Aerenchyma formation and increased accumulation of free proline in roots of xerophytic Aloe vera L. cultured in nutrient solutions

Abstract

Aloe vera L. is a xerophyte with a crassulacean acid metabolism pathway. Due to its importance in traditional medicine, healthcare, cosmetics and food products, aloe production is currently at an accelerated pace worldwide. Field production, however, has been limited by its sensitivity to chilling temperature and also by low yield and low plant quality. To overcome these limitations, this study tested the possibility of aloe cultivation in solution culture in a protected environment. Chinese Aloe or Buyecheng Aloe, both are A. vera, and were grown in four solution media or one of the media at different temperatures and pH levels. Time of primary root appearance and/or root growth was quantified. To identify possible mechanisms underlying aloe adaptation to aquatic stress, Chinese aloe were grown in a developing solution culture system and also in soil for 60 days and microstructure, ultra structure, genomic deoxyribonucleic acid (DNA) profile, free proline and protein contents of Chinese aloe roots adapted to aquatic stress were compared to those grown in soil. Results showed that aloes could be aquatically cultivated even though it is a xerophyte. Nutrient solutions, solution pH and cultivation temperature affected root initiation and root growth. Microstructure and ultra structure comparisons of roots produced in solution to roots grown in soil suggested that aloes adapted to aquatic stress by the formation of aerenchyma. Genomic DNA fragmentation of roots formed in solution showed apparent DNA ladders in the adaptive roots, suggesting that the aerenchyma formation was through programmed cell death. Meanwhile, free proline and protein contents in aquatic roots were significantly higher than those of roots grown in soil. As far as known, this is the first documentation of aerenchyma formation in a succulent crassulacean acid metabolism (CAM) plant for adaptation to aquatic stress. The formation of aerenchyma is structure along the increased of proline accumulation metabolically and could be the mechanisms underlying aloe adaptation to aquatic stress.