The genetic basis of water-use efficiency and yield in lettuce

Abstract

Background: Water supply limits agricultural productivity of many crops including lettuce. Identifying cultivars within crop species that can maintain productivity with reduced water supply is a significant challenge, but central to developing resilient crops for future water-limited climates. We investigated traits known to be related to water-use efficiency (WUE) and yield in lettuce, a globally important leafy salad crop, in a recombinant inbred line (RIL) lettuce mapping population, produced from a cross between the cultivated Lactuca sativa L. cv. Salinas and its wild progenitor L. serriola L.

Results: Wild and cultivated lettuce differed in their WUE and we observed transgressive segregation in yield and water-use traits in the RILs. Quantitative trait loci (QTL) analysis identified genomic regions controlling these traits under well-watered and droughted conditions. QTL were detected for carbon isotope discrimination, transpiration, stomatal conductance, leaf temperature and yield, controlling 4-23 % of the phenotypic variation. A QTL hotspot was identified on chromosome 8 that controlled carbon isotope discrimination, stomatal conductance and yield under drought. Several promising candidate genes in this region were associated with WUE, including aquaporins, late embryogenesis abundant proteins, an abscisic acid-responsive element binding protein and glutathione S-transferases involved in redox homeostasis following drought stress were also identified.

Conclusions: For the first time, we have characterised the genetic basis of WUE of lettuce, a commercially important and water demanding crop. We have identified promising candidate genomic regions determining WUE and yield under well-watered and water-limiting conditions, providing important pre-breeding data for future lettuce selection and breeding where water productivity will be a key target.

Keywords: Carbon isotope discrimination; Crop breeding; Lactuca sativa; Leafy vegetable; Quantitative trait loci; Salad; Sustainable agriculture; Water‐use efficiency.

Fig. 1

Diurnal transpiration of cultivated (L. sativa cv. Salinas) and wild (L. serriola) lettuce. Transpiration pattern (mmol m^− 2 s^− 1) (a), with example thermal images of cultivated and wild lettuce (b)

Fig. 2

Drought response of cultivated (L. sativa cv. Salinas) and wild (L. serriola) lettuce. Transpiration (a), stomatal conductance (b), leaf temperature (c), carbon isotope (d) and oxygen isotope discrimination (e). * indicate significant differences (see text for details)

Fig. 3

Correlations between water-use traits. Observed under well-watered conditions (a) and under drought 1 (b) and 2 (c) trials. Estimated using Spearman’s correlation, with scatterplot (bottom left) and significant r² correlation values (top right) shown. * indicates significance at P > 0.001 (***), P < 0.01 (**) and P < 0.05 (*). Transpiration (e), stomatal conductance (g_s), temperature measured by porometry (Temp), temperature measured by thermal imaging (IR), carbon isotope discrimination (Δ¹³C), whole fresh weigh (FW), dry weight (DW) and their ratio (FW:DW)

Fig. 4

QTL identification for water-use-associated traits in the RIL population. Bars represent each LG with position in centiMorgan on the left, LG number at the top of each bar and horizontal lines indicating marker positions. QTL are shown as filled boxes to the right of each LG representing the 1-LOD interval, with error bars showing the 2-LOD interval for QTL detected in the well-watered (blue), Dr1 (red) and Dr2 (black) trials. 13 C, Δ¹³C, FW, fresh weight; FW_ Lf56, fresh weight of fifth and sixth true leaves, FW:DW, fresh:dry weight ratio; gs, stomatal conductance, E, transpiration, E:DW, transpiration:(dry weight) ratio; Temp, leaf temperature measured via porometry, IR, leaf temperature measured via IR thermography

Fig. 5

Candidate gene mining of LG8 QTL. Illustration of LG 8, with the QTL investigated for candidate genes highlighted and gene information provided