Mesophyll conductance in cotton bracts: anatomically determined internal CO2 diffusion constraints on photosynthesis.

Jimei Han,Yali Zhang,Yujie Zhang,Zhangying Lei,Wangfeng Zhang,Marc Carriquí,Jaume Flexas

doi:10.1093/jxb/ery296

Abstract

Mesophyll conductance (gm) has been shown to affect photosynthetic capacity and thus the estimates of terrestrial carbon balance. While there have been some attempts to model gm at the leaf and larger scales, the potential contribution of gm to the photosynthesis of non-leaf green organs has not been studied. Here, we investigated the influence of gm on photosynthesis of cotton bracts and how it in turn is influenced by anatomical structures, by comparing leaf palisade and spongy mesophyll with bract tissue. Our results showed that photosynthetic capacity in bracts is much lower than in leaves, and that gm is a limiting factor for bract photosynthesis to a similar extent to stomatal conductance. Bract and the spongy tissue of leaves have lower mesophyll conductance than leaf palisade tissue due to the greater volume fraction of intercellular air spaces, smaller chloroplasts, lower surface area of mesophyll cells and chloroplasts exposed to leaf intercellular air spaces and, perhaps, lower membrane permeability. Comparing bracts with leaf spongy tissue, although bracts have a larger cell wall thickness, they have a similar gm estimated from anatomical characteristics, likely due to the cumulative compensatory effects of subtle differences in each subcellular component, especially chloroplast traits. These results provide the first evidence for anatomical constraints on gm and photosynthesis in non-leaf green organs.

Highlights

To reach the sites of carboxylation within chloroplasts of leaves of C3 plants, CO2 must diffuse through stomata and mesophyll
Lower assimilation rate (AN) in bracts than in leaves is due to co-limiting CO2 diffusion and biochemistry It is well known that leaves are the main photosynthetic organs in plant species, but numerous researchers have shown that non-leaf green organs are an important source of assimilated carbon (Tambussi et al, 2007; Redondo-Gómez et al, 2010; Pengelly et al, 2011; Hu et al, 2013; Jia et al, 2015; Zhang et al, 2015) and make a considerable contribution to terrestrial carbon exchange
Leaf thickness was 1.25 times larger than bract thickness and leaf density was 1.89 times larger than bract density (Table 3). These results suggest that higher density in the leaf mainly contributed to larger Leaf mass per unit area (LMA).A lower proportion of mesophyll and a higher fias due to random cell arrangement and lower cell numbers led to lower density in bracts (Table 3; Supplementary Fig. S3).While early studies have shown that there is a negative relationship between LMA and gm across broad functional groups and within species (Flexas et al, 2008; Niinemets et al, 2009; Galmés et al, 2011;Tosens et al, 2016), this is not consistent with our results, which show that bracts have lower LMA than leaves despite having a higher Tcw (Table 3)

Summary

Introduction

To reach the sites of carboxylation within chloroplasts of leaves of C3 plants, CO2 must diffuse through stomata and mesophyll. Regarding the liquid phase conductance, it is mainly constrained by the cell wall thickness (Tcw), the chloroplast dimensions, and the mesophyll and chloroplast surface area exposed to leaf intercellular air spaces (Sm/S and Sc/S) (Evans et al, 1994, 2009; Tosens et al, 2012b; Tomás et al, 2013) These anatomical structures have been observed to strongly differ between different species (Tomás et al, 2013; Peguero-Pina et al, 2016) or even within the same species growing under complex and variable growth environments (Terashima et al, 2011;Tosens et al, 2012a)

Objectives

Methods

Results

Discussion

Conclusion