discrimination analysis [29,107–110]. Several studies with a number of crop species have shown that mod-
erate water stress leads to an increase in TE as indicated by their level of^13 C discrimination ()
[88,98,111,112]. Water stress resulted in about 2‰ lower compared with well-irrigated plants of chick-
pea [107]. Similarly, for cowpea (Vigna unguiculata), it was shown that leaves sampled from field-grown
plants in a dry environment had about 1.5‰ lower than plants from irrigated conditions [113].
Under severe water deficit, TE is reported to decrease [103]. This is because leaves become less ef-
ficient with respect to water and CO 2 exchange; water can still be lost through the cuticle but CO 2 entry
through stomata is severely restricted, thus causing reduced TE [18]. In groundnut, the relation between
and TE can break down under severe drought conditions, which could be related to increased respira-
tory losses of carbon [74]. A similar response has been reported for sunflower [100]. Respiratory losses
of carbon can be as much as 40% under severe drought conditions [103].
842 SUBBARAO AND JOHANSENFigure 2 Relationship between transpiration efficiency (g dry matter per kilogram of water transpired) and
carbon isotope discrimination () under well-watered and moisture-deficient conditions for a range of peanut
cultivars grown in field conditions. (From Ref. 74.)