) is not independent of root activity, and it seems that there is a close correlation between gas exchange
in water-deficit environments and root attributes [137]. One way to overcome this problem of differences
in root attributes is to evaluate germ plasm lines under irrigated conditions, where differences in root
growth do not affect the leaf gas exchange characteristics and thus . In many crop species, variation in
Pi/Paandhas been reported among genotypes under irrigated conditions, indicating the existence of ge-
netic variation in the “baseline Ci” that is expressed under nonstress conditions [137].
In crops such as groundnut, there is a moderately positive correlation (r0.55) between and HI,
and thus selecting for low (high TE) could lead to selection of genotypes with low partitioning
[10,66,96]. This indicates that selection for high TE and HI, and thus yield potential, could be difficult
because of this negative association. However, the possibility of combining high HI and high TE requires
further research [10,96]. This highlights the need for physiologists and breeders to be aware of the po-
tential for negative associations between traits such as TE, partitioning of biomass, and root water uptake
attributes of roots.
As several factors can alter plant dry weight independently of , there may not always be a direct
association between and productivity [36]. However in many crops, the general trend in relationship
betweenand dry matter productivity is negative; that is, higher productivity under optimum condi-
tions (e.g., irrigated) is associated with lower [140]. Thus, in crops where there is a positive associ-
ation between and dry matter production, it may be that high TE and potential for dry matter pro-
ductivity are incompatible. For crops such as wheat, barley, and beans, where differences in TE are due
mainly to differences in gs, there appears to be a positive correlation between and dry matter pro-
duction [132]. This indicates that selection for low could lead to selection of genotypes with low dry
matter accumulation capability and thus potential productivity. It was suggested that selection for low
will improve adaptation to drought [29], whereas selection for high should improve yield potential
[132]. However, it should still be possible to identify genotypes that do not comply with this general
relationship. For example, in barley, although there is generally a negative relationship between TE and
dry matter accumulation among the genotypes tested, certain genotypes deviate from this relationship
(Figure 5) [148].
For crops such as groundnut, and in cool-season grasses, where photosynthetic rates are the main
source of variation in TE, selection for low should lead to genotypes with high dry matter production
TRANSPIRATION EFFICIENCY AND GENETIC IMPROVEMENT 847
Figure 5 Transpiration efficiency and total biomass production in barley genotypes grown in a greenhouse.
(From Ref. 148.)