Physiological evaluation of drought-escaping and drought-tolerant mechanisms in short-duration pulses

Introduction

When Levitt first classified plant drought responses in 1972, he drew a line between escape, avoidance, and tolerance [1] . That framework still guides pulse breeding, but the physiological boundaries remain blurred in practice [2] . Short-duration pulse crops chickpea and lentil account for a large share of protein intake in South Asia and East Africa, and terminal drought is their biggest yield constraint [3] . Escape works through early phenology: plants set grain before moisture runs out [4] . Tolerance relies on osmotic adjustment and antioxidant defenses that keep cells functional as water potential drops [5] . Both carry trade-offs escape sacrifices yield potential, tolerance demands metabolic investment [6] . Most prior work evaluated these strategies separately [7] . Direct comparisons within the same experiment are uncommon [8] . This research placed escape-type and tolerance-type genotypes of chickpea and lentil under matched progressive drought, measuring water relations, phenology, and yield to quantify how each strategy manages water and whether both traits can coexist within breeding targets [9] .

Material and Methods

Material

Four pulse genotypes were selected: escape-type chickpea ICCV 2 (extra-short duration, ~85 days), tolerance-type chickpea ICC 4958 (documented osmotic adjustment capacity), escapetype lentil ILL 6002 (early maturing), and tolerance-type lentil ILL 10065 (deep-rooting, high OA). A drought-susceptible chickpea (ICC 1882) served as check [3,5] . Seeds were sourced from the Genetic Resources Unit, ICRISAT. Plants were grown in 15 L lysimeters filled with sandy loam soil (pH 7.1) at the Sutton Bonington Campus glasshouse, University of Nottingham, from April to August 2022. Ethics clearance for plant research was approved under institutional guidelines (Protocol UoN-SB/2022-019, 8 March 2022).

Methods

A completely randomized design with four replications was used (5 genotypes × 2 water regimes × 4 reps = 40 lysimeters). Wellwatered controls received irrigation to 80% FC. Stressed plants had irrigation withheld from 30 days after sowing (DAS) onward. Soil moisture was tracked gravimetrically by weighing lysimeters every two days. Leaf water potential was measured pre-dawn with a Scholander pressure chamber at 7-day intervals [10] . Osmotic adjustment was calculated as the difference in osmotic potential at full turgor between stressed and control plants [5] . Stomatal conductance was recorded with a porometer (AP4, Delta-T Devices, UK). Cumulative water use was computed from lysimeter mass changes. Days to flowering, podding, and maturity were recorded. At harvest, grain yield, biomass, and harvest index were determined [11] .

Seasonal variations

Mean day/night glasshouse temperatures rose from 18/11°C in April to 26/16°C in July, and photoperiod extended from 13.8 to 16.4 hours. The rising temperature accelerated phenology in all genotypes, but escape types showed a stronger photothermal response, reaching 50% flowering 8-12 days earlier than tolerant types under both water regimes [12] .

Results

Comprehensive interpretation

Escape and tolerance genotypes managed drought through distinctly different water-use patterns (Figure 2, Table 2). Escape types consumed 25-31% less total water than tolerant types because they completed their life cycle 19-23 days earlier (Table 1). Their leaf water potential remained above -2.3 MPa at maturity (Figure 1), not because they maintained turgor physiologically, but because they avoided the most severe stress phase entirely. Tolerant genotypes, by contrast, sustained stomatal conductance at 128-142 mmol m⁻² s⁻¹ at 28 DAS roughly double the susceptible check enabled by osmotic adjustment values of 0.41-0.48 MPa. This allowed continued carbon fixation through the stress period, producing 14-18% more grain than escape types despite using more water. The susceptible check used the most water overall but converted it into the least grain (WUE = 0.25 g L⁻¹), confirming that water consumption without physiological protection is wasteful (Figure 3).

Discussion

The 0.48 MPa osmotic adjustment in ICC 4958 matches the 0.40-0.55 MPa range from ICRISAT field trials [5] , confirming that lysimeters can replicate field-level stress responses. The lower OA in escape types (0.18-0.21 MPa) isn't a weakness these genotypes simply don't need high OA because they sidestep the period when it matters [4] . The 14-18% yield gap between escape and tolerant genotypes is narrower than the 30-40% differences from field experiments in central India, where terminal drought intensity varies more between seasons [8] . In years when drought arrives early, escape types lose their advantage; when late sharp stress occurs, their advantage grows [6] . The practical question for breeders is whether early phenology can combine with moderate osmotic adjustment in a single line, and recent QTL mapping in chickpea suggests this may be feasible [13,14] .

Limitations

This research used glasshouse lysimeters, which constrain root exploration and eliminate field-level spatial variability in soil moisture. The single-season design prevents assessment of genotype × environment interactions. Only two genotypes per crop were tested, and the susceptible check was chickpea only, limiting cross-species comparisons [15] .

Practical applications

Escape-type genotypes like ICCV 2 and ILL 6002 suit environments where terminal drought timing is predictable and arrives late. Tolerance types (ICC 4958, ILL 10065) are better for regions where drought onset varies between seasons. Breeders targeting maximum yield stability should look for moderate escape (early flowering without extreme shortening of grain fill) combined with enough osmotic adjustment to buffer unpredictable stress events [16,17] .

Conclusion

Drought escape and drought tolerance represent distinct but complementary strategies in short-duration pulses. Escape genotypes avoided severe stress through early maturity, using less water and achieving stable but moderate yields. Tolerant genotypes sustained physiological function under prolonged stress through osmotic adjustment, producing higher yields but consuming more water. Neither strategy alone appeared optimal across all scenarios. The susceptible check confirmed that water uptake without physiological protection results in large yield losses. Combining moderate escape phenology with functional osmotic adjustment within single genotypes offers the most promising path toward yield stability in environments with variable terminal drought. Multi-environment testing over several seasons would clarify the yield stability advantages of such combined-strategy lines.

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