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Abstract [Objectives]Introducing and screening leaf vegetable varieties tolerant to heat and humidity has great significance to ensure balanced supply of vegetables in summer and autumn in Hainan Province.[Methods]On the basis of the introduction of greenstem nonheading Chinese cabbage and darkgreenleaf nonheading Chinese cabbage varieties， the varieties tolerant to heat and humidity were identified by the investigation of agronomic traits and evaluation of heat injury index.[Results]The data analysis results showed that four greenstem nonheading Chinese cabbage varieties including Huaguan Qingcaigeng F1， Zhongguan Qinggeng， Hanshen No. 1 Qinggengcai F1 and Huaxiawang Piaocai have the characteristics of strong tolerance to heat and high yield， and four darkgreenleaf nonheading Chinese cabbage varieties including Baimeigui， Heijingang， Xiaxue Naibaicai and Siji Baicai have the characteristics of strong tolerance to heat and high yield.[Conclusions]These eight leaf vegetable varieties are suitable for popularization in summer and autumn in Hainan Province.
Key words Greenstem nonheading Chinese cabbage; Darkgreenleaf nonheading Chinese cabbage; Agronomic trait;Heat injury indices
Hainan Province has a tropical monsoon marine climate[1]with high temperature throughout the year， It has the rainy season from May to October and the dry season from November to April of the next year[2]. Vegetable industry is one of the pillar industries of Hainanюs agricultural development， and vegetable production income accounts for a higher proportion of household income[3]. The high temperature and heavy rain in summer and autumn in Hainan Province last for a long time[4]， and the winter and spring are relatively low in temperature and have less rain. Therefore， the production of vegetables in Hainan is the best in winter and spring， but the summer and autumn are the offseason in vegetable production[5]. In winter， Hainan Province serves as the countryюs "vegetable basket"， while in summer and autumn， it consumes the most expensive vegetables in the country[6]. The selfsufficiency rate of vegetables in summer and autumn in Hainan Province is less than 50%， which is far from meeting the market demand[7]. Therefore， it is necessary to improve vegetable production in summer and autumn， especially facility vegetable production in summer and autumn， which is of great significance to ensure the balanced supply of Hainan vegetable market， reduce the dependence on the foreign market， increase farmersю income and maintain the economic and social stability of the province. Up to now， there have been few reports on the screening of greenstem nonheading Chinese cabbage and darkgreenleaf cabbage Chinese varieties tolerant to heat and humidity in Hainanюs high temperature and high humidity climate. The introduction and selection of vegetable varieties tolerant to heat and humidity is a key issue to facility vegetable production in summer and autumn in Hainan Province. This study was conducted to screen varieties tolerant to heat and humidity through the investigation of agronomic traits and heat injury index in field based on the introduction of greenstem nonheading Chinese cabbage and darkgreenleaf nonheading Chinese cabbage， with an attempt to provide excellent variety resources for the vegetable production in summer and autumn in Hainan Province. Materials and Methods
Materials
In this study， 24 greenstem nonheading Chinese cabbage （Table 1） and 17 darkgreenleaf nonheading Chinese cabbage （Table 2） were used as experimental materials.
Investigation of agronomic traits
The experiment was conducted at the experimental base and laboratory of Tropical Crops Genetic Resources Institute， Chinese Academy of Tropical Agricultural Sciences from July to August， 2017. Full uniform and diseasefree seeds were selected and directly sown in the experimental base. The randomized block design was adopted， in which each treatment plot had an area of 2 m2 with three replicates. Conventional field management was on a regular basis. Five plants were selected from each replicate for the investigation of agronomic traits 25 d after planting.
Investigation of heat injury index
According to the method of Hu et al.[8]， the heat injury index was calculated and investigated. The grading standards for heat injury index are as follows：
Grade 0： No heat injury symptoms， normal plant growth;
Grade 1： Number of injured leaves<1/3 of all leaves on the whole plant;
Grade 3： 1/3 of all leaves on the whole plantnumber of injured leaves<1/2 of all leaves on the whole plant;
Grade 5： 1/2 of all leaves on the whole plantnumber of injured leaves<2/3 of all leaves on the whole plant;
Grade 7： Number of injured leaves◎2/3 of all leaves on the whole plant， not dead.
