Humidity gradients in the air spaces of leaves

Humidity gradients in the air spaces of leaves

Wong S. C., Canny M. J., Holloway-Phillips M., Stuart-Williams H., Cernusak L. A., Márquez D. A., Farquhar G. D. (2022)

Suan Chin Wong, Martin J. Canny, Meisha Holloway-Phillips, Hilary Stuart-Williams, Lucas A. Cernusak, Diego A. Márquez, Graham D. Farquhar,


Nature Plants 8: 971–978 –


Stomata are orifices that connect the drier atmosphere with the interconnected network of more humid air spaces that surround the cells within a leaf. Accurate values of the humidities inside the substomatal cavity, wi, and in the air, wa, are needed to estimate stomatal conductance and the CO2 concentration in the internal air spaces of leaves. Both are vital factors in the understanding of plant physiology and climate, ecological and crop systems. However, there is no easy way to measure wi directly. Out of necessity, wi has been taken as the saturation water vapour concentration at leaf temperature, wsat, and applied to the whole leaf intercellular air spaces. We explored the occurrence of unsaturation by examining gas exchange of leaves exposed to various magnitudes of wsat − wa, or Δw, using a double-sided, clamp-on chamber, and estimated degrees of unsaturation from the gradient of CO2 across the leaf that was required to sustain the rate of CO2 assimilation through the upper surface. The relative humidity in the substomatal cavities dropped to about 97% under mild Δw and as dry as around 80% when Δw was large. Measurements of the diffusion of noble gases across the leaf indicated that there were still regions of near 100% humidity distal from the stomatal pores. We suggest that as Δw increases, the saturation edge retreats into the intercellular air spaces, accompanied by the progressive closure of mesophyll aquaporins to maintain the cytosolic water potential.

Estimating stomatal and biochemical limitations

Estimating stomatal and biochemical limitations during photosynthetic induction

Deans R. M., Farquhar G. D., Busch F. A. (2019)

Ross M. Deans,Graham D. Farquhar,Florian A. Busch,

Plant Cell and Environment 42: 3227–3240 –


Understanding stomatal and biochemical components that limit photosynthesis under different conditions is important for both the targeted improvement of photosynthesis and the elucidation of how stomata and biochemistry affect plant performance in an ecological context. Limitation analyses have not yet been extensively applied to conditions of photosynthetic induction after an increase in irradiance. Moreover, few studies have systematically assessed how well various limitation analyses actually work. Here we build on two general ways of estimating limitations, one that sequentially removes the effect of a limitation (elimination) and one that uses a tangent plane approximation (differential), by including the ternary effect and boundary layer conductance so that they are consistent with gas exchange data. We apply them to the analysis of temporal and time-integrated limitations during photosynthetic induction, calculating limitations either independent of the time course (one-step) or make use of the entire time course (stepwise). We show that the stepwise differential method is the best method to use when time steps are small enough. We further show that the differential method predicts limitations near exact when the internal CO2 concentration stays constant. This last insight has important implications for the general use of limitation analyses beyond photosynthetic induction.

The distribution of open stomata was similar to the pattern of starch accumulation

Characterisation of Non-Uniform Photosynthesis Induced by Abscisic Acid in Leaves Having Different Mesophyll Anatomies

Terashima I., Wong S.-C., Osmond C. B., Farquhar G. D. (1988)

Ichiro Terashima, Suan-Chin Wong, C. Barry Osmond, Graham D. Farquhar,

Plant Environmental Biology Group, Research School of Biological Sciences,The Australian National University, P.O.Box 475, Canberra, A.C.T. 2601, Australia

Plant and Cell Physiology 29(3): – DOI: 10.1093/oxfordjournals.pcp.a077506


The effects of abscisic acid (ABA) on photosynthesis in leaves of Helianthus annuus L. were compared with those in leaves of Vicia faba L. After the ABA treatment, the response of photosynthetic CO2 assimilation rate, A, to calculated intercellular partial pressure of CO2, Pi, (A(pi) relationship) was markedly depressed in H. annuus. A less marked depression was also observed in V.faba. However, when the abaxial epidermes were removed from these leaves, neither the maximum rate nor the CO2 response of photosynthetic oxygen evolution was affected by the application of ABA.

