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Field Studies of the Conductance of Wheat Leaves and Transpiration
Author(s) -
Denmead O. T.,
Millar B. D.
Publication year - 1976
Publication title -
agronomy journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.752
H-Index - 131
eISSN - 1435-0645
pISSN - 0002-1962
DOI - 10.2134/agronj1976.00021962006800020026x
Subject(s) - transpiration , irradiance , canopy , stomatal conductance , environmental science , evaporation , water stress , vapour pressure deficit , atmospheric sciences , agronomy , botany , photosynthesis , biology , physics , meteorology , quantum mechanics
Light and water stress appear to be the main factors determining the short‐term variation of stomatal aperture in the field but their joint influences have seldom been studied in the natural environment. We have examined the effects of the irradiances and water potentials of leaves of field‐grown wheat plants ( Triticum aestivum L. ‘Heron’) on their conductances for water vapor and on canopy transpiration. Water potentials were measured by thermocouple psychrometry or estimated from water flow rates and plant resistances. Conductances were measured with a diffusion porometer or calculated from meteorological measurements inside the canopy. The latter data also permitted calculation of leaf irradiances and transpiration rates. Provided leaf water potentials were higher than certain critical values (established by us in a previous study), leaf conductance (c cm s ‐1 ) was linearly related to the net irradiance of the leaf (R 1 , W m ‐2 ) by the equation c = 0.05 + 0.0020 R 1 , with a correlation coefficient of 0.90 and a standard error of estimate of 0.11 cm s ‐1 . This relationship applied to green leaves at all stem positions in the canopy up to a net leaf irradiance of > 500 W m ‐2 . At and below the critical water potentials, stomatal aperture was controlled by leaf water status rather than irradiance. In cases of mild water stress, induced, for example, by an excessive evaporation demand, it appeared that leaf conductances decreased just sufficiently to keep water potentials at their critical levels, thereby limiting canopy transpiration to a maximum rate which, in our case, was 0.6 mm h ‐1 . In severe water stress, complete stomatal closure occurred at leaf water potentials about 10 bars less than the critical values and appeared to occur first in leaves at lower positions on the stem.