Plants that are tolerant of short periods of flooding in summer tend to respond to oxygen deprivation in the opposite manner to that which can be seen in the over wintering organs of flood-tolerant species. Instead of down-regulating metabolism there is usually observed an acceleration of glycolysis. In the growing season this type of response can maintain tissues with an adequate supply of metabolic energy that gives the plant time to improve root aeration through the development of adventitious roots and aerenchyma. It is essential in growing plants that aeration be restored by improved root aeration otherwise a rapid depletion of carbohydrate reserves by a hypoxia-induced acceleration of glycolysis (the Pasteur effect) will lead rapidly to cell death. The Pasteur effect can be an effective remedy in the short term for restoring ATP levels and will aid short-term anoxia tolerance and prolong the life of the anoxic tissues for a few days. This is often sufficient for overcoming short periods of flooding during the growing season but will not serve as a long-term strategy for the survival of inundated organs during long periods due to the rapid consumption of carbohydrate reserves (Crawford, 2003).
Under field conditions it is often difficult to determine to what extent the roots of flood-tolerant plants are subjected to an oxygen deficiency. All root tips contain very little free oxygen due to high consumption rates and the density of the tissues and ethanol can always be detected in the dense tissues of the root meristems. Consequently, any reduction in the availability of oxygen, as well as leading to higher levels of ethanol, will extend the zone of oxygen deprivation upwards from the distal regions of the root to other parts of the root system and increase the amount of root that accumulates ethanol. Flooding may initially induce a shortage of oxygen, but in summer flood-tolerant species usually can alleviate this condition and restore an adequate supply in a matter of days, by the development or enhancement of existing aeration tissues (aerenchyma), and the growth of adventitious roots (Joly, 1994).
A series of studies carried over many years in the Netherlands on the plants that grow on the banks of the Rhine have provided much information on the close match that exists between flooding risk and the ability of species to respond morphologically so that submergence does not expose them to the dangers of anaero-biosis (Voesenek et al., 2004). A comprehensive survey of the ethylene-induced elongation response in 22 plant species occurring in the Rhine flood plain showed that the capacity for stem and petiole elongation upon exposure to ethylene correlates positively with flooding duration and negatively with drought. Based on this analysis, it was concluded that the capacity to elongate is an important selective trait in field distribution pat terns of plants in flood-prone environments. However, rapid shoot elongation under water appears to be a favourable trait only in environments with shallow and prolonged flooding events, as the costs associated with this response make this an unviable strategy in sites with deep floods, or in sites where the floodwaters recede rapidly (Voesenek et al., 2004).
The genus Rumex has received particular attention, as it possesses a number of closely related species that coexist along the banks of the Rhine and exhibit varying capacities for petiole extension in relation to their location of the floodline. Rumex palustris which lives low down on the bank and closer to the water's edge than any of the other species shows rapid petiole elongation mediated by the integrated action of the plant hormones ethylene, auxin, gibberellin, and abscisic acid. By contrast, the closely related Rumex acetosa which lives in the upper zone of reduced flooding risk is unable to switch on petiole elongation when submerged. Mature plants, with their greater carbohydrate reserves, generally show greater tolerance of flooding than seedlings (Fig. 8.11).
Adaptations to submergence during the growing season are mostly based on restoring the potential for photosynthesis to inundated plants. When the species are submerged in the dark, death follows in most cases within 6-9 weeks irrespective of whether the plants live at high or low levels on the riverbank (Blom et al., 1994). It is this last finding which demonstrates most clearly the importance to the submerged plant of being able to maintain some photosynthetic capacity and generate oxygen if it is to survive inundation during part of the growing season. A similar effect can be seen in flooded over wintering cranberry plants (Vaccinium macrocarpon). They remain evergreen under water and even under ice. However, when the ice is covered with snow and light is excluded oxygen deprivation injury ensues (Schlüter & Crawford, 2003).
Once contact with air is established the efficiency of the aeration process will be dependent on the internal anatomy of the flooded species. The amount of aeren-chyma differs considerably between wetland species, but usually varies between 10% and 60% of the total volume of shoots, rhizomes and roots (Studer & Braendle, 1984). Obviously, long-term flooding resistance is intimately correlated with aerenchyma formation, as it is the basis whereby gas transport is facilitated from shoots to fine roots. Oxygen can be channelled down to support juvenile mature juvenile mature
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