Leaf trait covariation and controls on leaf mass per area (LMA) following cotton domestication

The process of domestication has driven dramatic shifts in plant functional traits including leaf mass per area (LMA). It remains unclear whether domestication has produced concerted shifts in the lower-level anatomical traits that underpin LMA and how these traits in turn affect photosynthesis. In this study, we investigated controls of LMA and leaf gas exchange by leaf anatomical properties at the cellular, tissue and whole leaf levels, comparing 26 wild and 31 domesticated genotypes of cotton (Gossypium). As expected, domesticated plants expressed lower LMA, higher photosynthesis and stomatal conductance, suggesting a shift towards the 'faster' end of the leaf economics spectrum. At whole-leaf level, variation in LMA was predominantly determined by leaf density (LD) both in wild and domesticated genotypes. At tissue level, higher leaf volume per area (V_leaf) in domesticated genotypes was driven by a simultaneous increase in the volume of epidermal, mesophyll and vascular bundle tissue and airspace, while lower LD resulted from a dilution effect of lower increased volume of palisade tissue and vascular bundle of high mass density by higher increased volume of epidermis and airspace of low mass density. The volume of spongy mesophyll exerted direct control on photosynthesis in domesticated genotypes but only indirect control in wild genotypes. At cellular level, a shift to larger but less numerous cells with thinner cell walls underpinned a lower proportion of cell wall mass, and thus a reduction in LD. Taken together, cotton domestication has triggered synergistic shifts in the underlying determinants of LMA but also photosynthesis, at cell, tissue and whole-leaf level, resulting in a marked shift in plant ecological strategy.