Geographic variation in shoot structure linked to fruit size in Camelia japonica

Shoot structure is critically important for efficient photosynthesis and the growth and survival and reproduction of shoots and individual trees. The generality of scaling relationships between multiple shoot traits, known as Corner’s rules, has been considered to reflect the biomechanical limits of trees and tree organs. Variation in fruit size within species would be expected to affect shoot structure in a similar fashion to leaves, by changing the mechanical and hydraulic stresses caused by the mass and water requirement of fruits. Yet, hypotheses related to variation in fruit size within species have rarely been focussed on in previous studies of shoot structure.

In their new study published in AoBP, Goto & Osada investigated the differences in shoot structure and their relationship with fruit size in Camelia japonica in southern Japan. C. japonica (Theaceae) is a shade-tolerant evergreen broad-leaved species native to warm-temperate forests in Japan and the southern and western coasts of the Korean Peninsula. C. japonica is known to produce larger fruits with thicker pericarps in more southern populations because warmer climates induce a more intensive arms race between fruit size and the rostrum length of its obligate seed predator, the camelia weevil Curculio camelliae.

Goto & Osada found that this geographic variation in fruit size resulted in thicker 1-year-old stems in more southern populations, to withstand heavier loads and increased water requirements of the fruit. Such patterns were constrained by the allometric relationships between stem diameter and total mass of the 1-year-old shoot complex, which did not differ across the trees of different latitudes.

Overall, these results strongly suggest that shoot allometry is constrained by fruits in C. japonica, and evolutionary changes in fruit size resulted in thicker 1-year-old stems to withstand the loads and/or water requirements of fruits. The authors conclude that the effects of fruit size on shoot structure and allometry within species are important to understand how the Corner’s rules are physiologically regulated and restricted both within and among species.