18
wheat with the development of short-straw cultivars. Fischer et al. ( 2014 ) also con-
cluded that there is little hard evidence that plant breeding has increased transpira-
tion efficiencies and that higher HI values account for much of the increased yield
in recent years. However, Berry et al. ( 2007 ) concluded that there appears to be little
scope for further increases in HI beyond 0.55 to 0.60 in crops that bear aboveground
grain because such crops depend on a stable structure to distribute leaf area, support
grain and prevent lodging.
4.3 Potential to Enhance Water Productivity in Dryland
Farming
Wani et al. ( 2012 ) stated that there is vast untapped potential to increase yields in
rainfed areas with appropriate soil and water interventions. They indicated that a
linear relationship is generally assumed between biomass growth and ET, which
describes water productivity between 1000 and 3000 m^3 t−^1 (1000 to 3000 kg water
per 1 kg grain) for grain production (Fig. 6 ). Below yield levels of 3 t ha−^1 , however,
they state that this relationship does not hold true and this coincides with the yield
levels of small and marginal farmers in dryland areas. Water productivities range
from 5000 to 8000 m^3 t−^1 when grain yields are as low as 1 t ha−^1. Wani et al. ( 2012 )
state that evidence from water balance analyses in farmers’ fields around the world
shows that only a small fraction, less than 30 % of precipitation, is used as plant
transpiration to support plant growth.
Fig. 6 Yield relationship between water productivity and yield of cereal crops in different climatic
conditions and management (Source: Wani et al. 2012 )
B.A. Stewart and S. Thapa