On the conservative behavior of biomass water productivity

The ever-increasing demand and competition for the finite water resource worldwide call for more efficient use of water in all sectors, including firstly agricultural food production. One important consideration is the existence of a limit to the amount of biomass a crop can produce per unit of water consumed. This article analyzes the theoretical background and the experimental evidence for the conservative behavior of the efficiency in water use by crops to produce biomass, i.e., biomass water productivity (WP b ), under variable environmental conditions. Particularly, WP b is approximately constant for a given crop species after normalization for evaporative demand of the atmosphere and air carbon dioxide concentration. A stepwise scaling up approach, from leaf to canopy, is undertaken to underline the processes involved at the different hierarchical levels of biological organization that lead to the conservative behavior of WP b . Starting at the leaf level, the basic gas exchange equations are outlined to demonstrate that the normalized photosynthetic WP b at the leaf scale is proportional to the ambient CO 2 concentration. New experimental evidence in support of that conclusion is presented for several C 3 and a C 4 crops. Additional factors are introduced to assess photosynthetic WP b at the canopy scale, including the extent of radiation capture and the role of respiration. The composition of biomass was then considered in the analysis of WP b over a season. The paper highlights the need to normalize WP b for differences in climate, specifically, in evaporative demand of the atmosphere to extrapolate WP b values between climatic zones, and in atmospheric CO 2 concentration to account for changes in CO 2 with time, when looking at the past and into the future. Two procedures for normalization for differences in evaporative demand are presented, and a procedure for normalization for changes in CO 2 concentration is derived for the leaf scale and shown to be applicable to canopy scale. Some knowledge gaps and research needs are pointed out and the potential offered by the near constancy of normalized WP b in crop simulation modeling is emphasized.