Effects Of Adventitious Roots And Discontinuous Aerenchyma On Submergence Tolerance Of Plants

As consequences of climate changes,there has been an increased frequency of extreme precipitation and flooding events in recent decades in the whole world.Additionally,it is predicted that extreme precipitation events over most temperate regions will very likely become more intense and more frequent in the future,particularly in the cold season,but also in the growing season.These events would result in more severe flood hazards in future.Consequently,these changes will affect the natural system and probably will result in changes in species distributions and abundances and in changes in timing of ecological processes in both terrestrial and aquatic systems.In light of these rapid changes,the aims of this thesis were to investigate(1)the traits related to oxygen uptake and transport of terrestrial plants,here represented by the model species Alternanthera philoxeroides,upon different submergence regimes,especially of traits related to aerenchyma and adventitious roots,and(2)the subsequent influences or benefits of these responses to the growth and survival of the same plants.In this study,in order to test the function of discontinuous aerenchyma in stems and the effects of discountinous aerenchyma on the flooding tolerance,we applied the pith cavity blocking method and measured the oxygen dynimics in submerged plants by microelectrodes,at the same time we measured the performance of submerged plants after blocking treatments.Additionally,it has found that a number of plants produce adventitious roots on the submerged parts,in this study,in order to test the oxygen uptake ability of adventitious roots,we measured the oxygen profiles in the water column towards the surface of adventitious rotos;moreover,we proned the adventitious roots which were produced during submergence,to estimated the contribution of adventitious roots on the flooding tolerance of plants.At last,to test the responces of adventitious roots to different water conditions and efftcs on the flooding tolerance we controled the oxygen concentration and nutrient availability in water column.The main results were presented as followed:Compartmented pith cavities,partitioned by nodal diaphragms,are among the traits that are likely to affect oxygen transport upon submergence.Despite the occurrence of these cavities in many plant species,it is still not clear whether and how the compartmented pith cavities affect gas transport and internal aeration in plants.In chapter 2,we found that,in partially submerged plants,blocking compartmented pith cavity of only one stem internode impeded gas transport in stems and strikingly reduced the oxygen concentration in pith cavities and cortices of submerged stem parts below the blocked internodes.It resulted in suppressed plant growth,decreased biomass allocation to leaves,and enhanced formation of aquatic adventitious roots on submerged stem parts.These results indicate that compartmented pith cavities,although discontinuous and partitioned by nodal diaphragms,play an indispensable role in gas transport and internal aeration in plants and are advantageous to plants in flooding and submergence tolerance.For adventitious roots,induced by many terrestrial plant species in response to flooding,there is no experimental evidence for the possible oxygen uptake function;or for how important this function may be for the survival of plants during prolonged submergence;and how the morphology of adventitious roots is affected by the oxygen and nutrient availability in the surrounding environment.In chapter 3,O2 profiles in the ambient water column of adventitious roots showed that adventitious roots absorbed oxygen from water;and that oxygen was subsequently transported to other tissues.Moreover,plants with intact adventitious roots had a more efficient carbohydrate economy and prolonged survival during submergence compared to plants of which all adventitious roots had been removed.In chapter 4,the results demonstrated both nutrient and dissolved oxygen deficiency impeded the growth of partially submerged A.philoxeroides,and caused the plant to produce adventitious roots with larger surface area: volume ratio.Nutrient availability did not affect the size of aerenchyma channels in adventitious roots,but low level of dissolved oxygen stimulated the formation of large-sized aerenchyma channels in adventitious roots.Large-sized aerenchyma channels in adventitious roots reduced the gas diffusion resistance and improved the oxygen availability in adventitious roots,thereby enhancing the roots' nutrient absorption capability.The widening of aerenchyma channels in adventitious roots improved the tolerance of A.philoxeroides to hypoxia.During partial submergence,the parts of plants above water surface can exchange the air with atmosphere and transport down to the lower parts which are under water surface.However,it is common that the oxygen can be consumed in the pathway,then it caused the parts of plants are more close to water surface to get more oxygen;in opposite,more far from the water surface gets less oxygen.The production of adventitious roots need some energy and nutrients,so we hyposizied that the decreased oxygen in stems affects the production of adventitious root.In our study,it was found the nutrient availability in water column affected the distribution of adventitious roots on the submerged stems;Under low nutrient conditions,the plants invest more on the production of adventitious roots and the nodes which were close to water surface produced more adventitious roots compared to the nodes far from the water surface.Through the research described in this thesis several knowledge gaps related to flooding tolerance of terrestrial plants have been filled.Future research on flooding tolerance of terrestrial plants should focus on a broader range of plant species in relation to long term and deep water submergence in field sites and on hormone and gene expression related to the flooding survival traits of terrestrial plants.