Grade 9： Lost economic value or dead.
X： Grade of heat injury; Xi： number of plants of grade X; A： the highest grade; N： total number of investigated plants.
Results and Analysis
Screening of greenstem nonheading Chinese cabbage
The characteristics of various Chinese cabbage varieties were counted 25 d after field planting， The plant height of Huaguan Qingcaigeng F1 was the highest， reaching 25.19 cm， and the plant height of Japan Qingjiangbaicai was the smallest， being 18.00 cm. Huaguan Qingcaigeng F1， Shanghaiqing （Jimaocai）， Datouaijiao Qingjiangbaicai， Yuanzhongfanyu Shanghaiqing， Fujing F1 Xiaobaicai， Jinchuan 82， Zhongguan Qinggeng， Datousiji Qingjiangbaicai and Degao 206 had no significant difference in plant height therebetween， but there was a significant difference from other 15 materials. The leaf length of Huaguan Qingcaigeng F1 was the largest， of 15.38 cm， and the leaf length of Jieguan Qinggengcai was the smallest， of 9.98 cm. There was no significant difference in leaf length between Huaguanqing Qingcaigeng F1， Xinxia Dongqing， Yuanzhongfanyu Shanghaiqing， Fujing F1 Xiaobaicai， Jinchuan 82 and Zhongguan Qinggeng， but there was a significant difference from other 18 materials. Hong Kong Tiancuiqinggengcai had the largest leaf width of 10.81 cm， and the leaf width of Japan Qingjiangbaicai was the smallest， of 6.52 cm. There was no significant difference in leaf width between Hong Kong Tiancuiqinggengcai， Guangyanxinxuan Qingjiangbaicai， Huaguan Qingcaigeng F1 and Xinxia Dongqing， but there was a significant difference from other 20 materials. Huaguan Qingcaigeng F1 had the largest petiole width of 2.31 cm， and the petiole width of Japan Qingjiangbaicai was the smallest， of 1.52 cm. There was no significant difference in petiole width between Huaguan Qingcaigeng F1， Fujing F1 Xiaobaicai and Zhongguan Qinggeng， but there was a significant difference in petiole width from other 21 materials （Table 3）. Fujing F1 Xiaobaicai had the highest fresh weight of 63.03 g， and Japan Qingjiangbaicai had the smallest fresh weight of 35.87 g. There was no significant difference in single plant weight between Fujing F1 Xiaobaicai， Luhengmeidi， Huaguan Qingcaigeng F1， Gaoda No. 6 Qinggengcai， Xinxia Dongqing， Zhongguan Qinggeng and Siji Datouqingjiangbaicai， but there was a significant difference from other 17 materials. The chlorophyll content of Hanshen No. 1 Qinggengcai F1 was the highest， of 48.54 SPAD， and the chlorophyll content of Datouaijiao Qingjiangbaicai was the lowest， of 36.23 SPAD. There was no significant difference in chlorophyll content between Huaguan Qingcaigeng F1， Xiashuai Qinggengcai， Fujing F1 Xiaobaicai， Jinchuan 82， Zhongguan Qinggeng， Taiguopinzhong 7， Jieguan Qinggengcai， Siji Datouqingjiangbaicai， Hanshen No.1 Qinggengcai F1， Huaxiawang Piaocai， Meidu （Chulu No. 2） and Deco 206， but there was a significant difference from other 12 materials. The yield of Fujing F1 Xiaobaicai was the highest， of 28 684.2 kg/hm2， and the yield of Japan Qingjiangbaicai was the lowest， of 13 258.35 kg/hm2. Fujing F1 Xiaobaicai， Datouaijiao Qingjianbaicai， Huaguan Qingcaigeng F1， Guangyantexuan Qingjiangbaicai， Zhongguan Qinggeng， Hong Kong Tiancuiqinggengcai， Hanshen No. 1 Qinggengcai F1， Huaxiawang Piaocai， Meidu （Chulu No. 2） and Degao 206 showed no significant difference in yield， but there was a significant difference in yield from other 14 materials （Table 3）.
