Yawei WU, Sha SONG, Xiumei HAN, Wei ZHENG, Hua YANG, Yuhua MA

1. Institute of Fruit Science of Guizhou, Guiyang 550006, China;

2. Weining County Administrative Committee, Weining 553100, China

Drought is a key issue affecting fruit tree production. In Guizhou Province, karst is widely distributed and water retaining capacity performs poorly because surface water is prone to seep through rock crevices.Due to engineering water shortage,irrigation source is hardly resolved for karst-based mountainous gardens.Hence,it is an important way for improving apple’s resistance to drought to select drought-resistant rootstocks. In China, Malus rockii is native to the south and develops in the moist environment. It is a popular apple rootstock. Malus robusta is native to the north of China, tolerant to drought and infertility, and widely applied in apple producing regions around Bohai Bay. Leaf transpiration is an important way of water loss of plants,representing 95%.Besides,the thickness of leaf palisade tissue-to-the thickness of spongy tissue ratio, CTR and SR are closely related to drought resistance of rootstocks. For example,the rootstock with thicker palisade tissue, higher thickness of leaf palisade tissue-to-the thickness of spongy tissue ratio, higher compacting of leaf tissue structure and smaller loose tissue proves higher in resistance to drought, which has been reported based on grape,walnut,peach,apple,litchi and banana[1-2].Researches available most focus on the relationship between leaf tissue structure and plant tolerance to drought from the perspective of plant leaf structure[3-5]and the evaluation on restoration capacity of plants releasing from water stress. Therefore, the research explored the changes of leaf structure of Malus rockii and Malus robusta after re-watering in order to investigate the relationship of rootstock leaf structure and drought resistance of Malus rockii and Malus robusta.

Materials and Methods

Materials

Malus rockii and Malus robusta are one year old collected from Weining County and Yantai Academy of Agricultural Sciences.

Methods

The one-year-old Malus rockii and Malus robusta were grown in a greenhouse and water was controlled for 6 d, followed by rewatering on the 7thd.The mature leaves in the same part at the same age were sampled on the 6thd of water controlling and the 2ndd of rewatering. Specifically, rootstocks were collected from 30 plants where 2 leaves were sampled from every plant,totaling 60 leaves. Then, the number and size of leaf stoma were observed from 30 leaves and the rest leaves were used for preparing paraffin sections. For example, the leaves were fixed with formalin-acetic acid-alcohol(FAA)[7]to observe leaf tissue structure. The section thickness was in the range of 8 -10 μm and dyed with safranin O and fast green, followed by sealing with neutral balsam. Subsequently, total thickness of leaf, palisade tissue thickness, and spongy tissue thickness of leaves were measured with a microscope graticule. Totaling 150 visual fields were observed of every rootstock, followed by computing cell tease ratio (CTR),spongy ratio (SR) and palisade tissue thickness-to-spongy tissue thickness ratio[1],as follows:

CTR (%) = Palisade tissue thickness/leaf thickness×100;

SR (% ) =Spongy tissue thickness/Leaf thickness×100;

The palisade tissue thickness-tospongy tissue thickness ratio = Palisade tissue thickness/Spongy tissue thickness.

Table 1 Characters of leaf structure

Results and Analysis

Comparisons of leaf structures of Malus rockii and Malus robusta

Malus rockii consists of two layers of palisade tissue and Malus robusta consists of three layers of columnar cells (Fig.1). The leaf thickness of Malus robusta is in 258-324 m, which is 21.13%-35.60% higher compared with Malus rockii and palisade tissue and spongy tissue keep higher, especially for palisade tissue. For example,the thickness of palisade tissue at drought and rewatering averages 124.67 m of Malus robusta, which is 43.78% higher compared with Malus rockii. Spongy tissue forms a layer next to the palisade cells in the leaf. It is round and loose. The spongy tissue thickness of Malus robusta is 127.73 m, which is 14.68% higher compared with Malus rockii and leaf stoma density is 545.00 per mm2, 9.67% lower compared with Malus rockii.

The changes of leaf responses of Malus rockii and Malus robusta to water

As shown in Table 2, both of length and area of leaf stoma of Malus rockii and Malus robusta increased.Specifically,stoma length of Malus robusta grew by 9.17%,which advanced 3.95 points compared with Malus rockii. As for stoma area, the increased proportion of Malus rockii(31.53%)is 10 times as high as Malus robusta. Besides, stoma width also differed. The stoma width of Malus rockii increased remarkably as high as 25.00%,but of Malus robusta reduced by 5.63%. Additionally, of CTR, SR and palisade tissue thickness-tospongy tissue thickness ratio of the two materials showed similarly by rewatering, of which the former two indices declined considerably, and SR increased dramatically. Besides, CTR of Malus robusta kept higher at drought or rewatering. In terms of response changes, however, CTR, SR and palisade tissue thickness-tospongy tissue thickness ratio of Malus robusta changed more significantly by rewatering, advancing 10.95, 12.78 and 21.63 points,respectively.

Conclusions and Discussions

Leaf morphological characteristics are a kind of adaptation of plants to environment,and the adaptation tends to be volatile upon plants. Leaf is an organ exposed to environment with a maximal area. It reacts the most sensitively and changes size and structures upon environment[8]. Specifically,drought-resistant plants have thicker leaves, whose palisade tissue develops well, and the stoma is small at a large number.It is notable that sunken stoma is a major character of leaf structure of drought-resistant plants.The research concluded that leaf of Malus robusta is thicker, with fewer stoma and a small area; palisade tissue develops well, consisting of three layers.Hence,the structure of drought resistance shows more significant compared with Malus rockii.

Furthermore, the growth of palisade tissue proportion is conductive to water retention in an arid environment.At drought, CTR increases, SR drops and palisade tissue thickness-tospongy tissue thickness ratio improves of leaves of Malus rockii and Malus robusta, preserving water. Based on the research, the responses of Malus rockii and Malus robusta differed to droughts. For example, leaf stoma of Malus rockii responses much significantly to water, but the response ofMalus robusta shows insignificantly.Leaf stoma is a key way of water loss,which is possibly a kind of adaptation for Malus robusta to adapt to environment. Besides, leaf tissue structure of Malus robusta changes more significantly, especially for spongy tissue which absorbs more water,so that SR and palisade tissue thickness-tospongy tissue thickness ratio both reached over 20%. It shows that the more advanced the spongy tissues,the higher drought resistance[9]. Soil moisture is a crucial reservoir for plants, and roots are major ways for plants to take in water. Malus robusta is a northern rootstock, whose roots are strong and water absorbing capacity performs well. Therefore, once plants experience from drought to rewatering, leaves are capable of being supplemented with water because of advanced water supplying capacity of roots.

Table 2 The characters of leaf stoma and mesophyll

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