Comprehensive evaluation of drought resistance of 10 main cultivars of almond (Amygdalus communis L.) in Xinjiang by means of leaf microstructure and ultrastructure

Abstract: 【Objective】The almond (Amygdalus communis L.) is one of the four most famous nuts in the world.It has a long history of cultivation, and has important nutritional and ecological value. As one of the origin of Almond in the world, Southern Xinjiang is rich in almond resistant resources. In this study, the drought resistant capacity of 10 different almond cultivars including 6 main local cultivars and 4 main American introduced cultivars collected from the Southern Xinjiang were evaluated by means of leaf microstructure and ultrastructure characteristics in order to provide a theoretical basis for the establishment of evaluation index system for drought resistance of almond, and to help understanding the drought resistant mechanism of almond from the anatomy aspects.【Methods】Fully-expanded matured leaves on one–year old twigs were sampled from 10 different almond cultivars on local peach rootstock from the southern Xinjiang. Leaf samples were fixed in FAA and 2.5% glutaraldehyde separately. Paraffin sections were made. At least 4 different leaves were used for cultivar, and the mesophyll cells structure and main vein structure were observed under the light microscopy; the scanning electronic microscopy was used for observing the ultrastructure of epidermis (4 different leaves for each cultivar, 2 of them used for observing upper epidermis, other 2 for observing lower epidermis) . Besides of the anatomical structures, the SLA (specific leaf area) value of each cultivar was also calculated at least from 20 representative leaves. Except for the regular leaf anatomical indicators, crystal cell density (number of crystal cells per um2 mesophyll tissue) , mucous cell distributed thickness (measured at least from 8 different points) and cell porosity ratio (ratio of intercellular space at mesophyll cells of the paraxial surface to the total area of mesophyll cells of the paraxial surface) etc., xerophytic plant related several leaf structural features were quantified. Thickness of cuticle, epidermis, palisade cell and total leaf were measured under 10×20 magnifications, and the thickness of main vain, vascular bundle, phloem and xylem vessel diameter etc. were measured at 10×4 magnifications by using Image-J software.Comparisons of total 20 leaf anatomical indicators between cultivars were made with a one-way ANOVA. The principal component analysis was used for further screening the main representative indicators.The drought resistant degrees of different cultivars were evaluated by subordinate function values analysis. Pearson correlation analysis was carried out to further understand the interrelations between anatomical indicators.【Results】Under the same drought environmental conditions, the leaves of the 10 different almond cultivars exhibited different degrees of drought resistant anatomical structure. The cuticle, palisade tissue cells, main veins and lateral vascular bundles were well developed. There were crystal cells in the mesophyll tissue, and mucous cells around the vascular bundles. The stomata were distributed only in the lower epidermis. Among the 20 anatomical structure indexes, 18 indexes were differnt significantly between cultivars (p < 0.05) , and the CV was 2.17% to 27.44%. There were significant differences in some anatomical features between local cultivars and American introduced cultivars, such as SLA, LT, CTR%, thickness of epidermis and cuticle, etc. Total 13 drought resistant-related anatomical indexes were selected among 18 indexes based on the published research papers related to leaf anatomy and drought resistance, and also the leaf structural features of xerophytic plants. The main component analysis was carried out to further screen the representative indexes from 13 drought resistant related indexes. The cumulative contribution rate of the first four principal components was about 83%, the main vein thickness (MVT) , specific leaf area (SLA) , cell tightness ratio (CTR%) and stomata density (SD) were selected as the four principal component for drought resistant indicators. The results of subordinate function values analysis showed that the drought resistance capacity of 10 almond cultivars was S14>S3> M7 > M8>M1> M2> S12 > S1 >S9> S11. According to the Pearson correlation analysis, there were negative significant correlations between SLA and TL, LCT, indicating that the almond with lower SLA had thicker leaf lamina and cuticles (p < 0.05) . There were close positive correlations between cuticle thickness, lateral vein density and mucous cell distributed thickness (p < 0.05) , and this was one of the drought adaptive strategies of almond for surviving in the extremely drought environment.【Conclusion】The leaf tissue structures of American introduced cultivars were very similar to each other and there were large differences in the leaf tissue structures among Xinjiang local cultivars.Among 10 almond cultivars, Xinjiang local cultivars of S14 and S3 showed higherger drought resistance than other cultivars; the drought resistance capacity of American introduced cultivars was moderate and local cultivars of S11, S9 showed weaker drought resistance.