The smaller the heat injury index， the stronger the heat resistance of the material. It was found from the comparison of the heat injury index between different greenstem nonheading Chinese cabbage varieties that Luhengmeidi， Huaguan Qingcaigeng F1， Xiashuai Qinggengcai， Zhongguan Qinggeng， Jieguan Qinggengcai， Hanshen No. 1 Qinggengcai F1 and Huaxiawang Piaocai had a lower heat injury index， belonging to the heattolerant greenstem Chinese cabbage varieties， but Guangyanxinxuan Qingjiangbaicai， Japan Qingjiangbaicai， Xinxia Dongqing and Siji Datouqingjiangbaicai had a higher heat injury index， belonging to the heatsensitive greenstem Chinese cabbage varieties （Table 3）.
Combined with the agronomic traits and heat injury index， the comprehensive analysis found that Huaguan Qingcaigeng F1， Zhongguan Qinggeng， Hanshen No.1 Qinggengcai F1 and Huaxiawang Piaocai have the characteristics of stronger heat tolerance and high yield， and are suitable for promotion in hot and humid seasons. Screening of darkgreenleaf nonheading Chinese cabbage
The varietal characteristics of various darkgreenleaf nonheading Chinese cabbage varieties were counted 25 d after field planting. The plant height of Taiwan Siji Gaojiaotianbaicai was the highest， of 34.11 cm， and Tai 9 had the lowest plant height of 12.44 cm. Taiwansiji Gaojiaotianbaicai had a significant difference in plant height from other 16 materials. Xiaxue Daibaicai， Tai 9， Teyounaibaicai and hybrid Youzhidabanbaicai had a smaller plant height， and showed a significant difference in plant height with other darkgreenleaf nonheading Chinese cabbage varieties. Taiwansiji Gaojiaotianbaicai had the largest leaf length of 16.44 cm， and Tai 9 had the smallest leaf length of 7.90 cm. The leaf lengths of Taiwansiji Gaojiaotianbaicai， Heijingang， Lijiansiji Zhongjiaoheiyetianbaicai 555， Meidafu Heiyebaicai and Guangyanxinxuan Aijiaoheiyebaicai were not significant， but were significantly higher than other 12 darkgreenleaf nonheading Chinese cabbage. Heijingang had the largest leaf width， of 14.56 cm， and Tai 9 had the smallest width of 9.46 cm. Heijingang， Baimeigui， Lijiansijian Zhongjiaoheiyetianbaicai 555， hybrid Youzhidabanbaicai， Siji Baicai， Yutu hybrid Heiyebaicai and Guangyanxinxuan Aijiaoheiyebaicai had no significant difference in leaf width therebetween， but had a significant difference from other 10 materials. Hybrid Youzhidabanbaicai had the largest petiole width of 2.47 cm， and Zhongjiaokuishanheiyechigengbai 316 had the smallest petiole width of 1.63 cm. There was no significant difference between hybrid Youzhidabanbaicai， Tai 9 and Xiaxue Naibaicai in petiole width， but they were significantly higher than other 14 materials （Table 4）.
