Evaluation Of Water Use Efficiency And The Physiological Mechanism And Regulation Of Efficient Water Use In Young Apple Genetic Resources

Serious water deficit is one of the major limitations to apple (Malus domestica Borkh.)cultivation in Northwest Loess Plateau, China. Thus, improving capacity of apple tree towithstand drought stress is of great economic importance. The present study determined thewater use efficiency (WUE) of31different apple cultivars and9different Chinese wildMalus rootstocks, as well as the physiological mechanism of high water use, all of whichcould provide useful information for the selection of proper scion-stock combination in fieldplanting and parents' selection in breeding programe in arid and semi-arid regions.Furthermore, we preliminarily studied the regulation mechanism of nitrogen (N), phosphorus(P), and exogenous abscisic acid (ABA) on WUE, which could be useful for the further studyin this field. The main results are as followes:1. According to the growth, WUE, and δ13C, all cultivars were divided into3classes,high WUE cultivars:'Golden Delicious','Qinguan', and 'Hanfu'; medium WUE cultivars:'Ralls','Gale Gala','Jonagold','Orin','Tsuqaru','Senshu','Naganofuji No.2','Mutsu','Hokkaido No.9','Honeycrisp','Cameo','Hirosakifuji','Redchief Delicious','GrannySmith','Judeline','Hokudo', and 'Jazz'; low WUE cultivars:'Cinano Red','Korin','Pinova','Ariane','Pacific Rose','Pink Lady','Ambrosia','Braeburn','Ryoka no Kisetsu','Modi', and 'Hongro'. The results showed that drought tolerance of the31apple cultivarswas not completely consistent with WUE: those with higher WUE are more drought-tolerant,and those with lower WUE are less drought-tolerant.2. According to growth, WUE, and δ13C,9rootstocks were divided into3classes: M.prunifolia and M. sieversii are high WUE species, M. robusta and M. toringoides are mediumWUE species, M. hupehensis, M. sieboldii, M. micromalus, and M. mandshurica are lowWUE species. Rootstock influences drought tolerance and WUE of 'Gale Gala' apple trees.Trees grafted onto M. prunifolia and M. sieversii had higher drought tolerance and WUE,trees grafted onto M. hupehensis had the lowest drought tolerance and WUE.3. Difference in WUE among different apple cultivars was associated with the differencein their leaf and stoma morphological and anatomical structures, photosynthetic capacity, antioxidant system, and endogenous hormones as well as the extent of those parameters'response to drought stress. Drought stress induced decrease in total leaf area (TLA), leafrelative water content (RWC), leaf water holding capacity (WHC), net photosynthetic rate(Pn), transpiration rate (Tr) and stomatal conductance (gs), and the extent of this effect washighest in 'Pacific Rose', medium in 'Gale Gala', and lowest in 'Qinguan'. Content of ABA,ratio of ABA/(IAA+ZR), δ13C, activities of superoxide dismutase (SOD), catalase (CAT),ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), monodehydroascorbatereductase (MDHAR) and glutathione reductase (GR), and content of reduced ascorbate (AsA)and reduced glutathione (GSH) in leaf and root were all increased by drought, and theincrease extent:'Qinguan'>'Gale Gala'>'Pacific Rose'. The results suggest that youngapple cultivars response to drought by changing their leaf and stoma morphologicalcharacters, endogenous hormones levels, and antioxidant defense system, thereby reducingevapotranspiration water loss and improving drought tolerance and WUE.4. Under drought condition, M. hupehensis induced greater decrease in relative heightgrowth rate (HGR), relative basal diameter growth rate (BGR), plant relative growth rate(RGR), total fresh biomass (TB), TLA, leaf chlorophyll content (Cchl), RWC, Pn, and PSIImaximum photochemical efficiency (Fv/Fm) of 'Gale Gala' trees, M. sieversii induced greaterincrease in WUE, δ13C, and ABA content. It suggested that WUE of apple trees are affectedby the rootstock genotype used. Three possible mechanisms were involved for rootstock inimproving apple trees' WUE under drought-stressed condition:(1) increase biomassallocation to root, thereby improving the capacity of root to absorb water and nutrients fromsoil;(2) ABA was synthesized rapidly in root tips when soil becoming dry, and then beentransported to guard cell to modulate stoma activity, decrease evapotranspiration water lossbut has no significant negative effect on photosynthesis;(3) antioxidant capacity ofantioxidant defense system in leaf and root were improved, which was helpful for apple treesto survival in drought-stressed condition.5.75mg N kg-1soil (NL) and23.3mg P kg-1soil (PL) significantly increased WUE ofdrought-stressed apple trees, but150mg N kg-1soil (NH) and46.6mg P kg-1soil (PH) hadno significant influence on WUE. Water deficit significantly decreased apple trees' growthand biomass accumulation as manifested by the significant decrease in in plant height (PH),basal diameter (BD), above-ground biomass (AB), root biomass (RB), TLA, TB, and Pn.Under both well-watered and drought-stressed conditions, NL and PL induced significantincrease in PH, BD, AB, RB, and TB, but NH and PH had little or no significant influence onthese parameters. Fv/Fmwas significantly decreased by drought, NL and PL increased Fv/Fm,improved drought tolerance; but NH and PH had little or no significant influence on Fv/Fm. The results suggest that there is a balance between WUE (or drought tolerance) and supplylevel of N and P; proper supply of N and P was useful for plant growth and biomassaccumulation, thereby improving drought tolerance and WUE; On the contrary, deficient orexcessive supply of N and P had negative effect on plant drought tolerance and WUE. Twopossible mechanisms were involved for N and P in improving apple trees' WUE underdrought-stressed condition:(1) enhance root growth and increase biomass allocation to root,which were helpful for apple trees in abosorbing much more water from soil;(2) increaseTLA and Pn, thereby improving biomass production and WUE.6. WUE of 'Gale Gala' apple trees was significantly increased by10μmol L-1exogenous abscisic acid (ABA-L); but50μmol L-1exogenous ABA (ABA-H) had little or nosignificant influence on WUE. Under both watering regimes, ABA significantly inhibited thegrowth of 'Gale Gala' trees, as manifested by the decrease in TB, AB, and RGR, and thelarger extent of this effect was observed in drought-stressed trees. AB was significantlydecreased by ABA, but no significant influence was observed in RB, thereby increasing RSR.Pn, Tr, gs, andintercellular CO2concentration (Ci) were decreased by ABA, the decreaseextent was greater in Trthan that of Pn. Under well-watered condition, ABA had nosignificant influence on Fv/Fm, but Fv/Fmwas significantly increased by ABA when waterdeficient, which suggested that ABA could improve capacity of photochemisty under droughtcondition. Under drought condition, ABA content in root and leaf was increased, and ZRcontent was decreased. ABA treatment had significant influence on activity of SOD, CAT,APX, DHAR, MDHAR, and GR, and content of AsA and GSH; the extent of this effect wasassociated with the genotype of rootstock used. All in all, proper concentration of exogenousABA treatment could improve apple trees drought tolerance and WUE under droughtconditions. Three possible mechanisms were involved for exogenous ABA in improvingapple trees' WUE under drought-stressed condition:(1) increase RB and RSR, which washelpful for apple tree in absorbing much more water and nutrients from soil;(2) endogenoushormones levels were optimized to modulate stoma activity, thereby activating stomataregulation for WUE;(3) antioxidant capacity of antioxidant defense system in leaf and rootwere improved, which was helpful for apple trees to survival in drought-stressed condition.