A generalized scaling approach to improve variety selection in orchard fruit

Background/Question/Methods

Generalized scaling relationships are observed in plants between morphological characteristics such as branch diameter and mass. Broad invariance of scaling relationships across plant species indicates conserved patterns in plant growth and architecture. Such patterns can be used to better understand physiological limitations acting on plant development. Recently, a test of two orchard tree species (apple: Malus domestica, cherry: Prunus cerasus) reveals scaling relationships in agreement with generalized scaling expectations of the Metabolic Scaling Theory (MST). This finding broadens the scope of the MST beyond naturally-occurring plant species to include orchard trees that experience intense human manipulation such as grafting for complex genetics, pruning for biomass removal, and extreme manipulations of the agricultural environment for optimal growth and fruit production.  Despite conserved patterns in morphological characteristics, clear differences arise among the Malus varieties tested in regards to growth potential and fruit production. We propose a novel approach to breeding trials in horticulture that uses generalized scaling expectations to expose differences in growth and architecture that can lead to the selection of varieties for further research and commercial planting.

Results/Conclusions

The Malus varieties tested are all eight-year-old Golden Delicious apples. They differ by rootstock grafting that imposes various growth potentials leading to trunk diameters ranging from 5 - 19 cm. Of the six varieties tested, clear differences appear when standardizing by the generalized scaling expectations of the MST. When using the mass ~ diameter relationship at a branch level, three of the six rootstocks have slightly higher mass than expected by diameter while the other three have slightly lower mass than expected by diameter. The three rootstocks with higher mass ~ diameter relationships appear to be preferred for commercial planting for their relatively increased yield potential. All rootstocks expressed higher branch length than expected by mass and higher length than expected by diameter. However, some individuals overlap with the length ~ diameter expectation. When all rootstocks deviate significantly from scaling expectations of the MST, the mean estimated scaling exponent can be used to differentiate the rootstocks. Generalized scaling theory is a valuable tool for horticulture that reveals important differences among research varieties. Generalized scaling theory links physiological limitations of plant growth and architecture to the optimized growth, reproduction, and resource use goals of plant breeding.