Baimeigui had the largest single plant weight of 99.10 g， and the single plant weight of Zhongjiaokuishanheiyechigengbai 316 was the smallest， of 52.98 g. There was no significant difference in single plant weight between Baimeigui， Heijingang， Lijiansiji Zhongjiaoheiyetianbaicai 555 and hybrid Youzhidabanbaicai， but their values were significantly higher than other 13 darkgreenleaf nonheading Chinese cabbage varieties. The chlorophyll content of Tai 9 was the highest， being 51.54 SPAD， and the chlorophyll content of Taiwan Siji Gaojiaotianbaicai was the lowest of 27.66 SPAD. There was no significant difference in chlorophyll content between Tai 9， Teyounaibaicai， Lijiansiji Zhongjiaoheiyetianbaicai 555 and hybrid Youzhidabanbaicai， but their chlorophyll contents were significantly higher than other 13 darkgreenleaf cabbage varieties. The yield of hybrid Youzhidabanbaicai was the highest， of 26 258.15 kg/hm2， and the yield of Tai 9 was the lowest， of 12 572.85 kg/hm2. There was no significant difference in yield between hybrid Youzhidabanbaicai， Baimeigui， Heijingang， Teyounaibaicai， Xiaxue Naibaicai， Siji Baicai and Guangyanxinxuan Aijiaoheiyebaicai， but their yields were significantly higher than other 10 darkgreenleaf cabbage varieties （Table 4）. The smaller the heat injury index， the stronger the heat resistance of the material. It was found from the comparison of heat damage index between different darkgreenleaf nonheading Chinese cabbage varieties that the heat tolerance of Heiyekuishanbaicai is the strongest， Tai 9， Baimeigui， Heijingang， Xiaxue Naibaicai， Siji Baicai， Yutu hybrid Heiyebaicai and Zhongjiaokuishanheiyechigengbai 316 have stronger heat tolerance， and Taiwan Siji Gaojiaotianbaicai is the worst in heat tolerance （Table 4）.
Combined with agronomic traits and heat damage index， the comprehensive analysis found that Baimeigui， Heijingang， Xiaxue Naibaicai and Siji Baicai have stronger heat tolerance and higher yield cabbage， and are suitable for promotion in hot and humid seasons.
Conclusions and Discussion
Considering climatic factors， continuous high temperature above 35 °C， heavy rainfall and high humidity after rainfall are the main factors affecting the production of vegetables in Hainan Province in summer and autumn. Due to the lag in the research on vegetable varieties tolerant to heat and humidity in summer and autumn in Hainan Province， the current varieties of vegetables growing in summer and autumn in Hainan are single， and the heattolerant varieties used are poor， which is difficult to meet the needs of summer and autumn. These remarkable problems not only affect the enthusiasm of growers， but also restrict the healthy development of summer and autumn vegetable production in Hainan. In this study， 24 greenstem nonheading Chinese cabbage varieties and 17 darkgreenleaf Chinese cabbage varieties were collected from domestic and foreign countries， and the agronomic traits and heat damage index were investigated. The results showed that four greenstem nonheading Chinese cabbage varieties including Huaguan Qingcaigeng F1， Zhongguan Qinggeng， Hanshen No. 1 Qinggengcai F1 and Huaxiawang Piaocai have the characteristics of strong heat tolerance and high yield， and four darkgreenleaf nonheading Chinese cabbage varieties including Baimeigui， Heijingang， Xiaxue Naibaicai and Siji Baicai have the characteristics of strong tolerance to heat and high yield. These eight leaf vegetable varieties are suitable for popularization in summer and autumn in Hainan Province.
References
[1]TANG SX， ZHAO ZZ， BI H， et al. Analysis of characteristics and development of climate resources in Hainan[J]. Journal of Hainan Normal University： Social Sciences， 2008， 21（3）： 343-346. （in Chinese） [2]LIU JB， PENG Y， CHEN QB. Climate suitability analysis and zoning of sugarcane planting in Hainan[J]. Chinese Journal of Agrometeorology， 2009， 30（2）： 254-256. （in Chinese）
[3]DONG XX， ZHANG YM， ZHANG C. Analysis of the influence of vegetable industry development on farmersю income in Hainan Province――Based on survey data of farmers in six counties[J]. Shandong Agricultural Sciences， 2015（1）： 155-158. （in Chinese）
[4]KUANG CX. Lack of vegetables in summer and autumn in Hainan is due to that farmers in the high temperature and rainy central part are not good at growing vegetables[N]. Hainan Daily， 2015-1-29. （in Chinese）
[5]KUANG CX. What is the reason for the lack of vegetables in Hainan Island Growing vegetables is severely restricted by the season[N]. Hainan Daily， 2015-1-29. （in Chinese）
