Living With Limited Water, Part II: Dynamics of Leaf Rolling, Leaf Water Homeostasis and Water Economy by Hybrid Rice

Change in leaf form or shape has often been enumerated as a potential means of reducing transpiration by plants experiencing water deficit. Because leaf rolling is the first and foremost visible physiological response to water deficit in rice plant, its dynamic nature and impact on leaf water homeostasis and water conservation were studied in hybrid rice (Oryza sativa L.) in depth. Marked diurnal fluctuations were observed in leaf rolling behavior over short as well as extended periods of time during the drying cycle. Leaves were found unrolled (open) during early and late hours of the day in watered plants, whereas stressed plants exhibited partial rolling (score >3) even in the morning, followed by complete rolling at noon (score 5). The rolling of leaves of watered plants fluctuated within a very narrow range (between score 1 to 2) throughout 20 days of drying cycle, whereas stressed plants, although they did not differ from control plants during the first 5 days of stress, displayed wide fluctuations thereafter, culminating into complete rolling (score 5) with no further diurnal variations in space and time. The pattern of changes in leaf water potential closely followed leaf rolling behavior. Examination of leaves by naked eye and hand lens revealed two distinct textures of surfaces in a given leaf blade, i.e. smooth or ridged, located opposite to each other on either side of the midrib. Quite often, the ridged portion rolled first, followed by the smooth one rolling over the former during the drying cycle as the water potential of the ridged portion was less by 0.48 MPa than the smooth portion at the completion of leaf rolling (score 5). Similarly, the water potential of the apical half portion of the leaf blade was less by 0.41 MPa in comparison with the basal half. In the main, two types of leaf rolling were observed: one was classified as acropetal, which consisted of helical and non-helical rolling, and the other was non-acropetal, characterized by either the ridged portion bending first or ridged and smooth portions bending almost simultaneously. Leaf rolling in etiolated leaves was quicker by 14 min than the green ones; however, darkness delayed the rolling (score 5) of green leaves by 32 min. The rate of transpiration decreased by 52% in manually rolled, excised turgid leaves, which reversed back to the initial rate of water loss once the leaves were unrolled (open). In the absence of water supply to the excised leaves, the spontaneously rolled leaves exhibited 87% water saving (only 13% loss in fresh weight) against 68% saving (32% loss) by leaves mechanically prevented from rolling, suggesting its important role in leaf water homeostasis and water economy during limited-water supply conditions. The findings carry biotechnological applications that may further provide important information for the design of strategies for future engineering of an improved external plant defense system against drought with increased water-use-efficiency in hybrid and super hybrid rice.