Most terrestrial plants cannot withstand complete submergence lasting over a few days; however, semi-aquatic plants such as certain rice genotypes can survive complete submergence for over two weeks, as described in the preceding section. The period of submergence endured depends on various environmental conditions and the plant’s growth stage.
Tolerance in rice of transient submergence caused by flash floods is mainly achieved by assuming a “quiescent” strategy when submerged until floodwater recedes. Recovery after submergence when floodwaters recede is dependent on metabolic changes that occur during and immediately following submergence.
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.
Re-oxygenation injury is well-documented for both animal and plant tissues. A highly reduced intracellular environment (including transition metal ions) and low energy supply, such as occurs during soil waterlogging, are the factors which favour ROS generation. Free radicals are formed soon after O2 re-enters, in a so-called oxidative burst. At the same time, activity of most plant antioxidant systems is compromised due to metabolic perturbations caused by the previous period of anoxia or severe hypoxia (Blokhina et al. 2003). Production of ROS upon reaeration might impede ion uptake by roots; H2O2 causes membrane depolarization and K+ efflux (Chen et al. 2007). Recovery of nutrient uptake upon re-aeration following anoxia in roots of wheat showed a short time lag (~ 4 h) before net K+ uptake accelerated. The time lags could have been associated with repair of general metabolism (e.g. lag in recovery of mitochondria), repair of membranes and membrane transporters, or the prevention of damage from ROS.
Recovery of root growth upon drainage is important for crops in rain-fed agriculture, as deep roots will be required to obtain sufficient water from the soil later in the season (e.g. wheat in Mediterranean climates). Seminal roots of wheat cease growing soon after waterlogging, and if waterlogging exceeds several days these roots show little capacity for re-growth upon drainage and soil aeration. Apices of the main axes of the seminal roots typically die, with any regrowth from laterals. Adventitious roots, by contrast, are able to grow in waterlogged soils and retain the potential of the main axes to elongate upon soil aeration. Resumption of growth of adventitious roots following drainage can be fast, so that these roots continue in their importance for future shoot growth. Nevertheless, the adventitious root system could not compensate for the severe inhibition of the seminal roots of wheat. For waterlogging-sensitive crops, even short periods of transient waterlogging can have longer-term adverse effects. In wheat, for example, 3 d waterlogging severely retarded development even in the longer term after drainage (Malik et al. 2002), highlighting the need for improved waterlogging tolerance in our crops.