Our results demonstrated that exogenous application of both GABA and CaCl2 can synergistically enhance the enzymatic/non-enzymatic antioxidant response in rose flowers. The increased flavonoid, carotenoid, phenolic, and antioxidant activity levels as well as higher SOD, CAT, and POD activity observed with combined GABA and CaCl2 application align with previous investigations showing similar antioxidant-elevating properties of these compounds. A number of investigations have ascertained that GABA can induce the activation of the antioxidant defense system in response to abiotic stresses, thereby mitigating oxidative damage caused by the production of reactive oxygen species (ROS) 36–38. Subsequent to the application of exogenous GABA, a significant rise in the activity of enzymatic antioxidants within leaves is observed. This is accompanied by a reduction in the accumulation of ROS, consequently enhancing the resilience of seedlings to adverse environmental conditions 39–41. In an in vitro experiment, it was demonstrated that GABA and proline reduced ROS levels. GABA exhibits a superior capacity for the removal of superoxide anions (O2-) and hydrogen peroxide (H2O2) compared to proline 42.
Ca, through its binding affinity with the phospholipid bilayer, exerts regulatory control over membrane architecture, signaling cascades, and membrane functionality. Consequently, this involvement facilitates the reinforcement and improvement of structural integrity within membrane organelles in plants mitigating adverse environmental changes 43. Previous studies have documented the beneficial influence of exogenous Ca application in alleviating environmental changes in plants. These advantageous effects have been attributed to physiological mechanisms, which include osmotic adjustment, improved antioxidant (enzymatic/non-enzymatic) responses, modulation of Na and K ion homeostasis, enhancement of proline accumulation, and facilitation of root and shoot growth 44–46. In addition, Ca assumes a pivotal role as a secondary messenger in orchestrating plant responses to adverse environmental conditions 47. There exists an interrelationship between this essential macronutrient and GABA concerning the activation of plant antioxidant responses 48. In essence, the accumulation of GABA under both biotic and abiotic stress conditions is intricately linked to the regulation of intracellular Ca2+ and its interaction with calmodulin (CaM).
Under non-optimal conditions such as drought, salinity, and temperature stress, plants exhibit an elevation in intracellular Ca2+ levels. This surge in Ca2+ concentration serves as a stimulatory cue for Ca2+/CaM to activate glutamate decarboxylase (GAD), culminating in the accumulation of GABA. Trobacher, et al. 49 reported that the regulatory network involving Ca2+/CaM, GAD, and GABA depends on MdGAD1 and MdGAD2 in apple. Their activity and spectral properties are modulated by Ca2+/CaM balance and acidic pH. Moreover, the application of exogenous Ca has been demonstrated to activate GAD, thereby promoting the accumulation of GABA in carrots and pears 50–52. This collective body of research underscores the interplay between Ca signaling and the regulation of GABA levels in plants, particularly in response to challenging environmental conditions.
Our findings indicated that PPO activity decreased in response to elevated CaCl2 and GABA levels. The reduced PPO activity provides evidence that GABA and CaCl2 mitigate oxidative damage by enhancing antioxidants as higher PPO is associated with senescence, decay, and quality loss in flowers 53–55.
Our findings indicate applying GABA and Ca could help maintain postharvest quality as treated samples exhibited prolonged vase life, diminished petal MDA content, and higher petal protein concentration. This was in agreement with previous investigations. Anthurium cut flowers, when subjected to a 1 mM GABA treatment, displayed a reduction in electrolyte leakage, H2O2, MDA, lipoxygenase (LOX), and phospholipase D (PLD) activity. Simultaneously, these treated flowers exhibited an increase in proline content and enhanced antioxidant enzyme activities 6,7. The application of GABA, both pre- and post-harvest, in the vase solution at ambient temperatures, resulted in enhanced quality and extended vase life of rose cut flowers, as reported by Mirzaei Mashhoud, et al. 5. Similar finding were reported by Babarabie, et al. 13. They observed an improved vase life of tuberose flowers treated with GABA. In a study on Narcissus tazetta cv. ‘Shahla-e-Shiraz’, it was observed that the activity of PPO experienced significant inhibition in the presence of GABA. Moreover, GABA played a crucial role in enhancing the relative water content of narcissus petals during storage by mitigating alterations in the cellular membrane stability index 12. Abdolmaleki, et al. 56 proposed the use of CaCl2 as a method for enhancing the postharvest longevity of roses. Their research revealed that ‘Dolce Vita’ roses treated with a CaCl2 and/or salicylic acid solutions exhibited significantly prolonged vase life. Our findings and these observed improvement can be attributed to the previously mentioned interplay between Ca and GABA, orchestrating plant responses under non-optimal conditions. The synergistic action of Ca and GABA appears to optimize these responses. In this context, the bolstered antioxidant response induced by Ca and GABA can effectively mitigate the rate of senescence. This results in the preservation of crucial cellular structures such as proteins and membrane functionality for an extended duration, ultimately contributing to the overall longevity and quality of postharvest cut flowers.
In the present study, several correlations were identified among the studied parameters. SOD and CAT were positively correlated. SOD plays a crucial role in the first line of defense against oxidative stress by converting superoxide anions into H2O2. Hydrogen peroxide is then efficiently converted into water and molecular oxygen by CAT, acting as a potent ROS scavenger 57. SOD exhibited a strong positive correlation with total antioxidant activity, underscoring its significant role in the plant's antioxidant system 58. Conversely, PPO showed a negative correlation with total antioxidant activity, CAT, and SOD, aligning with previous studies that reported a decrease in PPO activity following abiotic stress, associated with improved antioxidant capacity 34. Total phenolic compounds were negatively correlated with PPO, potentially due to the enzymatic activity of PPO oxidizing phenolic compounds 59. A positive correlation was observed between total phenolic compounds and enzymatic antioxidants—SOD, CAT, and total antioxidant activity. This suggests that polyphenols play a role in the plant's mechanism against ROS, similar to SOD and CAT, indicating that higher polyphenolic compounds may lead to increased total antioxidant activity 57,60. Ca exhibited positive correlations with SOD, CAT, and total antioxidant activity, while showing a negative correlation with PPO. Calcium is known to play a crucial role in the activation of antioxidant enzymes such as SOD and CAT 61. Ca has an inhibitory effect on PPO activity 62. This can result in an increased content of polyphenolic compounds and improved antioxidant responses, thereby enhancing vase life.