Root Growth And Morphology Of Switchgrass And Bushclover In Mixtures Under Different Water And Phosphorus Supply Conditions

Water and phosphorus?P?are important environmental factors affecting plant growth in arid and semi-arid regions.Root system is the main organ of plants to absorb water and nutrients.To evaluate root growth and morphology of introduced species when mixed with native species under different water and P conditions would provide a theoretical basis for assessing the ecological risk of introduced species.Here,we investigated the effects of soil water,P and mixture ratio interactions on root morphological traits of switchgrass?Panicum virgatum L.??an introduced C₄ perennial herbaceous grass?and bushclover?Lespedeza davurica S.??a C₃ perennial leguminous subshrub?in mixtures.The two species were grown in a pot experiment under two soil water regimes[75±5%field capacity,FC?high water,HW?and 35±5%FC?low water,LW?],three P treatments[P₀?no extra P?,P_0.05?0.05g P₂O₅per kg dry soil?and P_0.1?0.1g P₂O₅ per kg dry soil?]and 5 mixture ratios[12:0,8:4,6:6,4:8,0:12].The root biomass and morphological traits of the two species under all treatments were evaluated at the end of growth season.The main results are as follows:1.Root biomass?RB?of switchgrass and bushclover at each P and mixture ratio treatment was significantly decreased under LW comparing with HW?p<0.05?.Comparing to P₀treatment,P application increased RB of switchgrass and bushclover under HW,while generally had no effect on RB of both species under LW.Under different water and P treatments,RB of switchgrass increased as its mixture ratio decreased and reached the maximum at 4:8?switchgrass:bushclover?,and averagely increased 64.9%in mixtures comparing to monoculture under HW,while 24.5%under LW.RB of bushclover showed a decreased trend as its mixture ratio decreased after P application under HW,while those under LW had no significant difference among mixture ratios.The results showed that 4:8 mixture ratio of their mixtures could contribute to the formation of RB of both species,and the effects of mixture ratio on RB of switchgrass and bushclover were depressed under LW.2.Root average diameter?RAD?of switchgrass significantly decreased under LW comparing to HW,specific root length?SRL?increased under LW.RAD and SRL of bushclover under LW was opposite to those of switchgrass.These indicated that switchgrass showed an increase in length of small diameter under low water,and bushclover showed the opposite.Comparing to P₀ treatment,P application reduced RAD and increased SRL of switchgrass under LW,while those of bushclover under LW showed the opposite,indicating P application further increase the root length in small diameter of switchgrass and the opposite to bushclover.Under each water and P treatment,total root length increased as its proportion decreased in mixtures,and those of bushclover showed the opposite.Fine root length?root length in 0⁰.5 mm diameter class?of switchgrass and bushclover accounted for about 80%of the total root length.Mixture ratio has no significantly effect on fine root length proportion,while fine root length proportion of bushclover in mixtures were decreased comparing to monoculture,indicating that mixed plantation mainly reduced the fine root length of bushclover.3.Under LW,stele diameter:root diameter,vessel diameter and ratio of vessel area of switchgrass were increased comparing to those under HW,and vessel diameter of bushclover were decreased under LW.Comparing to P₀ treatment,P application increased stele diameter,stele diameter:root diameter and vessel diameter of switchgrass under LW,but decreased vessel diameter of bushclover under LW.Mixture ratio had no significant effect on the root anatomical traits of switchgrass.Under HW,the root cross-sectional area and xylem area of bushclover increased with the increase of its proportion in mixtures,but the mixture ratio had no significant effect on the ratio of xylem area.Under LW,there was no significant difference in root cross-sectional area,xylem area and ratio of xylem area among different mixture ratios,indicating that the influence of mixture ratio on root xylem area of bushclover was caused by root diameter.In conclusion,root biomass and the effects of P fertilizer and mixture ratio on root biomass of switchgrass and bushclover were depressed under low water regime.Under low water regime,switchgrass increased the root length in small diameter and vessel diameter,as opposed to bushclover.Under low water regime,P application further increased the root length in small diameter and vessel diameter of switchgrass and decreased the root length in small diameter and vessel diameter of bushclover.Switchgrass and bushclover mixture reduce the fine growth of bushclover,4:8 mixture ratio promotes root growth in mixtures.