Bin LIU,Zhangyou SHEN,Xiuqin GAN,Benhui WEI*,Yanying LI,Po HU,Yong SONG,Zhanqi YUAN,Zhixian JI,Yanyong WU,Chengying LAO

1.Institute of Economic Crops,Guangxi Academy of Agricultural Sciences,Nanning 530007,China;

2.College of Horticulture and Landscape Architecture,Hunan Agricultural University,Changsha 410128,China;

3.Institute of Soil Fertility and Environmental Resources,Jiangxi Academy of Agricultural Sciences,Nanchang 330200,China;

4.Institute of Crops and Nuclear Technology Utilization,Zhejiang Academy of Agricultural Sciences,Hangzhou 310021,China

Responsible editor:Tingting XU Responsible proofreader:Xiaoyan WU

Cassava (Manihot esculenta Crantz),also called as tree potato,is a species of Euphorbiaceae.It is an annual or perennial shrub.Cassava is one of the three important potato crops across the world.Cassava tuber,which is the most commonly-used organ,is rich in starch and enjoys a great reputation of "King of starch"[1-2].In the context of global food shortage and energy crisis,cassava,as a food,energy and forage crop,has attracted much attention around the world and has great development potential[3].The cassava cultivation is mainly concentrated in the subtropical regions of South China,such as Guangxi,Guangdong and Hainan.Cassava cultivation has a huge industrial and economic development potential[4].However,since the potential of cassava as food and deeply-processed industrial product has not been fully developed,its cultivation efficiency is relatively low,resulting in a decreased cultivation area in the main cassava-planting areas of China.To safeguard the sustainable development of China cassava industry,some experts have proposed that the cultivation area of cassava in China should be divided into area south of Tropic of Cancer,area north of Tropic of Cancer and south of 25°N and area north of 25° N and south of 30° N.In addition,they have also suggested that the cassava should be developed as a food crop and the cassava industry should be developed northward[5].For the northward-moved cassava cultivation,the overwintering safe storage of seed-stem of cassava is a key issue.The study on overwintering safe storage and regional division of seedstem of cassava for different cultivation regions throughout China will contribute to achieving the northward movement and sustainable development of cassava industry.

The study on storage of seedstem of cassava has been started in as early as 1960s.Zhang[6]found in Liuzhou of Guangxi,the seed-stem of cassava could be stored vertically in cellar with vents or be stored horizontally in cellar without vents for overwintering.Lu[7]pointed that in the frostfree areas,the seed-stem of cassava could be stacked in the open air for overwintering; while in the frosty areas,the seed-stem of cassava could be stored in cellars.Yu et al.[8]used the seed-stem of Huanan 205 and GR891 as the test material.They carried out artificially-simulated cold injury and freezing injury tests.Combing the field observation and analysis on relevant meteorological data,they found the critical temperature for occurrence of cold injury and freezing injury in seedstem of cassava was 4-5 ℃,and in cassava tuber was 5-6 ℃.Fan et al.[9]investigated the effects of open-air stacking and indoor stacking on the overwintering of seed-stem of cassava.They found the seed-stem of cassava that was stacked in the open air geminated more rapidly and more evenly.Moreover,the cassava that was stacked in the open air had a higher germination rate.

There are rare reports on the regional division of overwintering safe storage methods of seed-stem of cassava and the optimum overwintering storage methods for different cassavaplanting areas.Based on different climatic features of different cassavapanting areas,the seed-stem of cassava was stored with different methods for overwintering in Hepu,Nanning and Laibin of Guangxi,Jiangyong of Hunan and Hangzhou of Zhenjiang.The injury in seed-stem of cassava and survived bud number was observed and counted.Combining the meteorological data in different regions,the regional division of overwintering safe storage methods of seedstem of cassava was carried out.In addition,the optimum storage methods were proposed to different cassava-planting areas so as to provide guarantee for the sustainable development of cassava industry.

Material and Methods

Material

The tested cassava cultivar was Huanan 205.