[6]LENG ZL. Eight measures in Hainan Province to protect the "vegetable basket"[J]. China Fruit & Vegetable， 2011（5）： 57. （in Chinese）
[7]XIA GN. Hainan strengthens the construction of 150 000 mu vegetable base to solve the problem of seasonal supply shortage[N]. Hainan Daily， 2016 -4-10. （in Chinese）
[8]HU QQ， CHEN LZ， ZHANG YJ， et al. Studies on heat tolerance identification method for nonheading Chinese cabbage[Brassica campestris L. ssp. Chinensis （L.） Makino var. communis Tsen et Lee][J]. China Vegetables， 2011（2）： 56-61. （in Chinese）
Key words Greenstem nonheading Chinese cabbage; Darkgreenleaf nonheading Chinese cabbage; Agronomic trait;Heat injury indices
Hainan Province has a tropical monsoon marine climate[1]with high temperature throughout the year， It has the rainy season from May to October and the dry season from November to April of the next year[2]. Vegetable industry is one of the pillar industries of Hainanюs agricultural development， and vegetable production income accounts for a higher proportion of household income[3]. The high temperature and heavy rain in summer and autumn in Hainan Province last for a long time[4]， and the winter and spring are relatively low in temperature and have less rain. Therefore， the production of vegetables in Hainan is the best in winter and spring， but the summer and autumn are the offseason in vegetable production[5]. In winter， Hainan Province serves as the countryюs "vegetable basket"， while in summer and autumn， it consumes the most expensive vegetables in the country[6]. The selfsufficiency rate of vegetables in summer and autumn in Hainan Province is less than 50%， which is far from meeting the market demand[7]. Therefore， it is necessary to improve vegetable production in summer and autumn， especially facility vegetable production in summer and autumn， which is of great significance to ensure the balanced supply of Hainan vegetable market， reduce the dependence on the foreign market， increase farmersю income and maintain the economic and social stability of the province. Up to now， there have been few reports on the screening of greenstem nonheading Chinese cabbage and darkgreenleaf cabbage Chinese varieties tolerant to heat and humidity in Hainanюs high temperature and high humidity climate. The introduction and selection of vegetable varieties tolerant to heat and humidity is a key issue to facility vegetable production in summer and autumn in Hainan Province. This study was conducted to screen varieties tolerant to heat and humidity through the investigation of agronomic traits and heat injury index in field based on the introduction of greenstem nonheading Chinese cabbage and darkgreenleaf nonheading Chinese cabbage， with an attempt to provide excellent variety resources for the vegetable production in summer and autumn in Hainan Province. Materials and Methods
Materials
In this study， 24 greenstem nonheading Chinese cabbage （Table 1） and 17 darkgreenleaf nonheading Chinese cabbage （Table 2） were used as experimental materials.
Investigation of agronomic traits
The experiment was conducted at the experimental base and laboratory of Tropical Crops Genetic Resources Institute， Chinese Academy of Tropical Agricultural Sciences from July to August， 2017. Full uniform and diseasefree seeds were selected and directly sown in the experimental base. The randomized block design was adopted， in which each treatment plot had an area of 2 m2 with three replicates. Conventional field management was on a regular basis. Five plants were selected from each replicate for the investigation of agronomic traits 25 d after planting.
Investigation of heat injury index
According to the method of Hu et al.[8]， the heat injury index was calculated and investigated. The grading standards for heat injury index are as follows：
Grade 0： No heat injury symptoms， normal plant growth;
Grade 1： Number of injured leaves<1/3 of all leaves on the whole plant;
Grade 3： 1/3 of all leaves on the whole plantnumber of injured leaves<1/2 of all leaves on the whole plant;
Grade 5： 1/2 of all leaves on the whole plantnumber of injured leaves<2/3 of all leaves on the whole plant;
Grade 7： Number of injured leaves◎2/3 of all leaves on the whole plant， not dead.
Grade 9： Lost economic value or dead.
X： Grade of heat injury; Xi： number of plants of grade X; A： the highest grade; N： total number of investigated plants.
