Regulatory Effect of Exogenous γ-Aminobutyric Acid on Respiratory Rate through the γ-Aminobutyric Acid Shunt in Malus baccata (L.) Borkh. Roots under Suboptimal Low Root-Zone Temperature

Malus baccata (L.) Borkh. is one of the most widely used rootstocks in apple-producing region Northern China. However, early growing season, apple roots are often subjected to suboptimal low root-zone temperatures. The regulatory effects exogenous γ-aminobutyric acid (GABA) on both shunt (GABA shunt) and respiratory activity under temperatures remain unknown. To explore physiological basis for GABA alleviation low-temperature stress M. roots, following treatments were examined: temperature (potted parts seedlings maintained at 5 ± 0.5 °C; L); + (LG); vigabatrin (VGB; LV), which a specific active inhibitor transaminase (GABA-T). Each treatment was matched with control (18 °C/8 °C day/night; CK) comparison. Our results showed that L reduced root vitality, increased malondialdehyde (MDA) content, promoted accumulation GABA, activated shunt, inhibited total respiration rate (VTotal) by decreasing rates Embden–Meyerhof pathway (VEMP) tricarboxylic cycle (VTCAC). LG significantly content endogenous accelerated metabolism enhanced increasing VTotal, VEMP, VTCAC, cytochrome (VCP), thus alleviating damage stress. Meanwhile, contrasting observed LV treatment. These findings revealed improved tolerance spring regulating activity.