Girdling-like Effect Of Alternanthera Philoxeroides Induced By Partial Submergence

As the global climate change,heavy rainfall and flood disasters occure frequently.More and more terrestrial plants suffer from flooding stress,especially partial submergence.The flooding-induced changes of the plant environment from air to water result in oxygen deficit and energy crisis in submerged tissues and organs.Energy deficiency may inhibit the transportation of carbohydrates in submerged tissues and organs which is an active process consuming energy.Carbohydrates transportation hampered by phloem girdling on woody plants result in the accumulation of carbohydrates in stem part upon girdling site and the stem swelling up.Then for partially submerged plants,it deserves study whether carbohydrates transportation hampered by the short supply of energy in submerged tissues result in the accumulation of carbohydrates in abovewater stem part and the stem swelling up,as same as phloem girdling on woody plants.Although the submerged environment poses many challenges for plant functioning,there are adaptation traits to flooding stress.Studies have shown that both adventitious roots and aerenchyma contribute to improving oxygen content in partially submerged plants.Accordingly,adventitious roots and aerenchyma can ease the energy crisis slightly of submerged tissues.Thus it deserves discussion whether adventitious roots and aerenchyma affect the girdling-like effect of partially submerged plants that carbohydrates accumulate in abovewater stem part and the stem diameter increase.In our study,three comparative experiments were carried out with Alternanthera philoxeroides(Mart.)Griseb.The first experiment was to compare with the partially submerged plants and unsubmerged plants,in order to confirm whether partial submergence causes the girdling-like effect.The second experiment was to compare with partially submerged plants with and without adventitious roots.This goup was used to investigate how does the adventitious roots affect the girdling-like effect of partially submerged plants.The third experiment was to compare with the aerenchyma blocked and unblocked plants,to investigate how does the aerenchyma affect the girdling-like effect of partially submerged plants.The results got from the above three experiments show that:(1)Compared with unsubmerged plants,the abovewater stem of partially submerged plants swelled up and had more carbohydrates accumulated which indicated that the partial submergence caused the girdling-like effect of A.philoxeroides.As the energy deficiency in submerged tissues,the biomass growth and sink volume of underground roots declined sharply which hampered carbohydrates transportation to underground roots and resulted in carbohydrates accumulation in stem and leaves.Because the stem and leaves above the watersurface were in the air with sufficient oxygen and had more available carbohydrates,the total biomass growth,the main stem elongation,the leaf number growth and leaf photosynthesis of partially submerged plants were all not significantly different from unsubmerged plants(p > 0.05).Therefore girdling-like effect is positive for the tolerance of partial submergence of A.philoxeroides.(2)Both the plants with adventitious roots and without adventitious roots had an accumulation of carbohydrates in stem and leaves and carbohydrates content had no significant difference(p > 0.05).However,the abovewater stem diameter increment,the total biomass growth,the main stem elongation,the leaf number growth and leave photosynthesis of plants without adventitious roots were significantly less than those plants with adventitious roots(p < 0.05).The nitrogen content in leaves of plants without adventitious roots was significantly lower than plants with adventitious roots(p < 0.01),which resulted in the lower leaf photosynthetic capacity.Therefore the total amount of carbohydrates produced by leaves in plants without adventitious roots is less than plants with adventitious roots,which account for the discrepancy in biomass growth and abovewater stem diameter increment between plants with and without adventitious roots.For the partially submerged plants,the presence of adventitious roots is a key factor to have a positive effect on growth.(3)Both the aerenchyma blocked plants and unblocked plants had a girdling-like effect.There were no significant differences in abovewater stem diameter increment and the amount of the accumulation of carbohydrates between the aerenchyma blocked plants and unblocked plants(p > 0.05);Compared with adventitious roots,the discontinuous aerenchyma of A.philoxeroides had no prominent effect on the occurrence of the girdling-like effect.