Results and Analysis
Screening of greenstem nonheading Chinese cabbage
The characteristics of various Chinese cabbage varieties were counted 25 d after field planting， The plant height of Huaguan Qingcaigeng F1 was the highest， reaching 25.19 cm， and the plant height of Japan Qingjiangbaicai was the smallest， being 18.00 cm. Huaguan Qingcaigeng F1， Shanghaiqing （Jimaocai）， Datouaijiao Qingjiangbaicai， Yuanzhongfanyu Shanghaiqing， Fujing F1 Xiaobaicai， Jinchuan 82， Zhongguan Qinggeng， Datousiji Qingjiangbaicai and Degao 206 had no significant difference in plant height therebetween， but there was a significant difference from other 15 materials. The leaf length of Huaguan Qingcaigeng F1 was the largest， of 15.38 cm， and the leaf length of Jieguan Qinggengcai was the smallest， of 9.98 cm. There was no significant difference in leaf length between Huaguanqing Qingcaigeng F1， Xinxia Dongqing， Yuanzhongfanyu Shanghaiqing， Fujing F1 Xiaobaicai， Jinchuan 82 and Zhongguan Qinggeng， but there was a significant difference from other 18 materials. Hong Kong Tiancuiqinggengcai had the largest leaf width of 10.81 cm， and the leaf width of Japan Qingjiangbaicai was the smallest， of 6.52 cm. There was no significant difference in leaf width between Hong Kong Tiancuiqinggengcai， Guangyanxinxuan Qingjiangbaicai， Huaguan Qingcaigeng F1 and Xinxia Dongqing， but there was a significant difference from other 20 materials. Huaguan Qingcaigeng F1 had the largest petiole width of 2.31 cm， and the petiole width of Japan Qingjiangbaicai was the smallest， of 1.52 cm. There was no significant difference in petiole width between Huaguan Qingcaigeng F1， Fujing F1 Xiaobaicai and Zhongguan Qinggeng， but there was a significant difference in petiole width from other 21 materials （Table 3）. Fujing F1 Xiaobaicai had the highest fresh weight of 63.03 g， and Japan Qingjiangbaicai had the smallest fresh weight of 35.87 g. There was no significant difference in single plant weight between Fujing F1 Xiaobaicai， Luhengmeidi， Huaguan Qingcaigeng F1， Gaoda No. 6 Qinggengcai， Xinxia Dongqing， Zhongguan Qinggeng and Siji Datouqingjiangbaicai， but there was a significant difference from other 17 materials. The chlorophyll content of Hanshen No. 1 Qinggengcai F1 was the highest， of 48.54 SPAD， and the chlorophyll content of Datouaijiao Qingjiangbaicai was the lowest， of 36.23 SPAD. There was no significant difference in chlorophyll content between Huaguan Qingcaigeng F1， Xiashuai Qinggengcai， Fujing F1 Xiaobaicai， Jinchuan 82， Zhongguan Qinggeng， Taiguopinzhong 7， Jieguan Qinggengcai， Siji Datouqingjiangbaicai， Hanshen No.1 Qinggengcai F1， Huaxiawang Piaocai， Meidu （Chulu No. 2） and Deco 206， but there was a significant difference from other 12 materials. The yield of Fujing F1 Xiaobaicai was the highest， of 28 684.2 kg/hm2， and the yield of Japan Qingjiangbaicai was the lowest， of 13 258.35 kg/hm2. Fujing F1 Xiaobaicai， Datouaijiao Qingjianbaicai， Huaguan Qingcaigeng F1， Guangyantexuan Qingjiangbaicai， Zhongguan Qinggeng， Hong Kong Tiancuiqinggengcai， Hanshen No. 1 Qinggengcai F1， Huaxiawang Piaocai， Meidu （Chulu No. 2） and Degao 206 showed no significant difference in yield， but there was a significant difference in yield from other 14 materials （Table 3）.
