Premium
Estimation of canopy average mesophyll conductance using δ 13 C of phloem contents
Author(s) -
UBIEREREA,
MARSHALL JOHN D.
Publication year - 2011
Publication title -
plant, cell and environment
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.646
H-Index - 200
eISSN - 1365-3040
pISSN - 0140-7791
DOI - 10.1111/j.1365-3040.2011.02350.x
Subject(s) - canopy , photosynthesis , thuja , phloem , botany , stomatal conductance , conductance , photosynthetic capacity , horticulture , biology , chemistry , physics , condensed matter physics
Conductance to CO 2 inside leaves, known as mesophyll conductance ( g m ), imposes large limitations on photosynthesis. Because g m is difficult to quantify, it is often neglected in calculations of 13 C photosynthetic discrimination. The ‘soluble sugar method’ estimates g m via differences between observed photosynthetic discrimination, calculated from the δ 13 C of soluble sugars, and discrimination when g m is infinite. We expand upon this approach and calculate a photosynthesis‐weighted average for canopy mesophyll conductance ( c g m ) using δ 13 C of stem phloem contents. We measured gas exchange at three canopy positions and collected stem phloem contents in mature trees of three conifer species ( Pseudotsuga menziesii , Thuja plicata and Larix occidentalis ). We generated species‐specific and seasonally variable estimates of c g m . We found that c g m was significantly different among species (0.41, 0.22 and 0.09 mol m −2 s −1 for Larix , Pseudotsuga and Thuja , respectively), but was similar throughout the season. Ignoring respiratory and photorespiratory fractionations ( c Δ ef ) resulted in ≈30% underestimation of c g m in Larix and Pseudotsuga , but was innocuous in Thuja . Substantial errors (∼1–4‰) in photosynthetic discrimination calculations were introduced by neglecting c g m and c Δ ef . Our method is easy to apply and cost‐effective, captures species variation and would have captured seasonal variation had it existed. The method provides an average canopy value, which makes it suitable for parameterization of canopy‐scale models of photosynthesis, even in tall trees.