Acclimation of mango (Mangifera indica cv. Calypso) to canopy light gradients - scaling from leaf to canopy

Mango (Mangifera indica), a leading tropical fruit crop, is a prime candidate for intensification through modern orchard-management techniques, including canopy manipulation to improve light interception. This study investigated how leaf-level acclimation to light gradients within the canopy of a high-yield, dwarfing mango cultivar (Calypso™) could be used to examine integrated canopy-scale responses. We quantified foliar morphological, biochemical, and physiological traits across a range of canopy positions using this information to model canopy-scale productivity within digital-twin representations of mango under both conventional open-vase and espalier training canopy systems.

Key findings demonstrated that leaves exposed to higher light exhibited increased leaf mass per area (LMA), nitrogen content, and photosynthetic capacity (Asat), but decreased chlorophyll-to-nitrogen ratios and photochemical reflectance indices, reflecting trade-offs between light capture and photoprotection. Phenolic content increased under high irradiance, indicating investment in photoprotective compounds at the expense of net carbon gain. Modelled leaf-level net primary productivity (NPP) increased with light availability, following a Michaelis-Menten saturating response, with diminishing returns under high light. Digital modelling of canopy light interception revealed that espalier-trellis training enhanced light distribution efficiency per unit leaf area but resulted in a 6.5% reduction in total canopy NPP due to a smaller total leaf area. However, when normalized by leaf area, the espalier-trellis system showed a 3.6% productivity advantage over conventional canopies at the time of year modelled.

These results highlight the role of canopy structure and light-use efficiency play in determining orchard productivity. Integrating spatially explicit mechanistic models with LiDAR-derived canopy data offers a promising pathway for designing high-density, resource-efficient mango orchards. Future work should expand modelling to account for dynamic canopy shape throughout the growing season and evaluate the interaction of modified canopy structures with environmental stressors, particularly under climate variability.

Methods
Leaf samples from across a range of light environments were collected from an even-age cohort over 4 days at the Walkamin Research Station of the Queensland Department of Primary Industries, Mango Planting System Experiment (17° 8′ 17′′ S145° 25′ 41′′ E; elevation 599 m). The site has a 10-year mean daily minimum and maximum temperature of 17.2 ± 3.3 and 28.1 ± 3.5 °C respectively with a distinct dry/cool, wet/hot tropical climate.

After characterisation of the foliage-specific light environment, data on leaf-level gas exchange were collected in-situ using a portable photosynthesis analyser (LI-6400XT, LiCOR Biosciences, Lincoln NE, USA). Subsequently, leaves were detached, weighed for fresh weight, measured for lamina thickness, scanned  to determine leaf area, and analysed for chlorophyll content and light transmission/reflectance and by difference absorption for shortwave radiation (400-1000nm). Fresh leaves were then wrapped in aluminium foil and snap frozen in liquid nitrogen (~20 min after harvest) and held at −80°C until lyophilisation. Dried samples were then reweighed to determine dry leaf mass and ground to a fine powder before determination of basic biochemical characteristics including total C and N, δ13C, and total phenolic content.

For detailed methods see accompanying paper