43
Transpiration Efficiency: Avenues for Genetic
Improvement
G. V. Subbarao*
Dynamac Corporation, Kennedy Space Center, Florida
Chris Johansen†
International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Andhra Pradesh, India
835I. INTRODUCTION
In view of the increasing demand for water for nonagricultural uses (such as for urban and industrial uses),
and also to rationally redeploy available water resources for more areas of crop production, it is impor-
tant to optimize the use of water for crop production [1]. Agricultural research has a major responsibility
to develop and use techniques and practices that will result in more effective use of water in farming sys-
tems. This involves improvement of water use efficiency (WUE), defined here as aerial dry matter pro-
duction of a crop per unit of evapotranspiration (ET). Transpiration efficiency (TE) is a component of
WUE, being aerial dry matter production per unit of water transpired by the crop. The difference between
WUE and TE is important, as suppression of soil evaporation and transpiration by weeds can improve
WUE without improving TE, which is a direct measure of the crop species performance. Plant attributes
(canopy structure, rate of canopy development, etc.) and management means (manipulating plant popu-
lation, optimizing planting dates, fertilizer management, etc.) can modify soil evaporative losses (Es) rel-
ative to transpiration (T) and can therefore affect WUE to a greater extent than TE.
Generally, any means (either genetic or management) that promotes early canopy development and
radiation interception will reduce Esand increase T(as evaporational losses would be negligible once the
canopy closes), often with little or no increase in total ET [2,3]. For example, in Syria, erect chickpea lines
intercepted less solar radiation, thus permitting greater evaporative water losses during early growth, and
consequently they had a lower WUE value than chickpea lines with a prostrate habit [4]. Similarly, leaf-
less pea had a lower WUE than either semileafless or conventionally leafed types [5]. Leafless pea inter-
cepts less radiation than semileafless or conventionally leafed pea and therefore the crop suffers greater Es
losses. Fertilizer application can increase WUE [6], as it promotes greater leaf area development and re-
ducesEsrelative to T. In many legumes, a basal dose of nitrogen and phosphorus promotes the early growth
rate and thus minimizes Es[3]. Other management options such as improving water delivery systems, nu-
trient management approaches, and improved cultural practices could enhance WUE by minimizing Es.
Also, vapor pressure deficit (vpd) during the growing season plays a major role in determining the
WUE. When other factors are nonlimiting, the cost of producing dry matter (in terms of water) would be
*Current affiliation:Japan International Research Center for Agricultural Sciences, Ibaraki, Japan.
†Current affiliation:Consultant in Agricultural Research and Development, Dhaka, Bangladesh.