Stomatal Conductance and Water Relations in a Drought-Stressed Orchard
A 12-year-old mango orchard in Rajshahi, Bangladesh, shows midday wilting despite irrigation. Soil moisture at 30 cm is 18% (field capacity 28%), and leaf water potential at predawn is -1.2 MPa (normal -0.4 MPa). Stomatal conductance (gs) measured at 10:00 is 0.15 mol m-2 s-1, but at 14:00 it drops to 0.03 mol m-2 s-1 while leaf temperature rises to 38°C (air 34°C). CO2 assimilation (A) falls from 12 to 2 µmol m-2 s-1. Root zone temperature is 32°C. The orchard is on a sandy loam with 1.2% organic matter. The farmer applied 100 kg ha-1 urea one week ago but no mulch. You are asked to diagnose the physiological bottleneck and recommend immediate and long-term interventions.
1. Explain the physiological cause of the midday drop in gs and A, and why the leaf temperature exceeds air temperature. Calculate the approximate percentage reduction in A and gs.
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The predawn leaf water potential of -1.2 MPa indicates severe water deficit (normal -0.4 MPa), so soil water is not replenishing the plant overnight. As vapor pressure deficit (VPD) increases at midday, the plant responds by stomatal closure to conserve water, causing gs to fall from 0.15 to 0.03 mol m-2 s-1 (an 80% reduction). This closure restricts CO2 entry, so A drops from 12 to 2 µmol m-2 s-1 (an 83% reduction). Because transpirational cooling is reduced, leaf temperature rises above air temperature (38°C vs 34°C). The combination of low soil moisture and high VPD triggers stomatal limitation, not biochemical limitation. The high root-zone temperature (32°C) may also increase root respiration and reduce water uptake efficiency. Immediate intervention: apply mulch to reduce soil evaporation and moderate soil temperature, and irrigate lightly but frequently to raise soil moisture to at least 22%. Long-term: increase organic matter to improve water-holding capacity, and consider partial root-zone drying or regulated deficit irrigation to acclimate the trees. The urea application without incorporation or water may have volatilized, so fertigation would be more effective.
2. If the orchard manager wants to screen 20 new mango cultivars for drought tolerance using a rapid, non-destructive method, which two physiological parameters would you choose and why? Describe a simple field protocol and a decision rule.
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Choose predawn leaf water potential (Ψpd) and stomatal conductance (gs). Ψpd integrates soil water status and root uptake, while gs reflects stomatal regulation and water-use strategy. Both can be measured quickly with a pressure chamber and a porometer, respectively. Protocol: measure Ψpd on three fully expanded sun-exposed leaves per tree between 04:00 and 06:00, and gs at 10:00 and 14:00 on the same leaves. Compute the relative change in gs (Δgs = (gs10 - gs14)/gs10) and the maintenance of Ψpd above -0.8 MPa. Decision rule: cultivars with Ψpd > -0.8 MPa and Δgs < 50% are considered tolerant; those with Ψpd < -1.2 MPa and Δgs > 70% are susceptible. This screening is non-destructive and can be repeated across seasons. To avoid confounding, ensure all trees are on the same rootstock and soil type, and measure at the same phenological stage. The method does not measure yield but identifies physiological traits linked to drought adaptation.