The morphogenesis of fast growth in plants

Growth rate represents a fundamental axis of life history variation. Faster growth associated with C₄ photosynthesis and annual life history has evolved multiple times, and the resulting diversity in growth is typically explained via resource acquisition and allocation. However, the underlying changes in morphogenesis remain unknown. We conducted a phylogenetic comparative experiment with 74 grass species, conceptualising morphogenesis as the branching and growth of repeating modules. We aimed to establish whether faster growth in C₄ and annual grasses, compared with C₃ and perennial grasses, came from the faster growth of individual modules or higher rates of module initiation. Morphogenesis produces fast growth in different ways in grasses using C₄ and C₃ photosynthesis, and in annual compared with perennial species. C₄ grasses grow faster than C₃ species through a greater enlargement of shoot modules and quicker secondary branching of roots. However, leaf initiation is slower and there is no change in shoot branching. Conversely, faster growth in annuals than perennials is achieved through greater branching and enlargement of shoots, and possibly faster root branching. The morphogenesis of fast growth depends on ecological context, with C₄ grasses tending to promote resource capture under competition, and annuals enhancing branching to increase reproductive potential.