Abstract

Cell sizes are linked across multiple tissues, including stomata, and this variation is closely correlated with genome size. These associations raise the question of whether generic changes in cell size cause suboptimal changes in stomata, requiring subsequent evolution under selection for stomatal size. We tested the relationships among guard cell length, genome size and vegetation type using phylogenetically independent analyses on 67 species of the ecologically and structurally diverse family, Proteaceae. We also compared how genome and stomatal sizes varied at ancient (among genera) and more recent (within genus) levels. The observed 60-fold range in genome size in Proteaceae largely reflected the mean chromosome size. Compared with variation among genera, genome size varied much less within genera (< 6% of total variance) than stomatal size, implying evolution in stomatal size subsequent to changes in genome size. Open vegetation and closed forest had significantly different relationships between stomatal and genome sizes. Ancient changes in genome size clearly influenced stomatal size in Proteaceae, but adaptation to habitat strongly modified the genome–stomatal size relationship. Direct adaptation to the environment in stomatal size argues that new proxies for past concentrations of atmospheric CO2 that incorporate stomatal size are superior to older models based solely on stomatal frequency.

Highlights

  • Stomata are critical to the ability of plants to thrive on land

  • This intimate link to the uptake of CO2 means that stomata are pivotal in terrestrial primary productivity, but can be used to estimate how atmospheric CO2 has changed through time (Royer, 2001; Grein et al, 2013)

  • Recent work has shown that the size of stomata is important for whole-plant function because the geometry of stomata combined with constraints on how many stomata can be packed into an area of leaf means that leaves with large stomata tend to have lower maximum capacity to absorb CO2 (Franks & Beerling, 2009; Brodribb et al, 2013)

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Summary

Introduction

Stomata (the microscopic valves that regulate the evaporative loss of water while leaves absorb CO2) are critical to the ability of plants to thrive on land. Both the size and abundance of stomata are important because together they determine the maximum capacity of leaves to absorb CO2. This intimate link to the uptake of CO2 means that stomata are pivotal in terrestrial primary productivity, but can be used (when fossilized) to estimate how atmospheric CO2 has changed through time (Royer, 2001; Grein et al, 2013). The relationship with guard cell size is of particular interest because several authors have attempted to use the size of fossil guard cells to reconstruct evolutionary trends in genome size (Franks et al, 2012; Lomax et al, 2014) or degrees of ploidy (Masterson, 1994)

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