Biochar and K-fertigation modulate stomatal behaviour, ion homeostasis and water use efficiency in waxy maize seedlings under salt stress

Abstract

K-fertigation and soil amendment could ameliorate the adverse effects of saline stress on C₄ crop physiology and performance, yet the underlying mechanisms remained largely elusive. This study investigated the interactive effects of two K-fertigation regimes (full irrigation and deficit irrigation regime during K-fertigation (FIK and DIK)) combined with two soil amendments (no (CK) and wheat straw biochar (WSB) amendment) on ABA signal, non-stomatal limitation, gas exchange, water use efficiency (WUE) and ion homeostasis in C₄ crops waxy maize seedlings under two salt stress levels (non-salt stress, 0 mM NaCl and salt stress, 120 mM NaCl), totalling eight treatments, each with four biological replicates. The results indicated that salt stress and DIK conjointly reduced 48.19% gas exchange rates, 50.68% water relations and 94.34% leaf K/Na ratio, while increasing 6.37% leaf abscisic acid concentration ([ABA_l]), 22.59% non-stomatal limitation, 1.38% stomatal density and 16.54% carbon isotope discrimination. WSB notably increased 24.26% leaf photosynthetic capacity under salt stress and DIK, primarily by elevating 78.98% [ABA_l] and thereby reducing 4.07% non-stomatal limitation. Moreover, WSB improved 21.05% plant WUE and presented a synergistic effect when associated with the DIK regime, achieving a 7.96% higher plant WUE than either single factor. WSB reduced 30.41% leaf Na⁺ accumulation and increased 59.70% leaf K/Na ratio to recover salt-impaired ion homeostasis. These findings would provide some novel insights for efficient water use and ion homeostasis in C₄ waxy maize seedlings under soil drying and salinity and support further field investigation.