Methods

According to the climatic conditions of cassava-planting areas,the frost-free Hepu of Guangxi,light-frost Nanning and Laibin of Guangxi and heavy-frost Jiangyong of Hunan and Hangzhou of Zhenjiang were selected.There were several treatments for each test site.Before the coming of frost,the seed-stem of cassava was cut off at 5 cm beneath the ground surface.The spears at the tip were removed.Then the seed-stem of cassava was bundled (25 kg/bundle).The seed-stem of cassava was stored for overwintering as designed.After the beginning of spring next year,the bud number and survived bud number were counted.Then the average survival rate of buds was calculated.

Hepu,GuangxiThe test site was located in Shizipu Town.There were 4 treatments,including open-air stacking,open-air grass-covered stacking,open-air film-covered stacking and open-air shallow burying.For the open-air stacking,the seed-stem of cassava was stacked neatly and covered nothing at a leeward and sunlightshaded place.For the open-air grasscovered stacking,the seed-stem of cassava was stacked upright (with base downward to ground surface)and neatly at a leeward place.Then a layer of 5 cm grass was covered on the surface of seed-stem stacks.Some stones were placed above the grass so as to fix the grass.For the open-air film-covered stacking,the seed-stem of cassava was stacked vertically (with base downward to ground surface) and neatly at a leeward place.Then the film was covered to the upper half of the seed-stem stacks(lower half not covered).For the open-air shallow burying,a ditch(width,1.5 m;depth,0.5 m)was dug at a leeward and well-drained place.The seed-stem of cassava was then placed vertically (with base upward to ground surface)and neatly in the ditch.Then a layer of 30-40 cm fine soil was piled around the base of seed-stem.

Nanning,GuangxiThe test site was located in Guangxi Academy of Agricultural Sciences.A total of 7 treatments were designed,including openair stacking,open-air grass-covered stacking,open-air film-covered stacking,open-air shallow burying,storage inside greenhouse,live overwintering storage and indoor storage.For the open-air film-covered stacking,the seed-stem of cassava was stacked horizontally and neatly at a leeward and sunny place.Then the whole stacks were covered with film.The edge of the film was compacted with soil.A total of 3-4 vents were arranged.For the storage inside greenhouse,a greenhouse (length,20.0 m;width,6.0 m; height,2.5 m) was constructed at a leeward,flat and welldrained place.The drainage ditch was dug around the greenhouse to prevent water from flowing into greenhouse(the same below).The seed-stem of cassava was stacked together neatly(height of stack,1.5 m).For the live overwintering storage,the cassava plants with good growth were selected.They were not harvested before the winter.Then the quality of seed-stem was observed after the coming of spring next year.For the indoor storage,the seed-stem of cassava was neatly stacked in the room.

