Eco-physiological Responses Of Atriplex Canescens Under Different Soil Water Stress

Atriplex canescens is an evergreen shrub introduced from the United States. Trials onits ecophysiological and drought-resistance features were conducted. Atriplexcanescens were planted in the pottery basins which were divided into 4 treatments,namely: water content is the maximum field water-holding capacity (without watersoil stress); water content is 80-90% of the maximum field water-holding capacity(light soil water stress); water content is 50-60% of the maximum field water-holdingcapacity (medium soil water stress); water content is 20-30% of the maximum fieldwater-holding capacity (heavy soil water stress). The eco-physiological indexes weremeasured in July, August and September. The research on the adaptability of Atriplexcanescens under different soil water stress conditions will offer a scientific base forthe introduction and extension of this species in arid areas. The research result shows:1. Under soil water stress conditions, water potential (Ψs), free water (Va) andrelative water content(RWC) decreased, while bound water (Vs), water saturationdeficiency (WSD) and Vs/Va increased; water-holding capacity of leaves increased,while evapor-transpiration rate decreased. Tissue water content of Atriplex canescenswill not change under light and medium, no and heavy soil water stress conditions.Tissue water content under light and medium soil water stress conditions changedobviously, comparing with that under no and heavy soil water stress conditions. At thebeginning of soil water stress, Vs, Va as well as Vs/Va changed slightly under no, lightand medium soil water stress conditions, while changed dramatically under heavy soilwater stress conditions. There's almost no change in tissue water content. In August,there are obvious changes among different treatments under soil water stressconditions. Vs, and Vs/Va obviously raised while Va reduced, and tissue water contentalso changed. Comparing with that in July, water content of leaves in differenttreatments in August decreased by 18.06%, 2.57%, 4.82% and 9.94%, respectively.In August and September, diurnal average Tr under different treatments (light,medium, heavy and no soil water stress conditions ) are 80.90 mmolh₂Om^-2s^-1,76.37mmolh₂Om^-2s^-1,65.46 mmolh₂Om^-2s^-1,42.11 mmolh₂Om^-2s^-1 and 40.60mmolh₂Om^-2s^-1,33.92 mmolh₂Om^-2s^-1,30.44 mmolh₂Om^-2s^-1,23.29 mmolh₂Om^-2s^-1,respectively. It shows that the Tr of Atriplex canescens leaves takes a tendency ofdeclining under long-term soil water stress conditions.2. Under soil water stress conditions, the photosynthesis rate (Pn), water useefficiency(WUE) and light compensation point(LCP) of Atriplex canescens obviouslyreduced, and its dark respiration rate (DRR) weakened. In the treatments in July, theaccumulative and average value of daily Pn from high to low is: light soil water stress＞no soil water stress＞medium soil water stress＞heavy soil water stress, the valueare 7.162μmolm^-2s^-1,6.106μmolm^-2s^-1,6.103μmolm^-2s^-1 and 1.761μmolm^-2s^-1, respectively. In the treatments in August, the accumulative and average value ofdaily Pn from high to low is: light soil water stress＞medium soil water stress＞heavy soil water stress＞no soil water stress, the value are 7.378μmolm^-2s^-1,4.738μmolm^-2s^-1,1.794μmolm^-2s^-1 and 1.705μmolm^-2s^-1, respectively. The Pnvariation curve in September is similar to that in August. The only difference is that the Pn in different treatments in September are higher than that in August, which isrelated to lowered temperature and increased humidity in September. In the 4treatments, the diurnal variation of WUE in different months are : light soil waterstress＞medium soil water stress＞no soil water stress＞heavy soil water stress,which testified that the heavier soil water stress is, the lower WUE is. Thischaracteristic of Atriplex canescens is consistent with that of many local species. TheLCP of Atriplex canescens in different treatments (light, medium, no and heavy watersoil stress) are 314.8μmolm^-2s^-1,272.5μmolm^-2s^-1,176.7μmolm^-2s^-1,143.1μmolm^-2s^-1, respectively, which indicates that the LCP of Atriplex canescens is reducingwith the escalation of soil water stress. This