Effects Of Drought Stress In The Early And The Middle Growth Phase On The Growth And Physiological Indexes Of Pinus Sylvestris Var.Mongolica Seedlings

Pinus sylvestris L.var.mongolica Litv.,a geographic variant of Pinus sylvestris L.,is a native afforestation species in the north of China.Spring drought occurs frequently in North China,and sometimes there is continuous drought in spring and summer.In this study,four-year-old Pinus sylvestris var.mongolica seedlings were exposed to control(CK),early growth phase drought(EGD)and middle growth phase drought(MGD)treatments,growth index,relative conductivity,chlorophyll content,mineral elements,endogenous hormones,soluble sugar and starch content,dry matter content of roots,stems and needles as well as the seedling mortality were measured during and after the treatments.The aim was to study the effects of soil drought at different growth stages on the physiological functions and growth of roots,and the growth,physiology and nutrition of the aboveground organs of Pinus sylvestris var.mongolica seedlings,therefore providing theoretical basis and practical reference for the adaptability of Pinus sylvestris var.mongolica seedlings in the drought region.The main findings are as follows:1.After 3 weeks of drought in the early growth phase,the increase of the number of root tips was inhibited,leading to the decrease of root length and root volume.After 3 weeks of drought in the middle growth phase,the number of root tips,root length and root volume were also significantly lower than that in CK.Moreover,the number of root tips in MGD was still lower after rehydration.Drought stress in the middle growth phase probably caused inner damage to the roots of Pinus sylvestris var.mongolica seedlings,thereby leading to the lower root growth even after rehydration.2.Both EGD and MGD reduced the growth of needles and stem diameter.At the end of the experiment,the stem diameter and the length of the current-year needles in EGD were both greater than in MGD,indicating the drought in the middle growth phase had more negative impacts on the stem diameter and needle growth in Pinus sylvestris var.mongolica seedlings.3.The mortality of Pinus sylvestris var.mongolica seedlings in MGD was much higher than that in EGD.This may be related to the high growth rate with the high water demand during the middle growth phase.Soil drought during this period caused the severe water deficit and the death of the seedlings.4.The effect of drought in the middle growth phase on the current-year needles was larger than on the old needles in term of the electrolyte permeability.Irrespective of the age of the needles,electrolyte permeability in MGD was higher than in EGD.5.When the Pinus sylvestris var.mongolica seedlings were exposed to drought stress in the middle growth phase,the soluble sugar and starch content,chlorophyll content,endogenous hormones ABA and ZR,mineral element potassium,calcium and magnesium content,dry matter content in roots and leaves of Pinus sylvestris var.mongolica seedlings were more negatively affected.