Laibin,GuangxiThe test site was located in the Qianjiang Farm.There were total 11 treatments,including open-air stacking,open-air sugarcane leaves-covered and film-covered horizontal storage,open-air sugarcane leaves-covered and film-covered vertical storage,sugarcane leaves-covered and film-covered horizontal burying,sugarcane leaves-covered vertical burying,sugarcane leaves-covered horizontal burying,horizontal storage inside greenhouse,vertical storage inside greenhouse,sugarcane leavescovered horizontal burying inside greenhouse,sugarcane leaves-covered vertical burying inside greenhouse and sugarcane leaves-covered horizontal storage inside greenhouse.For the open-air sugarcane leavescovered and film-covered horizontal storage,the seed-stem of cassava was stacked horizontally and neatly(height of stack,1.5 m) at a leeward and sunny place.A layer of 10 cm sugarcane leaves was covered on the stacks,followed by plastic film.The edge of the film was compacted with stones.For the open-air sugarcane leaves-covered and film-covered vertical storage,the seed-stem of cassava was stacked vertically at a leeward and sunny place.A layer of 10 cm sugarcane leaves was covered on the stacks,followed by plastic film.The edge of the film was compacted with stones.For the sugarcane leavescovered and film-covered horizontal burying,a ditch (depth,1.5 m; length,2.0 m;width,1.5 m)was dug at a leeward and sunny place (the same below).The seed-stem of cassava was then placed horizontally and neatly in the ditch.A layer of 10 cm sugarcane leaves was covered on the stacks(sugarcane leaves surface a litter higher than the ditch mout),followed by plastic film.The edge of the film was compacted with stones.A total of 3 -5 vents were arranged.And then a layer of 10-12 cm soil was covered.The drainage ditch was dug around the ditch.For the sugarcane leaves-covered vertical burying,the seed-stem of cassava was placed vertically and neatly in the ditch which was located in a leeward and sunny place.A layer of 10 cm sugarcane leaves was covered on the stacks (sugarcane leaves surface a litter higher than the ditch mout),followed by a layer of 10-12 cm soil.The drainage ditch was dug around the ditch.For the sugarcane leaves-covered horizontal burying,the seed-stem of cassava was placed horizontally and neatly in the ditch which was located in a leeward and sunny place.A layer of 10 cm sugarcane leaves was covered on the stacks(sugarcane leaves surface a litter higher than the ditch mout),followed by a layer of 10-12 cm soil.The drainage ditch was dug around the ditch.For the horizontal storage inside greenhouse,the seed-stem of cassava was stacked horizontally and neatly in the greenhouse (height of stack,1.5 m).Then the greenhouse was well covered with film.For the vertical storage inside greenhouse,the seed-stem of cassava was stacked vertically and neatly in the greenhouse.Then the greenhouse was well covered with film.For the sugarcane leaves-covered horizontal burying side greenhouse,the seed-stem of cassava was placed horizontally and neatly in the ditch inside greenhouse.A layer of 10 cm sugarcane leaves was covered on the stacks(sugarcane leaves surface a litter higher than the ditch mout),followed by a layer of 10-12 cm soil.Then the greenhouse was well covered with film.For the sugarcane leaves-covered vertical burying inside greenhouse,the seed-stem of cassava was placed horizontally and neatly in the ditch inside greenhouse.A layer of 10 cm sugarcane leaves was covered on the stacks (sugarcane leaves surface a litter higher than the ditch mout),followed by a layer of 10-12 cm soil.Then the greenhouse was well covered with film.For the sugarcane leaves-covered horizontal storage inside greenhouse,the seed-stem of cassava was stacked horizontally and neatly in the greenhouse (height of stack,1.5 m).A layer of 10 cm sugarcane leaves was covered on the stacks.Then the greenhouse was well covered with film.

Jiangyong,HunanA total of 3 treatments were designed,including open-air stacking,storage inside cave and storage inside cellar.For the storage inside cave,the natural cave that faces south was selected.Inside the cave,a relatively ventilated,dry and flat area was used to store seed-stem of cassava.First,a layer of dry sand or hay was mulched.Then the seed-stem of cassava was stacked upright.At final,the cave was sealed properly to prevent the intrusion of cold air.For the storage inside cellar,the cellar(length,3.0 m;width,1.4 m;depth,1.2 m)was dug at a high-altitude,leeward,sunny,low-underground water level,difficultly-seeped and well-drained place.The cellar edges were vertical.At the bottom of cellar,the 5 cm deep surface soil was crushed and loosened.The seed-stem of cassava was arranged upright and inserted into the loosened soil(in depth of 3 cm).A layer of 10-15 cm hay was covered on the stacks,followed by a layer of 20-cm turtle-shaped soil.The snorkels were installed at both ends the cave to adjust the temperature and humidity.At final,the cellar mout was sealed with a layer of compacted turf mud.Above the cellar,a simple arched plastic canopy was constructed.The drainage ditch was dug around the canopy.

Hangzhou,ZhejiangThere were a total of 2 treatments,including openair stacking and vertical stacking inside greenhouse along with small shed.For the vertical stacking inside greenhouse along with small shed,a greenhouse(length,15.0-20.0 m; width,6.0 m;height,2.5 m) was constructed at a leeward,sunny and flat place.The drainage ditch was dug around the greenhouse to prevent rainfall flowing into greenhouse.Inside the greenhouse,a smaller plastic shed(length,20.0 m; width,2.5 m; height,2.0 m)was constructed.Inside the shed,the top 5-8 cm surface soil was loosened.The seed-stem of cassava was stacked vertically (base downward)and inserted into the loosened soil(in depth of 3-5 cm).Both of the large and small sheds were well sealed with edges of film compacted.