tendency is favorable for Atriplexcanescens under drought stress to maintain certain photosynthesis even in weaksunlight so as to ensure the survival of itself, and keep balance between maintainingwater content inside and its photosynthesis. DPR of Atriplex canescens in differenttreatments are quite different, 2.87μmolm^-2s^-1 under no soil water stress, 6.46μmolm^-2s^-1 under light soil water stress, 6.06μmolm^-2s^-1 under medium soil waterstress, 3.04μmolm^-2s^-1 under heavy soil water stress. Comparing with the DPRunder light soil water stress, DPR decreased by 6.19% under medium soil waterstress, by 52.94% under heavy soil water stress and by 55.57% under no soil waterstress. This shows that the DPR of Atriplex canescens is reducing with the escalationof soil water stress, and its physiological activities obviously became weak.3. Under soil water stress, the biomass of Atriplex canescens reduced, ratio betweenroot and stem increased, the main root becomes long, branches and lateral rootsreduced, and its growth is restrained. The biomass of seedlings and base stem indifferent treatments in July, August and September are: light soil water stress＞medium soil water stress＞no soil water stress＞heavy soil water stress. The averageheight of Atriplex canescens under light soil water stress from July to September is47.3cm, 1.13 times (41.77cm) of that under medium soil water stress, 1.31 times(36.23cm) if that under no soil water stress, 1.34 times (35.22cm)of that under heavysoil water stress. The average base stem of Atriplex canescens under light soil waterstress from July to September is 14.76mm, 1.16 times (12.78mm) of that undermedium soil water stress, 1.47 times (10.03mm) of that under no soil water stress, 1.5times (9.82mm) of that under heavy soil water stress. Comparing with that undermedium, heavy and no soil water stress, items under light soil water stress alldecreased: fresh biomass of main root reduced by 6.23%, 13.83%, 13.02%, andits dry biomass reduced by 26.07%,50.29%,51.49%, respectively; freshbiomass of lateral root reduced by 27.40%, 84.30%, 91.02%, and its drybiomass reduced by 39.07%, 89.62%, 94.87%, respectively ; fresh biomassof stem reduced by 2.57%, 37.56%, 31.49%, and its dry biomass reducedby 15.43%, 50.52%,49.59%, respectively; fresh biomass of leavesreducedby20.28%,46.81%,52.39%, and itsdrybiomassreducedby23.99%, 51.76%, 47.97%, respectively.Under light soil water stress, the ratio between fresh and dry biomass of Atriplexcanescens above the ground and underground are 0.75 and 1.51. With the soil waterstress becoming more serious, the ratio becomes higher. Comparing with that under light soil water stress, the ratio between fresh weight of root and stem under medium,heavy and no soil water stress increased by 11.41%, 17.55%, 68.71%, andbetween dry one increased by 24.24 %, 118.18 %, and 175.75 %,respectively.4 Under light and medium soil water stress, there's a slight change of Atriplexcanescens's physiological indexes, which shows its certain adaptability. Under no andheavy soil water stress, there's a great change of Atriplex canescens's physiologicalindexes, Tr and Cs increased while Pn, WUE and LS decreased, its growth andbiomass reduced substantially, which reveals it can hardly survive under extremelydry habitat. Through comprehensive analysis of various physiological indexes, theinitial conclusion was drawn out: Atriplex canescens cannot survive in the areawithout irrigation and or with scarce rainfall; the limitation scope of rainfall forAtriplex canescens's survival is under research; this species can only be extended inthe area with certain favorable rainfall. The trials on cultivation of Atriplex canescensconducted in Minqin Desert Control Experiment Station reveals that this speciesentails fine qualities as strong tolerance in drought, cold, salinity and low temperatureas well as high annual biomass. So it cab be extended in sandy areas with favorableirrigation condition, and on hilly in loess plateau with relatively high rainfall and lowevaporation. As a new substitute species for water/soil conservation and rangelandregeneration, it has a big potential to be extended. However, more research on withinwhich scope this species can be extended need to be done...