Increasing crop productivity when water is scarce—from breeding to field management

Abstract

To increase crop yield per unit of scarce water requires both better cultivars and better agronomy. The challenge is to manage the crop or improve its genetic makeup to: capture more of the water supply for use in transpiration; exchange transpired water for CO₂ more effectively in producing biomass; and convert more of the biomass into grain or other harvestable product. In the field, the upper limit of water productivity of well-managed disease-free water-limited cereal crops is typically 20 kg ha⁻¹ mm⁻¹ (grain yield per water used). If the productivity is markedly less than this, it is likely that major stresses other than water are at work, such as weeds, diseases, poor nutrition, or inhospitable soil. If so, the greatest advances will come from dealing with these first. When water is the predominant limitation, there is scope for improving overall water productivity by better matching the development of the crop to the pattern of water supply, thereby reducing evaporative and other losses and fostering a good balance of water-use before and after flowering, which is needed to give a large harvest index. There is also scope for developing genotypes that are able to maintain adequate floret fertility despite any transient severe water deficits during floral development. Marker-assisted selection has helped in controlling some root diseases that limit water uptake, and in maintaining fertility in water-stressed maize. Apart from herbicide-resistance in crops, which helps reduce competition for water by weeds, there are no genetic transformations in the immediate offing that are likely to improve water productivity greatly.

Introduction

The ideas of drought resistance and drought tolerance are giving way in the agricultural world to the idea of water productivity (“more crop per drop”). This change is a great advance because the latter can be quantified, with units of amount of crop yield per volume of water supplied or used, say, kg m⁻³ or kg ha⁻¹ mm⁻¹. Because it can be quantified it enables improvements to be charted, thereby encouraging faster progress.

Nevertheless, the idea of drought remains in widespread use—certainly in the mass media, and also among crop scientists. It comes with connotations of hardship, or, in poor agricultural communities, malnutrition or even famine. It is an idea that inevitably enters any discussion of the impact of the scarcity of water on food production. It is important therefore to be clear about what it means, for it means different things to different people depending on their time scale of interest (Table 1); debates can easily be at cross purposes.

Many explorations of water deficits by plant physiologists, biochemists and molecular biologists are rather more concerned with survival than production, as noted in Table 1. While it is true that a crop plant that does not survive severe water deficits will not produce any yield, the converse is rarely true. Thus the challenge provided by changing focus from “drought tolerance” to “water productivity” clarifies the targets of research, especially those carried out at time scales of hours to days. Some processes occurring at these time scales can strongly affect water-limited yields; others have little relevance, as I shall discuss later.

“Water productivity” can also mean different things to different people (see, for example, Kijne et al., 2003, Pereira et al., 2002). To an economist it might mean the monetary value of outputs divided by that of the necessary inputs. To a geographer or irrigation engineer, it might mean the value of crops produced in a catchment in relation to the water supply of that catchment. But the quintessence of the idea is that it is quantifiable. In this paper, I will concentrate primarily on improving water productivity on farm (with units of amount of crop yield per amount of water supplied or used), though with occasional reference to other aspects.