Theory (Craig and Gordon 1965; Dongmann
et al. 1974; Sternberg et al.
1986; Farquhar and Lloyd 1993) suggests that the oxygen isotope ratio (d
18 O) of plant material should reflect the evaporative
conditions under which the material was formed, so that differences in
stomatal conductance should show up in plant d 18 O. To
test this theory we measured the oxygen isotope ratio of organic matter from
flag leaves at anthesis and grain at harvest from eight cultivars of spring
wheat (Triticum aestivum L.) grown under irrigation in
each of three seasons in Mexico. The cultivars ranged widely in stomatal
conductance and in average grain yield, with which conductance was positively
correlated. Supporting theory, the oxygen isotope ratio of flag leaves (d
18 Ol) was found to correlate
negatively with stomatal conductance for two of the three seasons. The
significant correlations are 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 combine
to result in less enriched d 18
Ol. Yield (grain weight per square meter) was also found
to be significantly negatively correlated with d
18Ol in two of the three seasons.
d 18 Ol was as good a predictor of
yield as stomatal conductance, and significantly better than carbon isotope
discrimination. Correlations between grain d 18 O (d
18 Og) and physiological
parameters were less clear. Significant negative correlations between d
18 Og and 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 (ecp) for
cellulose and whole leaf tissue was assessed. ecp was
found to be variable, and more negative when d 18
Oc and d 18
Ol were lower. Cultivar means for d
13 C and d 18 O of whole leaf
material were found to be significantly positively related, and the factors
required to produce such a relationship are discussed.
Coupling of the atmosphere and sediment melts across the Archean-Proterozoic transition