Independent variation in photosynthetic capacity and stomatal conductance leads to differences in intrinsic water use efficiency

Fig. 1 Schematic of components of intrinsic water use efficiency,
where WUEintr is the ratio between A and gH2O for a genotype of
interest. A soybean genotype of interest has an increased WUEintr
(diamond; WUEintr¼26.9/0.395¼68.1lmol CO2 mol1 H2O) relative
to the average value measured for a reference genotype (triangle).
This difference is due to a combination of two shifts: (i) a decrease in
gH2O resulting in an increase in WUEintr, which has been termed
WUEgH2O (circle; WUEgH2O¼24.6/0.395¼62.3lmol CO2 mol1 H2O)
and (ii) the genotypes have different A to gH2O relationships, resulting
from an increase in photosynthetic capacity of the genotype of
interest relative to the reference genotype, which has been termed
WUEPC (square; WUEPC¼29.1/0.498¼58.4lmol CO2 mol1 H2O).
As the high WUEPC of the genotype of interest is maintained under
lower gH2O, photosynthetic capacity differences can compensate for
variation in water use. All points along a line passing through the
origin (e.g. the dashed line) have the same WUEintr. The A to gH2O
relationship and variation in WUEPC and WUEgH2O represent realistic
values for soybean genotypes.

Independent variation in photosynthetic capacity and stomatal conductance leads to differences in intrinsic water use efficiency in 11 soybean genotypes before and during mild drought

by Gilbert M. E., Zwieniecki M. A., Holbrook N. M. (2011)

Matthew E. Gilbert1,*, Maciej A. Zwieniecki2 and N. Michele Holbrook1


1 Organismic and Evolutionary Biology, Harvard University, Cambridge 02138, MA, USA

2 Arnold Arboretum of Harvard University, Cambridge 02138, MA, USA

===

In Journal of Experimental Botany 62(8): 2875–2887 – doi:10.1093/jxb/erq461 –

https://scholar.harvard.edu/holbrooklab/files/jeb_v62_no8_p2875.full_.pdf

Abstract

Intrinsic water use efficiency (WUEintr), the ratio of photosynthesis to stomatal conductance to water, is often used as
an index for crop water use in breeding projects. However, WUEintr conflates variation in these two processes, and
thus may be less useful as a selection trait than knowledge of both components. The goal of the present study was to
determine whether the contribution of photosynthetic capacity and stomatal conductance to WUEintr varied
independently between soybean genotypes and whether this pattern was interactive with mild drought. Photosynthetic
capacity was defined as the variation in WUEintr that would occur if genotypes of interest had the same stomatal
conductance as a reference genotype and only differed in photosynthesis; similarly, the contribution of stomatal
conductance to WUEintr was calculated assuming a constant photosynthetic capacity across genotypes. Genotypic
differences in stomatal conductance had the greatest effect on WUEintr (26% variation when well watered), and was
uncorrelated with the effect of photosynthetic capacity on WUEintr. Thus, photosynthetic advantages of 8.3% were
maintained under drought. The maximal rate of Rubisco carboxylation, generally the limiting photosynthetic process
for soybeans, was correlated with photosynthetic capacity. As this trait was not interactive with leaf temperature, and
photosynthetic capacity differences were maintained under mild drought, the observed patterns of photosynthetic
advantage for particular genotypes are likely to be consistent across a range of environmental conditions. This
suggests that it is possible to employ a selection strategy of breeding water-saving soybeans with high photosynthetic
capacities to compensate for otherwise reduced photosynthesis in genotypes with lower stomatal conductance.

Elevated temperature stimulates stomatal opening regardless of the CO2 assimilation status

Stomatal opening at elevated temperature: an underestimated regulatory mechanism?

by Feller U. (2006)

Urs Feller

Institute of Plant Sciences, University of Bern, Altenbergrain 21, CH-3013 Bern,
Switzerland

===

In Gen. Appl. Plant Physiol. XXXII: 19–31 (special issue) –

http://www.bio21.bas.bg/ipp/gapbfiles/pisa-06/06_pisa_19-31.pdfby

Summary

Climate models predict more frequent and more severe extreme events (e.g. heat waves, extended drought periods) in Europe during the next decades. The response of plants to elevated temperature is a key issue in this context. Stomatal regulation is not only relevant for the diffusion of CO2 from the ambient air into the leaves, but it plays also an important role for the control of transpiration and leaf cooling. The regulation of stomatal aperture by the water status (hydroactive and hydropassive feed-back) and by internal CO2 availability (CO2 feed-back) are well documented in the
literature, while the response of the stomates to elevated temperature was far less considered in the past.

Photosynthesis is negatively affected by elevated temperature, but the water loss via transpiration may still be high. In the experiments reported here, bean leaf segments were incubated in darkness floating on water in the range from 20 to 50°C and then analyzed immediately by taking a photograph with a digital microscope. Stomatal aperture was measured on these pictures in order to quantify stomatal opening. After the incubation for 30 min, the opening was 0.66, 2.76 and 4.28 µm at 23, 30 and 35°C respectively. This opening at elevated temperature was fully reversible. Abscisic acid (0.1 µM) in the incubation medium shifted the temperature for stomatal opening to higher values. It can be concluded that elevated temperature stimulates stomatal opening regardless of the CO2 assimilation status and that there is a trade-off between leaf cooling on one hand and limiting water loss during drought periods on the other hand.