Potential Drought Tolerance Traits and Their Measurements

Morphological Traits

These are visible plant structures that help conserve water or improve water uptake.

(a) Root Traits

• Deep and extensive root system → improves access to deeper soil water.
  • Measurement:
    • Soil coring method: take soil cores and wash roots.
    • Rhizotron technique: transparent walls to observe roots.
    • Shovelomics: field-based root phenotyping.
  • Examples: Chickpea, sorghum, sunflower.
• Root length density & root:shoot ratio → higher allocation to roots.
  • Measurement: dry weight of roots vs shoots.

(b) Leaf and Shoot Traits

• Leaf area & LAI (Leaf Area Index) → smaller leaves reduce transpiration. Measurement: Leaf area meter, or calculation using length × breadth × correction factor.
• Leaf rolling/folding → reduces exposed surface area. Measurement: Visual scoring (0 = no rolling, 5 = complete rolling). Examples: Rice, maize.
• Leaf waxiness / cuticle thickness → reduces cuticular transpiration. Measurement: Microscopy / visual scoring. Examples: Pearl millet, sorghum.
• Leaf shedding / senescence → shedding old leaves reduces transpiration. Measurement: Count fallen leaves, or use senescence scoring. Examples: Pigeonpea, cotton.

(c) Reproductive Traits

• Early flowering & maturity (drought escape). Measurement: Field observation of days to 50% flowering. Examples: Early-maturing wheat, mungbean.
• Retention of flowers & grain filling under drought. Measurement: Count flowers, pods, grains.

Physiological Traits

These traits reflect plant water status and efficiency.

(a) Relative Water Content (RWC)

• Indicates hydration of leaves.
• Formula:

• RWC = FW−DW / TW−DW × 100
• where FW = fresh weight, TW = turgid weight, DW = dry weight.

• Critical values: 80% = well-watered. <50% = severe drought.

(b) Leaf Water Potential (Ψleaf)

• Direct measure of leaf hydration status.
• Measurement: Pressure chamber (Scholander bomb) or thermocouple psychrometer.
• Example: Drought-tolerant crops maintain Ψleaf above –1.5 MPa.

(c) Stomatal Conductance & Transpiration

• Reflect stomatal regulation under drought.
• Measurement: Porometer or Infrared Gas Analyzer (IRGA).

(d) Canopy Temperature / Crop Water Stress Index (CWSI)

• Drought-stressed plants have hotter canopies due to stomatal closure.
• Measurement: Infrared thermometer, thermal imaging.
• CWSI formula:

• CWSI=Tc – Tw / Td−Tw
• where Tc = canopy temp, Tw = non-stressed baseline, Td = fully stressed baseline.

(e) Osmotic Adjustment

• Maintenance of cell turgor via solute accumulation (proline, sugars, K⁺).
• Measurement: Osmometer / solute analysis.

Biochemical Traits

These are molecular changes in stressed plants.

• Proline accumulation → osmoprotectant maintaining osmotic balance. Measurement: Colorimetric assay (ninhydrin method).
• Glycine betaine & soluble sugars → maintain osmotic adjustment. Measurement: Spectrophotometry, HPLC.
• Antioxidant enzymes (SOD, CAT, POD) → detoxify ROS produced during drought. Measurement: Enzyme activity assays.
• ABA (Abscisic Acid) concentration → drought stress hormone, induces stomatal closure. Measurement: ELISA, HPLC, GC-MS.

Agronomic / Yield Traits

These reflect field-level performance.

• Grain yield under stress vs irrigated conditions. Measurement: Harvest yield trials in rainfed vs irrigated plots.
• Harvest Index (HI) → efficiency of converting biomass into yield. Formula: HI = Grain yield / Total biomass × 100 
• Yield stability index / stress susceptibility index → compares yield under stress vs non-stress.

Molecular / Genetic Traits

Modern methods to identify stress-tolerant genotypes.

• Drought-responsive genes (DREB, CBF, NAC, MYB). Measurement: Gene expression analysis (qRT-PCR, RNA-seq).
• QTL mapping for root traits and WUE. Measurement: Molecular markers, GWAS, MAS (Marker Assisted Selection).
• Transcription factors & stress proteins (LEA proteins, dehydrins, HSPs).

Key Facts for Exams

• RWC: >80% (no stress), <50% (wilting).
• Leaf rolling index (rice): scored on 0–5 scale.
• Proline content increases 5–10× under drought.
• ABA levels can increase 10-fold during drought.
• CWSI = 0 (no stress), 1 (severe stress).
• Deep-rooted crops (sorghum, pigeonpea) tolerate terminal drought best.