Responses To Longterm Winter Flooding

Less attention has been given to understanding the consequences of high water tables in winter than during the growing season, possibly because the study of flooding tolerance has been strongly influenced by the need for research on annual crop plants. The relative neglect of winter studies into flooding tolerance is also possibly due to the erroneous assumption that marsh and bog plants owe their ability to survive entirely to their capacity to aerate their submerged organs by the downward movement of oxygen from emergent shoots.

Fig. 8.10 Efficacy of shoot aeration of underground organs in the common reed Phragmites australis. Note the effects of shoot submergence and de-submergence on oxygen diffusion rate from submerged rhizome apices to phyllospheres. Closed circles, apex of horizontal rhizome; open circles, apex ofvertical rhizome. Diffusion rate in agar remote from rhizome apices and roots was close to zero throughout the experiment. (Reproduced with permission from Armstrong et al., 2006.)

Fig. 8.10 Efficacy of shoot aeration of underground organs in the common reed Phragmites australis. Note the effects of shoot submergence and de-submergence on oxygen diffusion rate from submerged rhizome apices to phyllospheres. Closed circles, apex of horizontal rhizome; open circles, apex ofvertical rhizome. Diffusion rate in agar remote from rhizome apices and roots was close to zero throughout the experiment. (Reproduced with permission from Armstrong et al., 2006.)

However, those ecophysiological studies that have been carried out on wetland plants reveal considerable physiological and biochemical variation between species in their ability to withstand prolonged oxygen deprivation as a result of winter flooding.

8.3.1 Surviving long-term oxygen deprivation

Plants that over winter in flooded habitats have to adapt to more than a transitory deficiency in oxygen availability to their submerged organs. In many herbaceous species the perennating organs are buried for months in a water-saturated soil with no access to atmospheric oxygen. The underground organs (rhizomes, tubers, etc.) of many of these plants are remarkable for their tolerance of extended periods of waterlogging and oxygen deprivation. Experimental studies in which the overwintering organs, instead of being merely submerged in water, were incubated in completely anoxic environments have shown that in many amphibious species perennating organs can survive months of total anoxia (see also Section 3.6.2). Some of these species are able to resume shoot growth in spring from a depth in anaerobic mud without the need for an air supply, while others wait for the floodwaters to subside before resuming growth. Species that emerge from a depth of flooded soil or water under total anoxia (e.g. Schoenoplectus lacustris, Typha latifolia) are capable of sustaining a viable growing shoot under total anoxia for several weeks. This tolerance of prolonged anoxia is frequently accompanied by an ability to down-regulate metabolism and thus reduce the risk of exhausting their carbohydrate reserves and accumulating toxic metabolites (see Section 3.6.3).

In most species the general dieback of herbaceous vegetation in winter renders aeration adaptations inoperative. However, a number of wetland herbaceous species do not die down completely in winter and still retain some green basal shoots. Both reed sweet grass (Glyceria maxima) and a number of rushes (e.g. Juncus effusus) retain some basal green leaves and can be expected therefore to have a source of at least some oxygen to alleviate the effects of winter flooding on their roots.

In the common reed (Phragmites australis) and common club-rush (Schoenoplectus lacustris) the dead stalks that survive above the level of winter flooding can act as a ventilating system for the submerged rhizomes. When wind blows across the tops of broken reeds it acts in a manner similar to air being blown over the holes of a flute, creating a pressure differential across tall dead culms, sucking air into the underground system (Venturi convection). It has been demonstrated (Armstrong et al., 1992) that wind, by increasing Venturi-induced convection, can raise the oxygen concentrations in the rhizome system, thereby causing substantial fluxes of oxygen into both the root and rhizosphere.

Was this article helpful?

0 0

Post a comment