| Malus prunifolia is an endemic wild apple germplasm resource in our country withstrong adaptability to different natural conditions. It is also an important apple rootstock forgood grafting affinity ability and widely used in northern and northwestern China. Thisdiverse species has formed many biotypes (variant) as long-term natural hybridization,genetic mutation and environmental factors influence. Some biotypes are so similar inphenotypic that it is difficult to distinguish them from each other. All these lead to itsnomenclature is confusing and the taxonomy is fraught with problems. On the other hand, nocomprehensive and systematic evaluation has been made of its drought and salt tolerance.Therefore, in our study, through explored the physiological response and evaluated thetolerance in different biotypes of M. prunifolia under drought and salt stress, it provide animportant theoretical and practical value for apple stress breeding and cultivation. Meanwhile,we used SSR markers in combination with cluster and principal coordinate analysis toevaluate the genetic relationship among biotypes of M. prunifolia; the findings are helpful tooptimize the collection and management of apple germplasm resources, and also provide atheoretical reference for traditional morphological classifications of Malus prunifolia at themolecular level.The main results were as follows:1. Eight apple rootstocks (seven biotypes of M. prunifolia and M. sieversii) grafted with‘Fuji’ subjected to drought stress for12d. Compared with the control (0d), the leaf relativewater content (RWC) values decreased significantly in different rootstocks, and the amount ofleaves total chlorophyll as well as photosynthesis parameters also declined to varying degrees.M. prunifolia ‘dongbeihuanghaitang’, M. sieversii, and M. prunifolia ‘fupingqiuzi’ stillmaintained relatively high Pn (8.48,7.55, and6.34μmol m-2s-1, respectively) after9d stress.Two more-sensitive biotypes of M. prunifolia—‘honghaitang’ and ‘baihaitang’—which hadthe lowest Pn values, at2.54and3.82μmol m-2s-1, respectively. Moreover, WUEiwas thehighest,5.71, in the more-resistant M. prunifolia ‘fupingqiuzi’, compared with the lowestvalue,2.42, for M. prunifolia ‘honghaitang’. 2. The activities of ascorbate peroxidase (APX), peroxidase (POD), and catalase (CAT)changed significantly during drought treatment. For APX activity, all biotypes had a sharpincrease during the first3d and began to decline after9d stress. However, the maximumactivity varied by biotypes. The POD activity began to drop in M. prunifolia ‘honghaitang’, M.prunifolia ‘wulenghaitang’, and M. prunifolia ‘baihaitang’ after3d, but M. prunifolia‘fupingqiuzi’, M. prunifolia ‘dongbeihuanghaitang’, M. prunifolia ‘laoshannaizi’ and M.sieversii still maintanted an upward trend. CAT activity raised quickly in M. prunifolia—‘honghaitang’ and ‘baihaitang’ at the stress started, but it did not maintant very long;however, M. prunifolia ‘fupingqiuzi’, M. prunifolia ‘dongbeihuanghaitang’, and M. sieversiiincreased slowly during the first3d treatment, then raised at a faster rate and reached thepeak values on6d.3. All treated plants had elevated electrolyte leakage (EL) and malondialdehyde (MDA)contents after12d of stress. Compared with their corresponding control, values for EL inless-tolerant M. prunifolia ‘honghaitang’ and ‘baihaitang’ increased by46.37%and45.86%,MDA contents increased by30.04%and31.92%; Such damage was not as severe amonghighly drought-tolerant rootstocks, e.g., M. prunifolia ‘fupingqiuzi’, M. prunifolia‘dongbeihuanghaitang’, and M. sieversii, which EL values increased by23.33～32.94%andMDA contents increased by22.06～23.94%compared with the control. Cluster analysis cangroup these Malus biotypes into three categories. The high tolerance included two M.prunifolia—‘fupingqiuzi’ and ‘dongbeihuanghaitang’—plus M. sieversii Roem; moderatetolerance were three M. prunifolia—‘wuqiqiuzi’,‘wulenghaitang’, and ‘laoshannaizi’; lowtolerance were two M. prunifolia—‘honghaitang’ and ‘baihaitang’.4. Eleven apple rootstocks (nine biotypes of Malus prunifolia plus M. sieversii and M.hupehensis) were exposed to150mM NaCl for10d. Salt injury index (SI) showed that, fourbiotypes of M. prunifolia—‘honghaitang’,‘baihaitang’,‘dongbaiguo’, and‘neimengguhaihong’ were more susceptible than other biotypes after3days of exposure (SI≥0.10); after10d stress, M. prunifolia ‘fupingqiuzi’, M. prunifolia ‘dongbeihuanghaitang’, andM. prunifolia ‘laoshannaizi’, as well as plants of M. sieversii and M. hupehensis, hadrelatively low SI. However, three more sensitive biotypes of M. prunifolia—‘baihaitang’,‘wulenghaitang’, and ‘neimengguhaihong’—that had higher SI values of0.519,0.509, and0.625, respectively.5. By comparison with net photosynthetic rate (Pn), variation percentage (VP) ofelectrolyte leakage, relative water content (RWC), SI and biomass adverse resistancecoefficients (ARC), we grouped these biotypes into three classes. The high tolerance included:three for M. prunifolia—‘fupingqiuzi’,‘dongbeihuanghaitang’ and ‘laoshannaizi’—plus M. sieversii and M. hupehensis; the moderate tolerance were: M. prunifolia ‘wuqiqiuzi’ and‘dongbaiguo’; the low tolerance were four for M. prunifolia—‘honghaitang’,‘baihaitang’,‘wulenghaitang’, and ‘neimengguhaihong’. Except for M. prunifolia ‘honghaitang’, all ofthese groupings followed a pattern similar to that found with SI values.6. We used10pairs of simple sequence repeat (SSR) primers in combination withtouchdown PCR to evaluate the genetic relationships among15biotypes of Malus prunifolia.Out of170detected fragments,156(90.74%) were polymorphic. The average geneticsimilarity coefficient (GC) between pairs of biotypes was0.66. The greatest GC (0.88) wasfound between ‘fupingqiuzi’ and ‘wuqiqiuzi’, while the biotype pairing of ‘dongbaiguo’ and‘hebeiregunzi’ showed the least GC (0.53).7. Cluster analysis sorted all biotypes into three groups, which was then confirmed byprincipal component analysis (PCA). A dendrogram showed that some of the clusteredbiotypes were largely congruent for geographical distribution. However, the DNA patterns forsome biotype groups did not demonstrate relative agreement in their origin. Five biotypesarising from the Xinjiang Autonomous Region were dispersed within the dendrogram,reflecting their complex genetic backgrounds. |
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