Developing drought tolerant crops: hopes and challenges in an exciting journey

Additional keywords: crop modelling, cross-cutting issues, InterDrought, phenotyping, plant hydraulics.

Water has become a critical resource for agriculture production, as urban societies develop and the competition for water between agriculture, industrial and domestic sectors becomes more acute. This is especially the case in areas where water is already scarce, irrigation potential has reached a maximum, and crop production is constrained by limited rainfall. Climate change is adding another dimension of complexity to an already extremely complex situation. InterDrought IV provided a forum to discuss the issue of water limitation in crop production and to propose avenues to improve it under current and future climates. Here we summarise the important areas of discussion during the conference, as reflected in selected papers from this special issue.

While these are major challenges faced by pre- and breeders generally, they are of particular relevance to water-limited environments because drought scenarios vary across time and geographical scales, leading to large interactions between genotypes and environments within and between years. Typically the questions that arise are which type of drought pattern occur, how frequently (Kholová et al. 2013), and which adaptive traits are required where? Crop modelling has emerged in this conference as a key tool to guide us through the complexity of drought scenarios. A paper in this volume (Kholová et al. 2014) presents a model to predict the value of breeding for adaptive traits and the trade-offs that occur, using the example of grain and stover productivity in sorghum, adding to earlier papers on this topic (Sinclair et al. 2010; Vadez et al. 2012). An alternative approach presented at InterDrought IV is to study adaptive strategies in genetic resource collections using germplasm that evolved under low or high drought stress in order to ascertain which traits were selected for these contrasting selection pressures. This ecophysiological approach was applied to wild and domesticated Lupinus luteus collected along Mediterranean terminal drought stress gradients in a companion InterDrought IV special issue paper by Berger and Ludwig (2014).

An interesting contrast in InterDrought IV, compared with the previous meeting held in Shanghai in 2009, was the decrease in papers reporting the use of transgenic technologies to improve the adaptation of crops to drought. Blum (2013, 2014) summarises decades of research on transgenics and concludes that little progress has been made in delivering cultivars with improved drought tolerance. Drought adaptation is complex, likely to involve interaction among many genes and we interpret the reduction in papers offering transgenic solutions as an acceptance within the scientific community that there is no simple ‘magic gene’ solution for adaptation to drought. Instead, genomic selection has become an emerging breeding tool (Thudi et al. 2014). Of course, the issues of G×E interaction covered in the previous paragraph remind us of the complexity of drought adaptation and we suggest that genomic selection must be integrated with physiology, agronomy and other disciplines used to understand interactions in order to fulfil its potential.

Several papers in this special issue deal with increasing water productivity (Fereres et al. 2014; Kapanigowda et al. 2014). We have indeed acquired new insight on the ways to improve the water productivity of crops in InterDrought IV (Vadez et al. 2014), especially on the relationship between the capacity of plants to restrict water losses under high evaporative demand, and differences in the hydraulic characteristics of roots and shoots. A paper in this special issue deals with the influence of soil water holding capacity on the mediation of hydraulic and chemical signals at the plant level (Tramontini et al. 2014). Clearly, the hydraulic properties of plants are becoming an important research area, especially around the issue of improving water productivity.

Roots have long been a desired target in cultivar improvement for drought, but are difficult to work with and have received scant attention compared with other plant organs. This special issue is a rich repository of studies showcasing the importance of roots to improve the drought adaptation of several crops. Henry et al. (2014) outlines the importance of deep rooting in rice, a crop that usually has very shallow root systems, adding to earlier studies highlighting importance of root hydraulic conductivity (Henry 2013). This paper also highlights the impact of G×E interaction, indicating that root traits such as deeper roots and more roots at depth can indeed improve rice productivity, but not in all environments. Two studies in this issue highlight specific crop cases highlighting the importance of water extraction at depth in wheat (Ober et al. 2014) and potatoes (Puértolas et al. 2014). A review paper in a companion InterDrought IV special issue introduces new ways to study roots by modelling root architecture and anatomical traits in 3 dimensions (Lynch et al. 2014). Two more papers in this special issue use crop modelling to simulate the effect of altering root traits on water productivity (Christopher et al. 2014; Kholová et al. 2014). We conclude that the scientific community is progressing towards a much more detailed understanding of root system form and function, and anticipate that this will play an important role in improving the drought adaptation of our crops.

In summary, several key topics were debated in InterDrought IV and this special issue reflects the multidisciplinarity of this approach. The last paper in this issue (Turner et al. 2014) reflects on the progresses being made in these different research areas, taking stocks of the different presentation at InterDrought IV, highlighting hopes and challenges for the future.