Drought stress is one of the serious natural disasters that restrict agricultural production in China.Due to drought problems,fruit tree production,fruit quality decline,and even plant death have caused huge losses to our fruit tree industry.Nitrogen participates in the synthesis of chlorophyll,protein and other substances,and is one of the essential elements in the growth and development of fruit trees.Plant growth regulators play an important role in regulating root growth,nutrient uptake and stress resistance of fruit trees.Rational use of plant growth regulators can promote the growth of fruit trees and the absorption of soil mineral elements by regulating the photosynthesis and respiration of fruit trees and hormone levels,and alleviate the damage of drought stress.In this experiment,Malus hupehensis Rehd.seedlings and‘Xiangfu’(The rootstock is M.hupehensis)were used as test materials.Under the condition of moderate drought stress,exogenous spraying of methyl jasmonate(Me JA)and4-methylumbelliferone(4-Mu)and mixed spraying of two substances were used to explore the effects of two growth regulators on apple trees root configuration and nitrogen uptake and the possible mechanism of alleviating drought stress.The results are as follows:(1)In the natural growth state,spraying 2.5μM Me JA and 500μM 4-Mu could significantly promote the growth of M.hupehensis,increase the fresh weight of seedlings,regulate the root architecture,increase the total root length,expand the root surface area,increase the number of forks and root tips,etc.,effectively promote the occurrence and elongation of lateral roots of M.hupehensis,form a new root architecture based on the increase of total root length and root tip number,and improve root activity.It also has a significant effect on photosynthesis,which could regulate stomatal conductance,promote photosynthesis,and improve the ability of roots to absorb and utilize nitrogen,especially nitrate nitrogen.(2)PEG stress can inhibit the growth of M.hupehensis,reduce fresh weight and plant height,inhibit root elongation,reduce root activity,and is not conducive to lateral root formation.Under the condition of PEG stress,the absorption rate of NO3-and the inflow rate of NO3-decreased,Km increased,and the affinity became worse.The affinity for NH4+was enhanced,and the absorption rate and nutrient inflow rate were maintained at a high level.The content of nitrate nitrogen in roots,stems and leaves decreased significantly,and the content of ammonium nitrogen increased significantly,which was consistent with the absorption kinetic parameters.The activities of nitrate reductase,glutamate synthase and glutamine synthetase in roots,stems and leaves decreased significantly.Spraying 2.5μM Me JA and 500μM 4-Mu under the condition of PEG stress could significantly alleviate the inhibitory effect of PEG stress on the growth of M.hupehensis,promote seedling growth,increase fresh weight and plant height.Spraying Me JA and 4-Mu alone could increase the contents of chlorophyll a and carotenoids in leaves,improve root configuration,and promote lateral root formation and main root elongation.Regardless of single application or mixed application,it can significantly increase root activity,promote the uptake of NO3-by seedlings,improve the affinity of seedlings to nitrate nitrogen,maximum absorption rate and nutrient inflow rate.The content of nitrate nitrogen in roots,stems and leaves increased significantly,and the activities of nitrogen assimilation enzymes,nitrate reductase,glutamate synthase and glutamine synthase increased significantly.After spraying Me JA and 4-Mu alone or in combination,the absorption rate and affinity of NH4+in apple seedlings decreased,and the content of ammonium nitrogen in plants decreased.(3)Under the condition of drought stress,foliar application of 10μM Me JA and 2 m M4-Mu could promote the growth of new shoots and roots,increase the root-shoot ratio,improve root activity,promote lateral root formation,and expand root absorption area.Spraying Me JA could increase root volume and alleviate the inhibition of drought stress on root growth.Under the condition of drought stress,spraying Me JA and 4-Mu respectively and mixedly increased the chlorophyll content of apple saplings to varying degrees,enhanced the activities of glutamine synthetase,glutamate synthetase in roots and leaves and nitrate reductase in leaves;the content of nitrate nitrogen in roots and leaves was increased,and the content of ammonium nitrogen in roots and leaves after drought stress was reduced.In the experiment,the effect of spraying Me JA was the best.(4)Drought stress can reduce the activity of soil urease and nitrite reductase,and reduce the content of ammonium nitrogen in soil.Under the condition of water stress,spraying Me JA(10μM),4-Mu(2 m M)and their mixture could increase the activities of urease and nitrite reductase in soil,promote the uptake and utilization of soil available nitrogen by saplings,and remission drought stress. |