A low-cost method to rapidly and accurately screen for transpiration efficiency in wheat

Plant Methods Fletcher et al. Plant Methods (2018) 14:77 https://doi.org/10.1186/s13007-018-0339-y Open Access METHODOLOGY A low‑cost method to rapidly and accurately screen for transpiration efficiency in wheat Andrew Fletcher1, Jack Christopher2, Mal Hunter3, Greg Rebetzke4 and Karine Chenu1* Abstract Background: Wheat (Triticum aestivum L.) productivity is commonly limited by the availability of water. Increasing transpiration efficiency (biomass produced per unit of water used, TE) can potentially lead to increased grain yield in water-limited environments (‘more crop per drop’). Currently, the ability to screen large populations for TE is limited by slow, low-throughput and/or expensive screening procedures. Here, we propose a low-cost, low-technology, rapid, and scalable method to screen for TE. The method uses a Pot-in-Bucket system that allows continuous watering of the pots and frequent monitoring of water use. To investigate the robustness of the method across environments, and to determine the shortest trial duration required to get accurate and repeatable TE estimates in wheat, plants from 11 genotypes varying in phenology were sown at three dates and grown for different durations in a polyhouse with partial environmental control. Results: The method revealed significant genotypic variations in TE among the 11 studied wheat genotypes. Genotype rankings for TE were consistent when plants were harvested the same day, at the flag-leaf stage or later. For these harvests, genotype rankings were consistent across experiments despite changes in environmental conditions, such as evaporative demand. Conclusions: These results indicate that (1) the Pot-In-Bucket system is suitable to screen TE for breeding purposes in populations with varying phenology, (2) multiple short trials can be carried out within a season to allow increased throughput of genotypes for TE screening, and (3) root biomass measurement is not required to screen for TE, as whole-plant TE and shoot-only TE are highly correlated, at least in wheat. The method is particularly relevant in developing countries where low-cost and relatively high labour input may be most applicable. Keywords: Water use efficiency, Water use, Transpiration, Phenotyping platform, Crop adaptation, Crop improvement, Breeding, Drought, Water deficit, Water stress, Wheat, Triticum aestivum, Cereal Background Water availability is one of the primary limiting factors of yield for bread wheat (Triticum aestivum L.). With projected increase in water-stress events in some regions due to climate change [1, 2] and continuing global population growth, greater food production is needed. This can be achieved, in part, through greater *Correspondence: 1 Queensland Alliance for Agriculture and Food Innovation (QAAFI), The University of Queensland, 203 Tor Street, Toowoomba, QLD 4350, Australia Full list of author information is available at the end of the article crop yield and more efficient use of limited resources, such as water. Transpiration efficiency (TE), which is defined as the amount of biomass produced per unit of water transpired, has been suggested as a trait of interest to improve yield in drought-prone environments, in particular where crops rely on stored soil moisture [3–5]. In such environments, crops that are able to utilise available soil water more efficiently can maintain greater soil water reserves early during the crop cycle to use later in the season, when water can be more effectively used to produce grain yield (e.g. [6, 7]). In Australia, increases in yield with the release © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Fletcher et al. Plant Methods (2018) 14:77 of new wheat cultivars has been linked with increases in TE [8], suggesting that indirect selection for TE has occurred as a by-product of breeding for yield in the drought-prone environments of the Australian wheatbelt [9]. Crop simulation studies have also indicated major potential yield gains related to an increase in TE or its component traits [5, 10, 11]. Further, experimental work on carbon-13 isotope discrimination (CID) in leaf dry biomass, a surrogate trait for TE, has highlighted the potential of TE to increase yield in wheat [12]. Selection based on CID has resulted in the release of two high water-use efficiency cultivars in Australia, Drysdale and Rees [4, 13]. TE of a plant is typically expressed as grams of biomass produced (with or without the roots) per kilogram of water transpired. It is commonly measured in sealed pots that exclude soil evaporation and deep drainage. TE differs from ‘water use efficiency’ (WUE), a term used in agronomy and ecology, which typically refers to field measurements of biomass per unit of initial soil water plus in crop rainfall and any irrigation applied but which does not account for soil water evaporation, deep drainage and excludes root biomass [14]. TE can also be defined at the leaf level, and then corresponds to the ratio between carbon assimilation (photosynthesis) and water flux through the stomata. Leaf TE is commonly measured using gas exchange with point measurements in terms of both space (part of a leaf ) and time (seconds to minutes). As a result, TE measured on individual leaves is typically more variable than plant-level measurements [15]. Even with multiple leaf-level point measurements per plant in stable light conditions, TE measured on parts of single leaves may still be poorly correlated with whole-plant TE [15]. Overall, measurements on individual leaves are generally less suitable for large-scale phenotyping than plantlevel measurements. While genomics has the potential to accelerate crop adaptation, one of the current greatest bottlenecks is arguably the ability to phenotype large numbers of genotypes [16] for traits of importance for target production environments such as TE [3, 17]. TE at the plant level can be phenotyped with gravimetric methods in pots, by estimating the amount of water transpired by changes in pot weight over time. In this case, the watering and weighing can be performed manually [18] or with automated platforms [19–21]. TE can also be phenotyped indirectly, in either pot or field experiments, using CID measurements on dried laminas typically harvested between the ‘stem elongation’ stage and flowering [12]. In C3 species such wheat, CID is a stable trait that negatively correlates with TE [12, 22, 23]. In compa (...truncated)