A systematic and quantitative approach to improve water use efficiency in agriculture

As the competition for the finite water resources on earth increases due to growth in population and affluence, agriculture is faced with intensifying pressure to improve the efficiency of water used for food production. The causes for the relatively low water use efficiency in agriculture are numerous and complex, including environmental, biological, engineering, management, social, and economic facets. The complexity of the problem, with its myriads of local variations, requires a comprehensive conceptual framework of the underlying physical and biological processes as the basis to analyze the existing situation and quantify the efficiencies, and to plan and execute improvements. This paper proposes such a framework, based on the simple fact that the overall efficiency of any process consisting of a chain of sequential step is the product of the efficiency (i.e., output/input ratio) of its individual component steps. In most cases of water use, a number of process chains, both branching and merging, are involved. Means to integrate the diverging and converging chains are developed and presented as equations. Upscaling from fields to regions and beyond are discussed. This chain of efficiencies approach is general and can be applied to any process composed of chains of sequential steps. Here the framework is used to analyze the systems of irrigated and dryland crop production, and animal production on rangeland. Range of plausible efficiencies of each step is presented as tables, with values separately for the poor and for the good situation of circumstances, management and technology. Causes of the differences in efficiency of each step, going from water delivery to soil water extraction, transpiration, photosynthesis, and conversion to crop biomass and yield, and to animal product are briefly discussed. Sample calculations are made to demonstrate how modest differences in the efficiencies of the component steps are manifested as large to huge differences in the overall efficiency. Based on an equation quantifying the impact of changes in efficiency of component steps on the overall efficiency, it is concluded that generally, it is more effective to made modest improvements in several or more steps than to concentrate efforts to improve one or two steps. Hence, improvement efforts should be systematic and not overly concentrated on one or two components. The potential use of the same equation as the point of departure to optimize the allocation of economic resource among the component steps to maximize the improvement in the overall water use efficiency is elaborated on. The chain of efficiencies framework provides the means to examine the current levels of efficiency along the pathways of agricultural water use, to analyze where inefficiencies lie by comparing with the range of known efficiency values in the tables presented, to assess the potential improvements that may be achieved in various parts and their impact on the overall efficiency, and to aid in the optimal allocation of resources for improvements.

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Host status and reaction of <Emphasis Type="Italic">Medicago truncatula</Emphasis> accessions to infection by three major pathogens of pea (<Emphasis Type="Italic">Pisum sativum</Emphasis>) and alfalfa (<Emphasis Type="Italic">Medicago sativa</Emphasis>)

Ditylenchus dipsaci, the stem nematode of alfalfa (Medicago sativa), Mycosphaerella pinodes, cause of Ascochyta blight in pea (Pisum sativum) and Aphanomyces euteiches, cause of pea root rot, result in major yield losses in French alfalfa and pea crops. These diseases are difficult to control and the partial resistances currently available are not effective enough. Medicago truncatula, the barrel medic, is the legume model for genetic studies, which should lead to the identification and characterization of new resistance genes for pathogens. We evaluated a collection of 34 accessions of M. truncatula and nine accessions from three other species (two from M. italica, six from M. littoralis and one from M. polymorpha) for resistance to these three major diseases. We developed screening tests, including standard host references, for each pathogen. Most of the accessions tested were resistant to D. dipsaci, with only three accessions classified as susceptible. A very high level of resistance to M. pinodes was observed among the accessions, none of which was susceptible to this pathogen. Conversely, a high level of variation, from resistant to susceptible accessions, was identified in response to infection by A. euteiches.     

AlgU contributes to the virulence of <Emphasis Type="Italic">Pseudomonas syringae</Emphasis> pv. <Emphasis Type="Italic">tomato</Emphasis> DC3000 by regulating production of the phytotoxin coronatine

Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), which causes bacterial speck disease of tomato, has been used as a model pathogen to investigate the molecular basis of plant–pathogen interactions. The function of many potential virulence factors encoded in the Pst DC3000 genome and their modes of action are not fully understood. P. syringae is known to produce the exopolysaccharide alginate. Although AlgU, a sigma factor, is known to regulate the expression of genes such as algD related to alginate biosynthesis, the molecular mechanisms of AlgU in the virulence of Pst DC3000 is still unclear. To investigate the function of AlgU and alginate in plant–bacterial pathogen interactions, we generated ΔalgU and ΔalgD mutants. After inoculation with ΔalgU but not ΔalgD, host plants of Pst DC3000 including tomato and Arabidopsis had milder disease symptoms and reduced bacterial populations. Expression profiles of Pst DC3000 genes revealed that AlgU can regulate not only the expression of genes encoding alginate biosynthesis, but also the expression of genes related to type III effectors and the phytotoxin coronatine (COR). We also demonstrated that the ΔalgU mutant showed full virulence in the Arabidopsis fls2 efr1 double mutant, which is compromised in the recognition of PAMPs. Further, the application of COR was able to restore the phenotype of the ΔalgU mutant in the stomatal response. These results suggest that AlgU has an important role in the virulence of Pst DC3000 by regulating COR production.     

The chemotaxis regulator <Emphasis Type="Italic">pilG</Emphasis> of <Emphasis Type="Italic">Xylella fastidiosa</Emphasis> is required for virulence in <Emphasis Type="Italic">Vitis vinifera</Emphasis> grapevines

Type IV pili of X. fastidiosa are regulated by pilG, a response regulator protein putatively involved in chemotaxis-like operon sensing stimuli through signal transduction pathways. To elucidate the roles of pilG in pathogenicity of X. fastidiosa, the pilG-deletion mutant XfΔpilG and complemented strain XfΔpilG-C were generated. While all strains had similar growth curves in vitro, XfΔpliG showed significant reduction in cell-matrix adherence and biofilm production compared with wild-type X. fastidiosa and XfΔpilG-C. The genes pilE, pilU, pilT, and pilS were down-regulated in XfΔpliG when compared with its complemented strain and wild-type X. fastidiosa. Finally, no Pierce’s disease symptoms were observed in grapevines inoculated with XfΔpilG, whereas grapevines inoculated with the wild-type X. fastidiosa and complemented strain of XfΔpilG-C developed typical Pierce’s Disease (PD) symptoms. The results indicate that pilG has a role in X. fastidiosa virulence in grapevines.