Mechanism Of Carbon-water Combination Allocation Response To Drought Stress In Bothriochloa Ischaemum-soil System

Precipitation in arid and semi-arid regions of China is scarce and its seasonal distribution is uneven.The climate model predicts that the frequent drought in the region will seriously affect the growth of plants,and the shortage of water resources restricts the development of ecological construction in this region.Understanding the response mechanism of carbon-water allocation in the plant-soil system to drought stress and understanding the core of plant adapt the drought stress environment in this area.A typical grass species in arid and semi-arid areas,Bothriochloa ischaemum(B.ischaemum),was studied in this study.By using the control experiment was combined with the carbon-oxygen isotope tracer method,the B.ischaemum-soil systems response to drought stress under three water treatments were compared(sufficiently water supply CK,mild drought stress MS,severe drought stress SS).Physiological and ecological characteristics,migration process of photosynthate fixation,distribution,metabolic and transformation to the soil,water distribution,and transportation process and carbon-water coordination relationship of the B.ischaemum-soil systems were analyzed.The results are as following:1)The net photosynthetic rate of B.ischaemum under drought stress was significantly lower than that of adequate water supply.The water use efficiency of B.ischaemum in mild drought stress treatment was significantly higher than the other two treatments.The new leaf isotope ratio S13C is most correlated with water use efficiency,indicating that the stable carbon isotope characterization could represent the water use efficiency of B.ischaemum.2)H218O pulsed markers 6 and 24h,the stems and leaves of plants reached the maximum value in turn,and the ?18O values of all organs decreased significantly after 48h.The labeled water reached the bottom soil(20-27cm)at 216h in SS treatment and 96h in CK treatment,respectively,but only reached the soil at 15-20 cm depth in the MS.The semi-reduction period and tracer residual period in drought stresses were significantly higher than that in CK treatment,because that the transpiration rate decreased causing the water transport rate decreased.Severe drought stress reduced the proportion of water distribution in old leaves and delayed the migration of water to the ground.Mild drought stress increased the proportion of water use both in roots and aboveground parts.The proportion of water allocated to roots was significantly reduced in CK treatment.and the proportion of water distribution in the aboveground part increases.3)After 13C pulse mark 0-6h,24h,216h and 360h,the amount of 13C fixed by photosynthesis reached peak value in new leaves,stems,roots and rhizosphere soil.SS treatment decreased the total biomass,but the root/shoot biomass ratio increased significantly compared with CK treatment,indicating that drought stress increased the carbon input of underground.The photosynthetic carbohydrate fixation and distribution strategy are as following:in CK treatment,the photosynthetic products fixed in the leaves are adequacy,which mainly used for the aboveground growth.The allocation of 13C amount in MS treatment is balanced between the aboveground and underground.The amount of 13C of roots in SS treatment increased,and the 13C ratio of fine/coarse roots under drought stress was significantly higher than that in CK,indicating that root carbon demand was affected by root heterogeneity.The carbon requirement of fine roots under drought stress was higher than that of coarse roots.4)The fine root turnover rate of severe drought stress is the lowest and the labeled carbon is not converted into soil.The amount of 13C in roots and rhizosphere soil under mild drought was significantly higher than other treatments,indicating that the new photosynthetic carbon assimilation in fine roots increased.Stimulating the fine roots to accelerate the turnover rate leads to an increase in the carbon input ratio of the rhizosphere soil.The fine root turnover of adequate water treatment was positively correlated with 13C amount and biomass,indicating that the labeled carbon input accelerated the turnover rate of fine roots.The amount of 13C root respiration under two drought stress treatments was positively correlated with the tissue N concentration of fine roots,indicating that the root respiration stimulating roots increased the demand for labeled carbon.13C in fine roots was significantly correlated with root respiration,fine root biomass,and specific root length,indicating that newly synthesized photosynthetic products were the main carbon source for root respiration and fine root growth.5)Our evaluation of 21 drought resistance indicators of B.ischaemum showed that under a given moisture treatment gradient one can isolate an optimal indicator to express growth,morphology,and physiology,to improve the accuracy of depicting plant drought resistance and simplify the drought resistance indicator system.Driving mechanism of carbon-water allocation in plant-soil system under drought stress is that severe drought stress drives the limited carbon and water resources was priority allocated to roots.Plant-soil system in MS treatment improves the utilization efficiency of carbon and water resources by increasing fine root respiration,stimulating fine root growth,increasing the proportion of new carbon distribution in fine roots,changing the morphology characteristics of fine roots,and increasing the turnover rate of fine roots,promoting the optimal growth of above-and under-ground.Plant-soil system in CK treatment preferentially supplies photosynthetic products to the growth of the aerial parts,but controls and maintains the growth of roots.