Medial pith cells per meter in twigs as a proxy for mitotic growth rate ({Phi}/m) in the apical meristem

The Φ model for plant mating system evolution proposes a causal link between Φ, the number of mitoses that occur within a plant's lifetime from zygote to gamete production, and constraints on the evolution of inbreeding depression and thereby on the evolution of plant mating systems. Through its use of plant stature, the Φ model emphasizes the important role of morphology in creating developmental and genetic constraints on plant evolution. However, the estimation of Φ itself is likely to be extraordinarily complex. Here I describe a protocol for estimating Φ per meter linear growth by an apical meristem (Φ/m) using medial pith cells from mature internodes of twigs. While such cells are produced by the apical meristem, during internode elongation, these pith cells also undergo further mitoses, and thus their measurement can only approximate a "true" Φ/m via the application of a multiplier (the adjustment ratio) that partially corrects for the occurrence of cell divisions and cell growth beyond the apical meristem. I applied this method to Delonix regia (Caesalpiniaceae) and derived several adjustment ratios from the literature. Because variation in Φ/m can have profound evolutionary implications, I also examined interspecific and intraspecific variation as well as within-individual variation in Φ/m. Conifers apparently have lower Φ/m than do angiosperms, while 20% of the total variance in Φ/m for D. regia was found among individual trees, with the remainder found within trees. Given the large differences in stature between "high-Φ" plants such as trees and "low-Φ" plants such as herbs, these results support the idea that the total per-generation mutation rate for high-Φ plants is likely to be many times higher than that for low-Φ plants.