# Divergent physiological strategies distinguish tolerant and plastic genotypes in elite Australian rice lines under limited irrigation

## TL;DR

This study identifies two distinct water conservation strategies in rice genotypes under moderate water limitation, offering insights for breeding water-efficient rice without yield loss.

## Contribution

The paper reveals previously under-recognized physiological adaptations in rice genotypes under moderate water stress.

## Key findings

- Inherently tolerant genotypes maintain photosynthesis via lower stomatal conductance and increased cuticular wax.
- Adaptive genotypes show structural changes like enlarged leaf papillae to reduce water loss.
- Top-performing varieties exhibit superior intrinsic water use efficiency confirmed by δ13C analysis.

## Abstract

Water scarcity threatens global rice production, necessitating identification of genotypes with improved water use efficiency (WUE) whilst maintaining productivity. Previous drought studies typically imposed severe stress conditions that compromised yield and quality, creating a knowledge gap regarding rice responses to moderate water limitation during vegetative growth. Here we show that 18 temperate japonica and 2 indica rice genotypes employ two distinct water conservation strategies under controlled limited water conditions (60–65% field capacity): inherent physiological tolerance versus adaptive phenotypic plasticity.

We evaluated rice varieties under ponded and limited water treatments, integrating stomatal traits, chlorophyll fluorescence parameters, leaf carbon isotope composition (δ13C), and surface properties quantified via scanning electron microscopy and ATR-FTIR spectroscopy.

Inherently tolerant genotypes maintained stable photosynthetic performance through constitutively lower stomatal conductance and enhanced cuticular wax deposition. Conversely, adaptive genotypes exhibited pronounced physiological plasticity under water limitation. Notably, LW treatment induced significant enlargement of leaf surface papillae positioned over stomatal complexes, suggesting a potential structural mechanism contributing to reduced transpirational water loss. This represents a previously under-recognised adaptation in smooth-leaf Australian germplasm lacking protective trichomes. Mixed-effects modelling confirmed that photochemical traits and water-use traits responded most strongly to treatment, while reproductive and yield-related measurements indicated no major penalty under limited water. Carbon isotope discrimination (δ13C) validated superior intrinsic WUE in top-performing varieties.

These complementary strategies provide multiple pathways for breeding water-efficient rice adapted to Australian temperate production systems under moderate water limitation without substantial yield loss.