Evaluation of drought resistance of 12 kiwifruit cultivars based on leaf morphology and microscopic characteristics

Abstract：【Objective】The Actinidia chinensis is the most domesticated species of the genus Actinidia and more than 100 commercially valuable cultivars have been developed. However, few studies have evaluated the drought resistance of different kiwifruit cultivars based on the leaf morphology and microstructure. This research aimed to evaluate the drought resistance of different kiwifruit cultivars by observing and analyzing characteristics, such as leaf morphology, anatomical structure, stomata, epidermis and trichomes. Key indicators for evaluating the drought resistance of kiwifruit cultivars and assessing their drought resistance were identified.【Methods】We selected a total of 12 kiwifruit cultivars (belonging to A. chinensis var. chinensis and A. chinensis var. deliciosa), including Donghong, Guihai No. 4, Hort 16A, Jintao, Jinyan, Jinmei, Jinxia, Cuiyu, Jin Mei, Jinkui, Hayward and Xuxiang, as our observa-tion samples. We employed the multifunctional image analysis method, paraffin sectioning method and scanning electron microscopy technique to observe and record the leaf morphology, anatomical structure, stomata and epidermal trichomes of 12 kiwifruit cultivars. A total of 24 traits (designated as X1- X24) were documented. Subsequently, we conducted variance analysis to compare the significant differences in the 24 traits among different cultivars. Then, using principal component analysis, we identified the key traits related to drought resistance from the original set of indicators. Finally, we employed the membership function method to comprehensively evaluate the drought resistance of different kiwifruit cultivars.【Results】There were significant differences in leaf morphology, anatomical structure, stomatal apparatus and the hair of the epidermis among different kiwifruit cultivars. Morphologically, the leaf was heart-shaped or oval-shaped, leaf length (X1) ranged from 10.94 cm to 17.28 cm, leaf width (X2) ranged from 11.39 cm to 16.35 cm, leaf shape index (X3) ranged from 0.93 cm to 1.06 cm, petiole length (X4) ranged from 6.87 cm to 15.91 cm, petiole diameter (X5) and leaf area (X6) ranged from 3.51 mm to 4.58 mm and from 95.03 cm2 to 208.36 cm2 . For stomatal apparatus, the length (X7) ranged from 18.77 μm to 29.21 μm, the width (X8) ranged from 12.64 μm to 18.57 μm, the macroaxis (X9) ranged from 8.46 μm to 16.31 μm, and the density (X10) was between 168.70 and 339.63 per square millimeter. In the villi of lower epidermis, all cultivars had fluff, the length (X11) ranged from 249.41 μm to 324.10 μm, the pedestal density (X12) was between 9.37 and 31.83 pedestals per square millimeter, the villus density (X13) was between 54.93 and 113.95 roots per square millimeter, and the number of villis per pedestal (X14) was between 5.57 and 7.30. For sake of anatomical structures, these cultivars had similar anatomical characteristics. The structure of the leaves from the lower epidermis to the upper epidermis was composed of lower epidermal cells, sponge tissue, palisade tissue and upper epidermal cells. Calcium oxalate crystals were scattered in the mesophyll cells. The leaf veins mainly formed protrusions on the lower epidermis, which were vascular bundles, thin-walled tissues and mechanical tissues from the inside to outside. Lateral vein diameter (X15) ranged from 595.04 μm to 860.71 μm, leaf thickness (X16) ranged from 177.68 μm to 264.53 μm, thickness of upper epidermis cell (X17) ranged from 15.36 μm to 23.37 μm, the first layer of palisade tissue cell density (X18) was between 75.33 and 113.00 per square millimeter, thickness of lower epidermis cell (X19) ranged from 10.03 μm to 20.35 μm, thickness of palisade tissue (X20) ranged from 75.16 μm to 123.60 μm, thickness of spongy tissue (X21) ranged from 49.81μm to 85.68 μm, the ratio of X20 to X21 (X22) ranged from 1.19 to 2.16, tightness of leaf tissue structure (X23) ranged from 0.41 to 0.52, and looseness of leaf tissue structure (X24) ranged from 0.24 to 0.36. The coefficient of variation (CV) of the 24 traits ranged from 8.04% to 35.80%, and the correlation analysis showed that there were significant or extremely significant correlations among the different traits. Principal component analysis showed that the cumulative contribution rate of the first 7 principal components reached 93.046%, effectively retaining most of the information of the 24 indicators. Eight key drought- resistance indicators were selected, including X2 (leaf width), X3 (leaf shape index), X9 (macroaxis), X14 (number of villis per pedestal), X17 (thickness of upper epidermis cell), X19 (thickness of lower epidermis cell), X22 (thickness of palisade tissue/thickness of spongy tissue) and X23 (tightness of leaf tissue structure). Furthermore, the drought resistance of different cultivars was compared by subordinate function method. The order of drought resistance was as follows: Xuxiang ＞ Jinmei ＞ Jinxia ＞ Hayward ＞ Jinyan ＞ Jinkui ＞ Jin Mei ＞ Donghong ＞ Cuiyu ＞ Jintao ＞ Guihai No. 4 ＞ Hort 16A.【Conclusion】The differences in leaf morphology, stomata, epidermal hair and anatomical structure of different kiwifruit cultivars were revealed. Leaf width, leaf shape index, stomatal length axis, number of villis per pedestal, thickness of upper epidermis, thickness of low-er epidermis cell, thickness of palisade tissue/thickness of spongy tissue, and tightness of leaf tissue structure were selected to evaluate drought resistance. The drought-resistant cultivars such as Xuxiang, Jinmei and Jinxia were screened by the method of subordinate function analysis, which could provide reference for genetic breeding, variety selection and production management in the future.