Development of apoplastic barriers in roots of selected rice genotypes during drought

Drought is a common form of abiotic stress that plants experience under natural condition, which primarily affects the uptake of roots with water. The internal apoplastic barriers in the roots are thought to regulate/constrain the radial water flow via the cellular gaps in the roots (apoplastic pathway). In this study, a screenhouse experiment was conducted using four cultivars of rice namely: Jasmine, Arabon, Basmati, and UPLRi-3 to examine the development of apoplastic barriers in the roots under three water regimes: well-watered, drought, and rewatered conditions in rice roots, apoplastic barriers are usually found in the rigid exodermis, lignified sclerenchymatous layer, and suberized endodermis. Statistical analysis revealed that all of these tissues significally responded to the water regimes. Compared to normal or well-watered conditions, drought imposition for three weeks caused an increase in exodermal and endodermal cell sizes by 15.65% and 6.57%, respectively, and an increase in lignification of sclerenchymatous layer by 37.25%. Pronounced suberization of root endodermis was observed during drought imposition with a mean thickness of 3.17 um. The four varieties did not significantly differ in exodermal and endodermal cell sizes which ranged from 17.55-17.99 um and 11.74-12.27 um, respectively. The drought-tolerant variety UPLRi-3 developed the thickest lignifications of sclerenchymatous layer (2.15 uml), followed by Jasmine (1.9 um), and lastly Arabon and Basmati (1.59 um and 1.51 um, respectively). The varieties UPLRi-3, Arabon, and Jasmine produced high suberin depositions in endodermis, which were not significantly different from each other (3.76 um, 3.51 um, and 3.49 um, respectively). On the other hand, Basmati, the drought-sensitive variety, produced the lowest (2.81 um) deposition of suberin. These results suggest that rice roots develop barriers to prevent internal water loss thereby protecting the root from rapid dying during drought.