Starch-iodine tests revealed that photosynthesis was not uniform over the leaves of H. annuus treated with ABA. The starch content was diffferent in each bundle sheath extension compartment (the smallest subdivision of mesophyll by veins with bundle sheath extensions, having an area of ca. 0.25 mm² and ca. 50 stomata). In some compartments, no starch was detected. The distribution of open stomata, examined using the silicone rubber impression techniques, was similar to the pattern of starch accumulation. In V.faba leaves, which lack bundle sheath extensions, distribution of starch was more homogeneous.

These results indicate that the apparent non-stomatal inhibition of photosynthesis by ABA deduced from the depression of A(pⁱ) relationship is an artifact which can be attributed to the non-uniform distribution of transpiration and photosynthesis over the leaf. Intercellular gaseous environment in the ABA-treated leaves is discussed in relation to mesophyll anatomy.

A more precise physical approach to the electrical resistance analogy for gas exchange, resulting in a more accurate calculation of gas exchange parameters (stomatal conductance)

An improved theory for calculating leaf gas exchange more precisely accounting for small fluxes

Marquez D. A., Stuart-Williams H., Farquhar G. D. (2021)

Diego A. MárquezHilary Stuart-Williams, Graham D. Farquhar,

Nat. Plants 7: 317–326 –


The widely used theory for gas exchange proposed by von Caemmerer and Farquhar (vCF) integrates molar fluxes, mole fraction gradients and ternary effects but does not account for cuticular fluxes, for separation of the leaf surface conditions or for ternary effects within the boundary layer.

The magnitude of cuticular conductance to water (gcw) is a key factor for determining plant survival in drought but is difficult to measure and often neglected in routine gas exchange studies. The vCF ternary effect is applied to the total flux without the recognition of different pathways that are affected by it.

These simplifications lead to errors in estimations of stomatal conductance, intercellular carbon dioxide concentration (Ci) and other gas exchange parameters.

The theory presented here is a more precise physical approach to the electrical resistance analogy for gas exchange, resulting in a more accurate calculation of gas exchange parameters. Additionally, we extend our theory, using physiological concepts, to create a model that allows us to calculate cuticular conductance to water.

Leaf Conductance in Relation to Assimilation

Influence of Irradiance and Partial Pressure of Carbon Dioxide

Leaf Conductance in Relation to Assimilation in Eucalyptus pauciflora Sieb. ex Spreng

Wong S. C., Cowan I. R., Farquhar G. D. (1978)

Suan C. Wong, Ian R. Cowan, Graham D. Farquhar,

Plant Physiology 62(4): – DOI:


Rates of assimilation and transpiration in Eucalyptus pauciflora Sieb. ex Spreng were measured at various ambient partial pressures of CO2 and various irradiances and were used to estimate leaf conductance and intercellular partial pressure of CO2. The responses of leaf conductance and rate of assimilation to change in intercellular partial pressure of CO2 were expressed in terms of feedback. They are small in the sense that their combined effect was to reduce disturbances in intercellular partial pressure of CO2 by 30% only. The magnitude of the feedback had no influence on the system as affected by irradiance, because the direct responses of conductance and rate of assimilation to changes in irradiance in the range 0.25 to 2 millieinsteins per meter per second were such that intercellular partial pressure was maintained almost constant.

Genomic regions for canopy temperature and their genetic association with stomatal conductance

Genomic regions for canopy temperature and their genetic association with stomatal conductance and grain yield in wheat

Rebetzke G. J.,  Rattey A. R., Farquhar G. D., Richards R. A., Condon A. G. (2013)

Greg J. Rebetzke A C , Allan R. Rattey A , Graham D. Farquhar B , Richard A. Richards A and Anthony (Tony) G. Condon A

A CSIRO Plant Industry, PO Box 1600, Canberra, ACT 2601, Australia.

B Australian National University, PO Box 475, Canberra, ACT 2601, Australia.