The smaller the heat injury index， the stronger the heat resistance of the material. It was found from the comparison of the heat injury index between different greenstem nonheading Chinese cabbage varieties that Luhengmeidi， Huaguan Qingcaigeng F1， Xiashuai Qinggengcai， Zhongguan Qinggeng， Jieguan Qinggengcai， Hanshen No. 1 Qinggengcai F1 and Huaxiawang Piaocai had a lower heat injury index， belonging to the heattolerant greenstem Chinese cabbage varieties， but Guangyanxinxuan Qingjiangbaicai， Japan Qingjiangbaicai， Xinxia Dongqing and Siji Datouqingjiangbaicai had a higher heat injury index， belonging to the heatsensitive greenstem Chinese cabbage varieties （Table 3）.
Combined with the agronomic traits and heat injury index， the comprehensive analysis found that Huaguan Qingcaigeng F1， Zhongguan Qinggeng， Hanshen No.1 Qinggengcai F1 and Huaxiawang Piaocai have the characteristics of stronger heat tolerance and high yield， and are suitable for promotion in hot and humid seasons. Screening of darkgreenleaf nonheading Chinese cabbage
The varietal characteristics of various darkgreenleaf nonheading Chinese cabbage varieties were counted 25 d after field planting. The plant height of Taiwan Siji Gaojiaotianbaicai was the highest， of 34.11 cm， and Tai 9 had the lowest plant height of 12.44 cm. Taiwansiji Gaojiaotianbaicai had a significant difference in plant height from other 16 materials. Xiaxue Daibaicai， Tai 9， Teyounaibaicai and hybrid Youzhidabanbaicai had a smaller plant height， and showed a significant difference in plant height with other darkgreenleaf nonheading Chinese cabbage varieties. Taiwansiji Gaojiaotianbaicai had the largest leaf length of 16.44 cm， and Tai 9 had the smallest leaf length of 7.90 cm. The leaf lengths of Taiwansiji Gaojiaotianbaicai， Heijingang， Lijiansiji Zhongjiaoheiyetianbaicai 555， Meidafu Heiyebaicai and Guangyanxinxuan Aijiaoheiyebaicai were not significant， but were significantly higher than other 12 darkgreenleaf nonheading Chinese cabbage. Heijingang had the largest leaf width， of 14.56 cm， and Tai 9 had the smallest width of 9.46 cm. Heijingang， Baimeigui， Lijiansijian Zhongjiaoheiyetianbaicai 555， hybrid Youzhidabanbaicai， Siji Baicai， Yutu hybrid Heiyebaicai and Guangyanxinxuan Aijiaoheiyebaicai had no significant difference in leaf width therebetween， but had a significant difference from other 10 materials. Hybrid Youzhidabanbaicai had the largest petiole width of 2.47 cm， and Zhongjiaokuishanheiyechigengbai 316 had the smallest petiole width of 1.63 cm. There was no significant difference between hybrid Youzhidabanbaicai， Tai 9 and Xiaxue Naibaicai in petiole width， but they were significantly higher than other 14 materials （Table 4）.
Baimeigui had the largest single plant weight of 99.10 g， and the single plant weight of Zhongjiaokuishanheiyechigengbai 316 was the smallest， of 52.98 g. There was no significant difference in single plant weight between Baimeigui， Heijingang， Lijiansiji Zhongjiaoheiyetianbaicai 555 and hybrid Youzhidabanbaicai， but their values were significantly higher than other 13 darkgreenleaf nonheading Chinese cabbage varieties. The chlorophyll content of Tai 9 was the highest， being 51.54 SPAD， and the chlorophyll content of Taiwan Siji Gaojiaotianbaicai was the lowest of 27.66 SPAD. There was no significant difference in chlorophyll content between Tai 9， Teyounaibaicai， Lijiansiji Zhongjiaoheiyetianbaicai 555 and hybrid Youzhidabanbaicai， but their chlorophyll contents were significantly higher than other 13 darkgreenleaf cabbage varieties. The yield of hybrid Youzhidabanbaicai was the highest， of 26 258.15 kg/hm2， and the yield of Tai 9 was the lowest， of 12 572.85 kg/hm2. There was no significant difference in yield between hybrid Youzhidabanbaicai， Baimeigui， Heijingang， Teyounaibaicai， Xiaxue Naibaicai， Siji Baicai and Guangyanxinxuan Aijiaoheiyebaicai， but their yields were significantly higher than other 10 darkgreenleaf cabbage varieties （Table 4）. The smaller the heat injury index， the stronger the heat resistance of the material. It was found from the comparison of heat damage index between different darkgreenleaf nonheading Chinese cabbage varieties that the heat tolerance of Heiyekuishanbaicai is the strongest， Tai 9， Baimeigui， Heijingang， Xiaxue Naibaicai， Siji Baicai， Yutu hybrid Heiyebaicai and Zhongjiaokuishanheiyechigengbai 316 have stronger heat tolerance， and Taiwan Siji Gaojiaotianbaicai is the worst in heat tolerance （Table 4）.
