Exogenous-osmoprotectant of Casuarina equisetifolia leaf extract enhancing the root anatomical adaptations of Indonesian local rice under drought

Proline is widely distributed in higher plants. In this study, the function of proline as an osmoprotectant in maintaining cell osmotic stability and structural integrity during drought was analyzed in rice roots. The cultivar of Boawae Seratus Malam(BSM), Gogo Jak(GJ), Kisol(KM), Situ Bagendit(SB) and Ciherang(CH) were exposed to the fraction of transpirable soil water (FTSW) method with FTSW 1 level (control), and FTSW 0.2 (drought) with a combination of exogenous osmoprotectant from Casuarina equisetifola leaves extract in three levels: 0%(low), 50%(moderate), and 100%(high). The root anatomical characteristics were analyzed using the paraffin embedding method. Plants undergo root structural changes when exposed to drought. In plants without exogenous osmoprotectant, the epidermal width was in the range of 20 to 30 μm. GJ and SB cultivars showed shrinking in size by 10 to 20 μm in epidermis when exposed to moderate osmoprotectants and increased by 15 to 30 μm in high osmoprotectants. Under drought and exogenous osmoprotectants, the root diameter, root area, cortex radius, vascular cylinder area, number of metaxylem, and diameter and width area of metaxylem showed a significant positive correlation. The epidermal and sclerenchyma thickness and the amount of xylem tend to show non-significant response under drought and osmoprotectants. These changes in root anatomical structure are presumed caused by amino acid compounds, particularly proline in exogenous osmoprotectants acting as a signaling molecule and cell wall protein hydrolysate as well as an inducer for the activation of genes in the biosynthesis of hormones and proteins included in the formation of the cell wall and cytoskeleton during root vegetative growth.