As detailed in Section 18.5, complete submergence restricts light intensity and gas exchange, slows O2 and CO2 exchange between shoot tissue and floodwater. Reduced photosynthetic activity, together with excessive growth during submergence, result in severe carbohydrate starvation and consequently, death and disintegration of most tissues when flooding persists for longer duration.
Visual symptoms of stress generally start developing soon after desubmergence, with sensitive genotypes showing leaf senescence and decay, followed by mortality within a few days after desubmergence.
Excessive growth during submergence is common, and due to accumulation of the phytohormone ethylene. Submerged plants tend to elongate excessively, an “elongation escape” adaption that allows their leaves to maintain contact with air until the floodwaters are too deep. This elongation capacity is mediated through ethylene which suppresses ABA synthesis but enhances synthesis and sensitivity to GA, resulting in leaf and internode elongation (Das et al. 2005). Ethylene accumulation also triggers chlorophyll degradation and leaf senescence (Ella et al. 2003b), rendering leaves less fit for photosynthesis both underwater and upon resumption of contact with air after desubmergence.
The sudden aeration and exposure to high illumination upon desubmergence causes oxidative stress resulting from ROS generated in leaves that have limited capacity for photosynthesis following submergence (Ella et al. 2003a).
Recovery after submergence therefore, depends on maintenance of carbohydrate reserves, during and shortly after flooding (Das et al., 2005), and the maintenance of a functional photosynthetic system. In rice, as explained in Section 18.5, tolerance of submergence is conferred by an ethylene response-like transcription factor SUB1A. Induced by ethylene that accumulates within plants during submergence, SUB1A disrupts the elongation escape strategy typical of most lowland rice varieties through suppressing GA-promoted elongation, and also slows ethylene-induced leaf senescence (Bailey-Serres et al. 2008). Survival and recovery are enhanced in two ways: (i) less energy is consumed on elongation growth and carbohydrates are conserved, and (ii) leaf senescence is prevented. Thus, plants continue photosynthesis while underwater, and can resume optimal rates of carbon fixation upon re-exposure to air and high illumination and so minimise ROS damage after desubmergence. The sudden exposure to high O2 and high light increases the generation of ROS. The ability to recover quickly and produce new tillers following desubmergence is important because only these new tillers will become effective in contributing to grain yield.