Plant production in water-limited environments

Water-limited environments confound efforts to intensify plant production needed to address the impending shortfall in global food production capacity (Fischer et al., 2014). The additional influence of anthropogenic global warming on the extent of water limitation in key production environments, not only through anticipated effects on precipitation, but also via increased evaporative demand associated with higher temperature and vapour pressure deficit (e.g. Lobell et al., 2013), further exacerbates this global challenge, although increased atmospheric CO₂ concentration and changes in seasonal climate patterns and crop system management can generate off-setting benefits (Parent et al., 2018; Hunt et al., 2019; Hammer et al., 2020).

How to enhance the effective use of available water remains a critical research question for plant production in water-limited environments. A central challenge for researchers and policymakers is to devise technologies that lend greater resilience to agricultural production under this particular environmental stress. This special issue on this topic arises from keynote and session lead presentations at the sixth Interdrought conference (IDVI). As it was not possible to proceed with the IDVI Congress as a face-to-face meeting in Mexico City in March 2020 as planned due to the COVID-19 pandemic, the scientific programme of the IDVI was delivered via a digital platform (available at https://interdrought2020.cimmyt.org/). IDVI builds on the successful series of Interdrought congresses in Montpellier 1995, Rome 2005, Shanghai 2009, Perth 2013, and Hyderabad, 2017.

IDVI facilitated the development of concepts, methods, and technologies associated with solutions to the global challenge of improving crop production under drought-prone conditions. The congress embraced the Interdrought philosophy of presenting and integrating results of both applied and basic research with a clear transdisciplinary approach (Tardieu et al., 2017) (Box 1). The review and research papers highlighted here encompass the disciplines ranging over molecular biology, genetics, physiology, modelling, breeding, and agronomy, and the scales of biological organization from molecular to crop systems.

The IDVI congress was structured around five subject themes which set the order for the papers presented in the review and research paper sections of this special issue, and each is briefly outlined here.

Optimizing dryland crop production: crop design

The concept underpinning design at the crop and cropping system scale relates to identifying genetic (G) and management (M) factors that optimize crop performance for a given production environment (E). Of course, in variable and water-limited E, every season is different and usually not predictable. While it is plausible to type environments quantitatively (Chenu et al., 2011), such envirotyping does not enhance season-scale predictability. As the optimal G×M depends on the E experienced, it will vary from year to year (Tardieu, 2012). This generates trade-offs between productivity/profitability and risk associated with G×M options that must be considered along with the farmer’s attitude to risk in consideration of crop design (Hammer et al., 2020). Traits that might confer yield stability or resilience over E (and M) might be particularly advantageous. In this issue, Xie et al. (2021) explore the value of root plasticity in rice in this regard and, while they find some promise, they also note complex interactions that confound the outcome. Hein et al. (2021) review the possibilities for enhancing potential genetic gain in stress resilience via high-throughput phenotyping of drought and heat stress resilience, physiological traits that are difficult to measure using traditional methods.

Water capture, transpiration, and transpiration efficiency

Water captured by a crop as transpiration (T) and the efficiency with which that is utilized in forming biomass (TE) are critical to crop growth and yield in water-limited environments. In reviewing plant traits for plant productivity in water-limited situations, Condon (2020) used the identity

where W is total crop water use (which includes losses such as to soil evaporation and deep drainage) and HI is the crop harvest index (i.e. mass of harvestable product as a ratio of total crop biomass), to emphasize the importance of ensuring that as much water as possible is actually transpired by the crop rather than being wasted. Deeper and/or more efficient root systems to increase T (Sponchiado et al., 1989; Lopes and Reynolds, 2010; Thorup-Kristensen et al., 2020) as well as various avenues to improving TE, such as limited maximum transpiration (Vadez et al., 2014; Messina et al., 2015), enhancements to photosynthetic biochemistry and responsiveness, or greater mesophyll conductance (Leakey et al., 2019), are relevant traits for enhanced productivity in water-limited environments. The utility of trait combinations is supported by pre-breeding in wheat (Reynolds et al., 2009) as well as the results of crop simulation modelling (Condon et al., 2004; Messina et al., 2015; Leakey et al., 2019; A. Wu et al., 2019), although the modelling also enables a more dynamic perspective on the regulation and timing of water limitation in the crop cycle, which can be critical for many aspects of adaptation to water limitation (e.g. Borrell et al., 2014). In this issue, Li et al. (2021) review wheat root system architecture traits and their genetic regulation, and discuss opportunities for targeted genetic improvement. Vadez et al. (2021) report detailed studies yielding novel findings on factors influencing TE, such as species interactions with soil type, which suggest that a deeper understanding of root system differences and their interaction with soil hydraulics is required. The research by Franco-Navarro et al. (2021) shows that macronutrient chloride improved drought resistance in tobacco plants by water deficit avoidance and tolerance mechanisms. Chloride treatment specifically improved leaf turgor, photosynthesis performance, and water use efficiency, indicating an influence of chloride nutrition on TE.

