Drought Response in Two Wheat Cultivars
Two spring wheat cultivars, A and B, are grown in lysimeters under identical conditions. Water is withheld for 12 days. On day 12, cultivar A has leaf water potential −1.6 MPa, stomatal conductance 0.08 mol m−2 s−1, and 8% xylem embolism; cultivar B has −2.4 MPa, 0.18 mol m−2 s−1, and 25% embolism. Shoot biomass relative to well-watered controls is 82% for A and 65% for B. Leaf abscisic acid is 2.5-fold higher in A. After rewatering, A recovers to 60% of control conductance within 3 days, whereas B recovers to 95%. In a field trial with a terminal drought, A outyields B by 12%; in a trial with intermittent droughts, B outyields A by 18%. The breeder asks which cultivar to recommend for a region with unpredictable rainfall and whether embolism resistance should be a selection target.
1. Interpret the trade-off between water saving and carbon gain in the two cultivars. Which is better adapted to terminal drought and which to intermittent drought? Use the embolism and recovery data.
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A is conservative: higher ABA, lower stomatal conductance, higher (less negative) leaf water potential, and only 8% embolism. It saves water and outyields B in terminal drought, but its stomata reopen slowly after rewatering (60% recovery), so it is less able to exploit relief. B spends water: higher conductance, lower water potential, 25% embolism, lower biomass under terminal drought, but rapid recovery (95%) and better performance with intermittent droughts. Thus A is better for terminal drought and B for intermittent drought; embolism resistance alone does not capture the recovery trait.
2. Evaluate whether embolism resistance should be a primary selection target for the unpredictable-rainfall region. Propose a selection strategy that accounts for both traits.
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Embolism resistance should not be the only target for unpredictable rainfall. The region needs both hydraulic safety and rapid recovery after rain, so selection should combine low embolism, fast stomatal reopening, and maintained yield. A breeding strategy: phenotype a large set at moderate water potential for percent loss of conductance (PLC), measure recovery of conductance 3 days after rewatering, and score ABA sensitivity; then test lines in both terminal and intermittent drought trials. Selecting only for high ABA or low conductance risks slow recovery and reduced carbon gain, as in A. B’s embolism level is a warning, so a cross or selection index that penalizes high PLC while rewarding recovery is more appropriate.