Combined with agronomic traits and heat damage index， the comprehensive analysis found that Baimeigui， Heijingang， Xiaxue Naibaicai and Siji Baicai have stronger heat tolerance and higher yield cabbage， and are suitable for promotion in hot and humid seasons.
Conclusions and Discussion
Considering climatic factors， continuous high temperature above 35 °C， heavy rainfall and high humidity after rainfall are the main factors affecting the production of vegetables in Hainan Province in summer and autumn. Due to the lag in the research on vegetable varieties tolerant to heat and humidity in summer and autumn in Hainan Province， the current varieties of vegetables growing in summer and autumn in Hainan are single， and the heattolerant varieties used are poor， which is difficult to meet the needs of summer and autumn. These remarkable problems not only affect the enthusiasm of growers， but also restrict the healthy development of summer and autumn vegetable production in Hainan. In this study， 24 greenstem nonheading Chinese cabbage varieties and 17 darkgreenleaf Chinese cabbage varieties were collected from domestic and foreign countries， and the agronomic traits and heat damage index were investigated. The results showed that four greenstem nonheading Chinese cabbage varieties including Huaguan Qingcaigeng F1， Zhongguan Qinggeng， Hanshen No. 1 Qinggengcai F1 and Huaxiawang Piaocai have the characteristics of strong heat tolerance and high yield， and four darkgreenleaf nonheading Chinese cabbage varieties including Baimeigui， Heijingang， Xiaxue Naibaicai and Siji Baicai have the characteristics of strong tolerance to heat and high yield. These eight leaf vegetable varieties are suitable for popularization in summer and autumn in Hainan Province.
References
[1]TANG SX， ZHAO ZZ， BI H， et al. Analysis of characteristics and development of climate resources in Hainan[J]. Journal of Hainan Normal University： Social Sciences， 2008， 21（3）： 343-346. （in Chinese） [2]LIU JB， PENG Y， CHEN QB. Climate suitability analysis and zoning of sugarcane planting in Hainan[J]. Chinese Journal of Agrometeorology， 2009， 30（2）： 254-256. （in Chinese）
[3]DONG XX， ZHANG YM， ZHANG C. Analysis of the influence of vegetable industry development on farmersю income in Hainan Province――Based on survey data of farmers in six counties[J]. Shandong Agricultural Sciences， 2015（1）： 155-158. （in Chinese）
[4]KUANG CX. Lack of vegetables in summer and autumn in Hainan is due to that farmers in the high temperature and rainy central part are not good at growing vegetables[N]. Hainan Daily， 2015-1-29. （in Chinese）
[5]KUANG CX. What is the reason for the lack of vegetables in Hainan Island Growing vegetables is severely restricted by the season[N]. Hainan Daily， 2015-1-29. （in Chinese）
[6]LENG ZL. Eight measures in Hainan Province to protect the "vegetable basket"[J]. China Fruit & Vegetable， 2011（5）： 57. （in Chinese）
[7]XIA GN. Hainan strengthens the construction of 150 000 mu vegetable base to solve the problem of seasonal supply shortage[N]. Hainan Daily， 2016 -4-10. （in Chinese）
[8]HU QQ， CHEN LZ， ZHANG YJ， et al. Studies on heat tolerance identification method for nonheading Chinese cabbage[Brassica campestris L. ssp. Chinensis （L.） Makino var. communis Tsen et Lee][J]. China Vegetables， 2011（2）： 56-61. （in Chinese）