Drought-Induced Anatomical Compensation Stabilizes Gas Exchange in Sorghum

Abstract

Drought limits carbon gain in crops, yet the functional role of bulliform cells in regulating gas exchange and water-use efficiency remains unclear. Here, we tested whether natural variation in bulliform size alters carbon assimilation rate and whole-leaf water-use efficiency in Sorghum bicolor. Two locally adapted ecotypes with contrasting bulliform cell abundance were examined under well-watered, mild water deficit, and severe (total) drought conditions using integrated anatomical, hydraulic, gas exchange, and time-resolved leaf rolling measurements. Under well-watered conditions, the large-bulliform ecotype (PI-533871, F) exhibited thicker leaves, wider interveinal spacing, greater hydrenchyma, higher stomatal density, and smaller xylem area than the small-bulliform ecotype (PI-656076, A). Under mild water deficit, ecotype F underwent pronounced developmental remodelling of leaf structure, whereas ecotype A showed comparatively little anatomical change. Despite these shifts, carbon assimilation and stomatal conductance were maintained in both ecotypes as leaf water potential declined, and water-use efficiency did not differ between ecotypes or treatments. Leaf rolling showed transient, time-dependent differences between ecotypes and did not predict steady-state gas exchange. Ecotype F maintained higher water content per unit leaf area, consistent with coordinated anatomical compensation that stabilized physiological function. When well-watered plants were subsequently subjected to total drought, ecotype-specific physiological divergence emerged, consistent with a possible contribution of pre-existing hydrenchyma differences to drought responses. Although stomatal conductance declined similarly in both lines, the large bulliform ecotype showed higher carbon assimilation rate and exhibited increased water-use efficiency relative to A, suggesting that its greater hydrenchyma investment could have slowed the physiological decline under extreme water limitation. These findings highlight the importance of considering drought intensity, drought timing relative to leaf development, and coordinated trait responses when interpreting bulliform-associated variation and selecting for drought resilience.