Vegetative and reproductive growth

While capture and efficient use of water are critical for crop growth and biomass accumulation, the development and growth of reproductive (or storage) organs is critical to crop yield. This determines the HI term in the identity above. In grain crops, the product of the yield components grain number (GN) and grain size (GS) determines yield. GN is the major yield component and has been related to plant growth rate and extent of partitioning to reproductive organs during a critical period around flowering for a range of species (Vega et al., 2001). In water-limited situations, studies on targeted water stress conditions invoked from before flowering to late grain filling on a set of successful commercial maize hybrids released over decades (Campos et al., 2006) demonstrated that GN was most sensitive to water deficit effects around flowering, while GS was affected by deficits during grain filling. That study also showed that genetic yield gains under stress at flowering were associated with increased kernels per ear and reduced anthesis–silking interval (ASI) (Edmeades et al., 1993). The ASI indicator has also been used in public maize breeding for drought tolerance, leading to the release of improved cultivars in Africa (Banziger et al., 2006). Messina et al. (2019) reviewed the causes of reproductive failure under drought in maize and developed a dynamic model of grain set and abortion, which captured the interplay among effects on assimilate availability, silk extension, and timing of source–sink interactions in a manner that captured process knowledge (e.g. Zinselmeier et al., 1995; Cárcova et al., 2003; Oury et al., 2016; Turc et al., 2016) and was able to predict grain abortion, GN, and GS observations of the study by Campos et al. (2006). In this issue, Messina et al. (2021) show how advances in aspects of this reproductive resilience, rather than changes in root system architecture and water capture, was the major contributor to long-term yield improvement in water-limited situations in maize.

Breeding for water-limited environments

Advances in genomics, phenomics, and data analytics continue to proffer rapidity in breeding crops better adapted to water-limited environments. While some major advances have been made in industry (Cooper et al., 2014a, b) by a clear definition of product targets and integration of advanced technologies, such as managed environments, precision phenotyping, dense genomic profiles, genetic understanding of key adaptive traits, and genetic prediction methodologies, breeding programmes at this scale and level of organization and investment have not yet been feasible in the public sector. Advances in crop growth modelling methodology are being used to evaluate the integrated effects of multiple traits for product concepts and guide their development and evaluation. Recent developments (Technow et al., 2015; Messina et al., 2018) have linked crop growth models (CGMs) with whole-genome prediction (WGP) and demonstrated a potential for improved genetic gain over WGP alone in situations where G×E interactions are significant in a breeding programme. Cooper et al. (2014b) suggest that for the foreseeable future, plant breeding methodology will continue to unfold as a practical application of this scaling of quantitative biology. The foundations of the quantitative dimension will be integration of quantitative genetics, statistics, gene to phenotype knowledge of traits embedded within crop growth and development models. This view is perhaps reinforced by the paucity of quality leads from an extensive industry biotechnology programme to enhance maize agronomic traits, including drought tolerance, through transgenic manipulation (Simmons et al., 2021). Of the few potentially useful events discovered, overexpression of the transcription factor zmm28, which enhances nitrogen uptake and utilization efficiency, generated stable yield increases when subjected to extensive field testing in a range of production environments (J. Wu et al., 2019). In this issue, both Reynolds et al. (2021) and Kholova et al. (2021) identify the need to advance these integrated next-generation breeding technologies in public breeding programmes, such as those co-ordinated via the CGIAR centres.

Managing cropping systems for adaptation to water-limited environments

Managing cropping systems for adaptation to water-limited environments involves integrating effects of varying technological possibilities associated with system components and identifying and exploiting their interactions at cropping system scale (Beres et al., 2020). The technological possibilities might be genetic or management based. At crop level, the G×M interactions of controlled crop stature and fertility management to increase HI (green revolution) and enhanced reproductive resilience (ASI) and increased plant density (US maize) are well known examples. In this issue, Otegui et al. (2021) show how the introduction of genetically modified insect-tolerant maize enabled a change in sowing date of maize in Argentina to facilitate more favourable water deficit patterns through the crop cycle. At cropping system scale in water-limited environments, the studies by Kirkegaard and Hunt (2010) on system-level water productivity in wheat cropping systems in Australia highlight the synergistic interactions of interventions. They found that the combination of conservation tillage, fallow weed control, a legume break crop, earlier sowing, and long coleoptile length generated benefits to system-level water productivity beyond that achievable with any intervention alone. In this issue, Hunt et al. (2021) extend this approach by incorporating potential for genetic improvement to deliver flowering time stability and adjusted sowing times.

It is evident that the Interdrought theme of a trans-disciplinary approach operating across the scales of biological organization from molecular to crop systems is at the fore in the contributions to this issue. The integrated G×E×M×S approach (Box 1), where S is system context, establishes the construct needed to frame and pursue research questions as we move forward with the challenge of improving plant production in water-limited environments.

References