Genetic variation in transpiration response to evaporative demand in faba bean

Mandour, Hend and Dodd, Ian and Carmo-Silva, Elizabete (2022) Genetic variation in transpiration response to evaporative demand in faba bean. PhD thesis, Lancaster University.

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Abstract

Limiting maximum transpiration rate (TR) under high vapour pressure deficit (VPD) is considered a water conservation strategy, with genotypes expressing this trait considered desirable in high VPD environments where water deficits commonly develop later in the growing season. While breeding companies have incorporated this trait into some crops, there is uncertainty on the best way to phenotype this trait, its underlying physiological mechanisms, and genetic regulation, which hitherto remain unknown for faba bean (Vicia faba L.). Thus, this thesis aimed to 1) understand whether variation in this trait was consistent at single leaf versus whole-plant levels. 2) identify whether restricting transpiration under high VPD was associated with low plant hydraulic conductance and/or tissue abscisic acid (ABA) levels. 3) identify genetic variation in TR response to VPD in 165 faba bean recombinant inbred lines (RILs) derived from two parental lines with contrasting water use and other physiological traits (Mélodie/2 & ILB 938/2). Two British faba bean cultivars (Masterpiece and Robin Hood) were grown in well-watered soil in a semi-controlled glasshouse with diurnally fluctuating VPD and light conditions. In the same plants, whole-plant transpiration was measured gravimetrically under these conditions, single leaf transpiration was measured using an infra-red gas analyzer that regulated VPD around the leaf and whole-plant transpiration was measured in a gas exchange chamber that regulated VPD around the shoot. Transpiration response to VPD consistently varied between the cultivars across the three measurement approaches and fitted a segmented transpiration model with a break-point (BP ) averaging 3.05 and 2.33 kPa for Masterpiece and Robin Hood, respectively then stabilized, decreased, or slightly increased at a diminished rate as VPD increased further. Statistical analysis of model variables (Slope 1, the BP, and Slope 2 values) revealed no significant differences according to the measurement approach, indicating that different instruments can be used according to their availability. The response was also consistent across different times of the year that varied in light conditions, temperature, and VPD. In both cultivars, limited transpiration rates under high VPD coincided with decreased root hydraulic conductance and higher root ABA concentrations. The lower VPD break-point of Robin Hood was correlated with lower root hydraulic conductance and higher root ABA and VII root xylem sap ABA concentrations than Masterpiece at the applied VPD. Thus, genotypic differences in transpirational responses to high VPD in faba bean were more closely associated with root hydraulic conductance and root ABA concentrations than stem hydraulic conductance, leaf ABA and xylem sap ABA concentrations. Measuring whole-plant TR and hydraulic conductance response to VPD in the whole-plant gas exchange chamber revealed contrasting TR responses in the parents of the RILs, with TR of Mélodie increasing linearly with VPD whereas ILB938/2 limited its TR after 2.02 kPa. The higher leaf water potential of Mélodie/2 than ILB 938/2 at the two tested VPDs indicates better control in water status at the leaf level than ILB 938/2. Almost 90 % of the RILs limited their TR at high VPD with a BP range of 1.5<BP<3 kPa and about 10 % had a linear TR response to VPD. Genotypic variation in the BP may allow specific cultivars to be developed for differing water-deficit environments. QTL analysis identified thirteen QTLs contributing to minimum and maximum transpiration, whole-plant and root hydraulic conductances traits on faba bean chromosomes 1 and 3, while one locus associated with break-point transpiration was identified on chromosome 5. These QTLs harboured many abiotic stress-responsive genes, thus they can be used as potential targets for marker-assisted breeding to genetically improve faba bean performance under water-limited environments particularly. Taken together, the limited TR response under high VPD in faba bean is controlled by restricted root hydraulic conductance and higher root ABA accumulation. Further research of cross-talk between different hormones and aquaporins activity seem essential to understand how plant transpiration and/or hydraulic conductance decline at elevated VPD.