity on irradiance and leaf area distribution in apple trees

Photosynthetic irradiance and leaf area distributions were determined in individual trees of ten year old 'Royal Gala'/MM.106 apple planted at 4.5 x 2.0 m and 5.0 X 3.0 m. Since tree height, across-row profile, and management history were similar, the extent of canopy discontinuity between trees along the row in response to tree spacing represented the main difference between the planting systems. Total daily photosynthetic photon flux (PPF) to deep within canopies was higher in the wider-spaced trees because attenuation of PPF from mid to lower canopy regions was less pronounced than with close-spaced trees. The differences were evident under both diffuse and direct light conditions, although in direct light, decline in irradiance occurred mainly in the eastern and southern aspects of the canopy. The patterns of irradiance under partially cloudy conditions were very similar to those in direct light. Light interception was 59% by trees spaced 3.0 m apart in the row and 64% by trees spaced 2.0 m apart in the row. LAI was significantly higher in trees planted at 4.5 m x 2.0 m, the difference caused by greater shoot leaf area in the mid and upper canopy regions. LAD was higher in all parts of the canopy in the closer-spaced trees. Many of the irradiance and leaf canopy properties of trees planted at 5.0 x 3.0 m approached optimum values modelled for efficient orchard production systems. The roles of the conical tree form and discontinuity between adjacent trees along rows or in beds are discussed in relation to orchard canopy efficiency.