In Functional Plant Biology 40: 14–33 –


Stomata are the site of CO2 exchange for water in a leaf. Variation in stomatal control offers promise in genetic improvement of transpiration and photosynthetic rates to improve wheat performance. However, techniques for estimating stomatal conductance (SC) are slow, limiting potential for efficient measurement and genetic modification of this trait. Genotypic variation in canopy temperature (CT) and leaf porosity (LP), as surrogates for SC, were assessed in three wheat mapping populations grown under well-watered conditions. The range and resulting genetic variance were large but not always repeatable across days and years for CT and LP alike. Leaf-to-leaf variation was large for LP, reducing heritability to near zero on a single-leaf basis. Replication across dates and years increased line-mean heritability to ~75% for both CT and LP. Across sampling dates and populations, CT showed a large, additive genetic correlation with LP (rg = –0.67 to –0.83) as expected. Genetic increases in pre-flowering CT were associated with reduced final plant height and both increased harvest index and grain yield but were uncorrelated with aerial biomass. In contrast, post-flowering, cooler canopies were associated with greater aerial biomass and increased grain number and yield. A multi-environment QTL analysis identified up to 16 and 15 genomic regions for CT and LP, respectively, across all three populations. Several of the LP and CT QTL co-located with known QTL for plant height and phenological development and intervals for many of the CT and LP quantitative trait loci (QTL) overlapped, supporting a common genetic basis for the two traits. Notably, both RhtB1b and RhtD1b dwarfing alleles were paradoxically positive for LP and CT (i.e. semi-dwarfs had higher stomatal conductance but warmer canopies) highlighting the issue of translation from leaf to canopy in screening for greater transpiration. The strong requirement for repeated assessment of SC suggests the more rapid CT assessment may be of greater value for indirect screening of high or low SC among large numbers of early-generation breeding lines. However, account must be taken of variation in development and canopy architecture when interpreting performance and selecting breeding lines on the basis of CT.

Oxygen isotope ratio of leaf and grain material correlates with stomatal conductance and yield

Oxygen isotope ratio of leaf and grain material correlates with stomatal conductance and yield in irrigated, field-grown wheat

by Barbour M. M., Fischer R. A., Sayre K. D., Farquhar G. D. (2000)

Margaret M. Barbour, R. Anthony Fischer, Ken D. Sayre, Graham D. Farquhar,


In Aust. J. Plant Physiol. 27: 625–637 – ISSN : 0310-7841


To test the theory that the oxygen isotope ratio (δ18O) of plant material reflects the evaporative conditions under which the material was formed, so that differences in stomatal conductance should show up in plant δ18O, measurements were made of δ18O of organic matter from flag leaves at anthesis and grain at harvest from 8 cultivars of spring wheat grown under irrigation in 1992/93, 1993/94 and 1994/95 in Mexico. The cultivars varied widely in stomatal conductance and average grain yield, with which conductance was positively correlated. Supporting the theory, δ18O of flag leaves (δ18Ol) was negatively correlated with stomatal conductance for 2 of the 3 years. The significant correlations were consistent, with high conductance cultivars having lower leaf temperatures and kinetic fractionation factors, and higher vapour pressure fractionation factors and Péclet numbers, all of which combined to result in less enriched δ18Ol. Yield (grain weight/m2) was also significantly negatively correlated with δ18Ol in 2 of the 3 years. δ18Ol was as good a predictor of yield as stomatal conductance, and significantly better than carbon isotope discrimination. Correlations between grain δ18O (δ18Og) and physiological parameters were less clear. Significant negative correlations between δ18Ogand stomatal conductance, leaf temperature and yield were found only during the first season. By measuring the oxygen isotope ratio of cellulose extracted from leaf samples, the difference in fractionation factors (εcp) for cellulose and whole leaf tissue was assessed. εcp were found to be variable, and more negative when δ18O for cellulose and δ18Ol were lower. Cultivar means for the carbon isotope composition and δ18O of whole leaf material were significantly positively correlated, and the factors required to produce such a relationship are discussed.

Relationship between stomatal conductance and light intensity, derived from experiments using the mesophyll as shade

Relationship between stomatal conductance and light intensity in leaves of Zea mays L., derived from experiments using the mesophyll as shade

by Raschke K., Hanebuth W. F., Farquhar G. D. (1978)

  • Klaus Raschke, William F. Hanebuth, Graham D. Farquhar,

MSU-ERDA Plant Research Laboratory, Michigan State University, East Lansing, USA


In Planta 139: 73-77 –


Attached leaves of Zea mays were illuminated with monochromatic light, with either the upper or the lower epidermis facing the light source.

The mesophyll absorbed between 99.5 and 99.6% of the red or blue light used. An inversion of the light direction therefore caused a 200- to 250-fold change in the quantum flux into each epidermis. This variation in quantum flux did not affect stomatal conductance.

Stomatal conductance was however correlated with intercellular CO2 concentration, ci, and the relationship between stomatal conductance and ciappeared also to remain the same if changes in ci were brought about by changes in atmospheric CO2 concentration instead of light.

A close inspection of the data showed that stomata of the upper (adaxial) epidermis exhibited a small increase in conductance (<0.1 cm s-1) in response to blue light that was superimposed on the dominating response to ci.