Results and Analysis

Hepu,Guangxi

As shown in Table1,among the 4 storage methods in Hepu,Guangxi,the survival rates of buds were all higher than 90.00%.The treatment effects of open-air shallow burying and filmcovered stacking were the best.The treatment effect of open-air grass-covered stacking ranked second.The survival rate of buds was lowest in the open-air stacking.The variance analysis showed there were no significant differences in survival rate of buds among the 4 treatments.

Nanning,Guangxi

Among the 7 storage methods in Nanning,Guangxi,the survival rate of buds in seed-stem of cassava that was stacked in the open air was lowest,and was significantly lower than those of other storage methods(P ＜0.05;the same below).The buds in the seedstem of cassava that was stored in greenhouse had the highest survival rate,which was significantly higher than those of live overwintering storage and open-air stacking.However,there were no significant differences in survival rate of buds among storage in greenhouse,open-air film-covered stacking,open-air shallow burying,indoor storage and open-air grass-covered stacking.

Laibin,Guangxi

Table3 showed,in Laibin,Guangxi,the survival rates of buds in seed-stem of cassava that was stored in greenhouse were almost all higher than 89.60%,and were all higher than those of cassava that was stored in the open air.Among the 11 storage methods,the sugarcane leaves-covered horizontal burying inside greenhouse had the highest survival rate of buds(98.79% ).The variance analysis showed there were significant differences in survival rate of buds between sugarcane leaves-covered horizontal burying inside greenhouse and sugarcane leaves-covered horizontal storage inside greenhouse,horizontal storage inside greenhouse and vertical storage inside greenhouse.Among the open-air storage treatments,the survival rates of buds for open-air sugarcane leaves-covered and film-covered horizontal storage and open-air sugarcane leaves-covered and film-covered vertical storage were all higher than 80.00%,and they were significantly higher than those for other open-air storage treatments.All of the buds in seed-stem of cassava that was stacked in the open air were dead,but there were certain amounts of buds survived for the other storage treatments that had taken certain protective measures.It was indicated that some protective measures must be taken for the overwintering storage of seedstem of cassava in Laibin,Guangxi.

Jiangyong,Hunan

As shown in Table4,the survival rate of buds for storage inside cave was as high as 82.20%,which was higher (P ＜0.05)than that for storage inside cellar by 10.80%.The buds in seed-stem of cassava that was stacked in the open air were all dead.

Hangzhou,Zhejiang

Table5 showed in Hangzhou,the average survival rate of buds for vertical stacking inside greenhouse along with small shed was 71.71%.But the buds in seed-stem of cassava that was stacked in the open air were all dead.

Table1 Effects of different storage methods on seed-stem of cassava in Hepu,Guangxi

Table2 Effects of different storage methods on seed-stem of cassava in Nanning,Guangxi

Table3 Effects of different storage methods on seed-stem of cassava in Laibin,Guangxi

Table4 Effects of different storage methods on seed-stem of cassava in Jiangyong,Hunan

Table5 Effects of different storage methods on seed-stem of cassava in Hangzhou,Zhejiang

Discussion

Cassava is a tropical crop,but it can also be planted in temperate[10].However,the overwintering safe storage methods of seed-stem of cassava must be mastered.The cultivation of cassava in China can be divided into 3 areas,main cassava-panting area(south of Tropic of Cancer),secondary cassava-panting area (north of Tropic of Cancer and south of 25° N) and marginal cassava-planting area(north of 25° N and south of 30° N).The marginal cassava-planting area is the mainly northward-moved cassavaplanting area[5].Different ecological zones have their own distinctive climatic features.The objective laws should be respected.In addition,considering the overwintering safe storage of seed-stem of cassava,the household storage and concentrated storage should be combined.In according to the requirements by industrial development and planned cultivation area of cassava,the overwintering safe storage method should be adopted rationally and arranged coordinately,thereby improving the overwintering storage of seed-stem of cassava.

