Planta

, Volume 177, Issue 2, pp 207–216

Separating the contribution of the upper and lower mesophyll to photosynthesis in Zea mays L. leaves

  • S. P. Long
  • P. K. Farage
  • H. R. Bolhár-Nordenkampf
  • U. Rohrhofer
Article

DOI: 10.1007/BF00392809

Cite this article as:
Long, S.P., Farage, P.K., Bolhár-Nordenkampf, H.R. et al. Planta (1989) 177: 207. doi:10.1007/BF00392809

Abstract

The appearance of transverse sections of maize leaves indicates the existence of two airspace systems serving the mesophyll, one connected to the stomata of the upper epidermis and the other to the stomata of the lower surface, with few or no connections between the two. This study tests the hypothesis that the air-space systems of the upper and lower mesophyll are separated by a defined barrier of measurable conductance. A mathematical procedure, based on this hypothesis, is developed for the quantitative separation of the contributions made by the upper and lower halves of the mesophyll to carbon assimilation using gasexchange data. Serial paradermal sections and three-dimensional scanning-electron-microscope images confirmed the hypothesis that there were few connections between the two air-systems. Simultaneous measurements of nitrous-oxide diffusion across the leaf and of transpiration from the two surfaces showed that the internal conductance was about 15% of the maximum observed stomatal conductance. This demonstrates that the poor air-space connections, indicated by microscopy, represent a substantial barrier to gas diffusion. By measuring the CO2 and water-vapour fluxes from each surface independently, the intercellular CO2 concentration (ci) of each internal air-space system was determined and the flux between them calculated. This allowed correction of the apparent CO2 uptake at each surface to derive the true CO2 uptake by the mesophyll cells of the upper and lower halves of the leaf. This approach was used to analyse the contribution of the upper and lower mesophyll to CO2 uptake by the leaf as a whole in response to varying light levels incident on the upper leaf surface. This showed that the upper mesophyll was light-saturated by a photon flux of approx. 1000 μmol·m-2·s-1 (i.e. about one-half of full sunlight). The lower mesophyll was not fully saturated by photon fluxes of nearly double full sunlight. At low photon fluxes the ci of the upper mesophyll was significantly less than that of the lower mesophyll, generating a significant upward flux of CO2. At light levels equivalent to full sunlight, and above, ci did not differ significantly between the two air space systems. The physiological importance of the separation of the air-space systems of the upper and lower mesophyll to gas exchange is discussed.

Key words

Gas exchange (maize leaf)Intercellular air-spaceLeaf anatomy and photosynthesisPhotosynthesis (upper, lower mesophyll)Stomatal conductanceZea (gas exchange)

Abbreviations and symbols

A

net leaf CO2 uptake rate

Aupperapp.and Alowerapp.

net rates of CO2 uptake across the upper and lower surfaces

Aupper and Alower

derived net rates of CO2 uptake by the upper and lower mesophyll

Aupward

net flux of CO2 from the lower to upper mesophyll

ca, ca, upper and ca, lower

the CO2 concentrations in the air around the leaf above the upper surface and below the lower surface

cN2O

the concentration of N2O in the air around the leaf

ci, ci, upper and ci, lower

the mesophyll intercellular CO2 concentration of the whole leaf, the upper mesophyll and the lower mesophyll

gi

leaf internal conductance to CO2

gs, gs, lower and gs, upper

the stomatal conductance of the whole leaf, the lower surface and the upper surface

g

the total conductance across the leaf

Q

the photosynthetically active photon flux density

Copyright information

© Springer-Verlag 1989

Authors and Affiliations

  • S. P. Long
    • 1
  • P. K. Farage
    • 1
  • H. R. Bolhár-Nordenkampf
    • 2
  • U. Rohrhofer
    • 2
  1. 1.Department of BiologyUniversity of EssexColchesterUK
  2. 2.Institut für PflanzenphysiologieUniversität WienWienAustria