Cassava is a strategic crop with China’s characteristics and has attracted much attention from Chinese government.The development of China’s cassava industry requires more high-quality seeds and high-quality cultivar supports.To improve the cassava cultivars and single yield of cassava,the storage of seed-stem of cassava in China can be divided into 3 regions in according to meteorological data and study results,including frostfree region(non-facility storage region),light to heavy-frost region(simple-facility storage region)and heavy-frost and low-temperature region (installed protective storage region).In the region south of 21.6° N,including Hainan,Zhanjiang of Guangdong and local areas of South Guangxi,such as Hepu,Qinzhou,South Fangchenggang and South Yulin,the average annual temperature is higher than 22 ℃with average temperature in January higher than 14 ℃; there is no or light frost in winter; the air humidity is relatively high.This region is the main base for breeding cassava and is also known as the overwintering safe storage region of seed-stem of cassava.This region has low latitude but has no frost in winter.The cultivation of cassava and overwintering storage of seed-stem can be in accordance with the traditional technologies.Non special facilities are required.The seed-stem of cassava can be buried shallowly in the open air or stacked in the open air with film covered for overwintering.In terms of safe overwintering of seed-stem of cassava,the light to heavy-frost region(simple-facility storage region) ranks second.This region can also be further divided into I and II regions.The I region(21.6° N - 22.8° N,Hepu-Nanning) includes Chong-zuo,Yulin and South Nanning of Guangxi,Pu’er of Yunnan and Shanwei of Guangdong.This region is located between tropics and subtropics.The climate is relatively warm.The average annual temperature is around 21.6 ℃,and average temperature in January is higher than 12.8 ℃.Moreover,frost occurs less frequently in winter.However,there may be frosty weather in a certain year.In order to ensure the safe overwintering of seed-stem of cassava,some simply protective storage technologies should be adopted,such as storage inside greenhouse and open-air film-covered stacking.The II region (22.8° N - 23.8° N,Nanning-Laibin) includes Wuzhou and Guigang of Guangxi,Honghe and Wenshan of Yunnan and Huizhou of Guangdong.This region has subtropical climate.The weather is relatively warm.The average annual temperature ranges from 18.1 ℃to 21.2 ℃.In January,the temperature is higher than 10.4 ℃.There are relatively frequent frosty weathers in winter.So the simply protective facilities should be provided for the overwintering safe storage of seed-stem of cassava.The open-air sugarcane leaves-covered and filmcovered horizontal (vertical) storage inside greenhouse,sugarcane leavescovered horizontal storage inside greenhouse and vertical burying inside greenhouse can be selected for the overwintering storage of seed-stem of cassava.The heavy-frost and lowtemperature region(installed protective storage region)is north of 23.8°N,including North Guangxi,North Guangdong,North Yunnan,Hunan,Jiangxi and Zhejiang.This region is the marginal region for planting cassava.This region can also be called as the insecure region for overwintering storage of seed-stem of cassava.The temperature is relatively low in winter,so frost,freezing or low temperature occurs every year in this region.So the overwintering safe storage of seedstem of cassava is relatively difficult in this region.During the storage of seedstem,facilities are essential.In protected improperly,the seed-stem of cassava may be necrosed wholely.The results of tests carried out in Jiangyong and Hangzhou showed,the survival rates of buds in seed-stem of cassava that was stored in special facilities were all higher than 70.00%.Therefore,in the heavy-frost and lowtemperature region,the storage inside cold-resistant cave or cellar or vertical stacking inside greenhouse along with small shed should be adopted for the overwintering safe storage of seedstem of cassava.

Conclusions

The overwintering safe storage of seed-stem of cassava in China can be divided into frost-free region,light to heavy-frost region and heavy-frost and low-temperature region.In the frostfree region south of 21.6° N (Maoming-Hepu -Jinghong),the open-air shallow burying or film-covered stacking can be adopted for the overwintering storage of seed-stem of cassava.In the light to heavy-frost region(21.6°N-23.8°N,south of Qingyuan-Laibin-Lincang),storage inside greenhouse,open-air film-covered stacking or vertical burying inside greenhouse can be adopted.In the heavy-frost and low-temperature region north of 23.8°N (north of Qingyuan-Laibin-Lincang and south of Hunan -Jiangxi -Zhejiang),the seed-stem of cassava can be stored in cold-resistant caves,cellars or greenhouse along with small shed.

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