HAN Ming-li, SHEN Wei-xin, ZHAO Gen, CHEN Li-ping

Huzhou Academy of Agricultural Sciences, Huzhou 313000, PRC

Abstract As a newly-emerged healthy vegetable, ice plant (Mesembryanthemum crystallinum) has won widespread popularity among consumers in Huzhou, Zhejiang. This paper formally introduces the nutritional and biological characteristics of Mesembryanthemum crystallinum, and also summarizes its key cultivation techniques under the climatic conditions of North Zhejiang, including seed propagation, cuttage breeding, transplanting, field management, pest control, harvest, and seed preservation.

Key words Mesembryanthemum crystallinum; Health vegetable; Cultivation techniques

1. Introduction

With the improvements in living standards and social development, consumers have gradually shifted their pursuit of vegetable quantity for their pursuit of vegetable quality. Healthy vegetables are popular among Chinese consumers because of their unique f lavors, rich nutrients, and special medicinal values. Some of these healthy vegetables have developed new cultivars for mass production. Being highly resistant to diseases and pests, healthy vegetables are also a new favorite for farmers and can bring them great social and economic benefits. Many new healthy vegetables are now being planted for the market on a massive scale and have gained widespread popularity among consumers.

In general, Mesembryanthemum crystallinum is an annual succulent of the Aizoaceae family, Mesembryanthemum genus. Its stems and back of its leaves are covered with large, glistening, transparent particles, ref lected in its common names of ice plant, African ice plant or crystalline ice plant[1]. It is also a facultative CAM (crassulacean acid metabolism) plant with strong resistance to cold, drought, and salinealkaline soils. Its life cycle is usually completed within 6 months, depending on environmental conditions. Ice plant is native to the drought regions of Namibia, South Africa. It is a high-class food material in France because of its glittering and translucent appearance, fresh and crisp flavor, and saline taste. Ice plants immediately gained widespread popularity among consumers upon initial naturalization in China. Farmers in Shouguang (Shandong) were the first to grow ice plants in greenhouses with a yield of 29 985.00～37 481.26 kg/hm2based on purchasing orders from Guangdong, Taiwan, Hainan and South Korea[2]. Normally, ice plants are priced at 30 yuan/kg, but there was a time the price rose to 70 yuan/kg in Beijing, Shanghai, and Nanjing, generating high economic revenues. Farmers in Deqing (Huzhou, Zhejiang) started growing ice plants in the last two years and obtained high economic benefits based on the average market price of 30 yuan/kg[3].

In this research, ice plants were naturalized and grown in a greenhouse to investigate the key cultivation techniques involving light, water management, fertilization, and salinity, in order to provide technological support for ice plant production and popularization.

2. Nutrient value of ice plant

Ice plant is a highly nutritious vegetable, good for human health. Analyses of ice plants indicate that most nutrients are in the stems and leaves[4]. Fresh stems and leaves covered with large crystalloid bladder cells or water vesicles are edible and can be stir-fried or made into salad. Simple cooking methods could reduce nutrients loss. It is reported that ice plants are rich in malic acid, amino acid, carotene, sodium, potassium, and calcium elements. The slight saline taste is a reflection of the plant’s sodium elements, a kind of natural plant salt beneficial to human bodies. Ice plants also contain D-pinitol, inositol, retinol, etc. D-pinitol can stimulate the secretion of insulin, and therefore is conducive to the reduction in blood glucose, the absorption of creatine, and the improvement in liver functions. Inositol can help burn fat, reduce the level of cholesterol, and prevent fatty liver and arteriosclerosis. Ice plant is a good supplier of inositol in contrast to normal vegetables.

Additionally, many Africans use the crushed leaves of ice plants as a soap substitute or medicinal herb. In Japan, ice plants have been domesticated by researchers for large-scale planting and cosmetic applications because of their excellent moistening effects and rich vitamin C content. Skincare products containing ice plant substance are highly praised by female cosmetics consumers[5].

3. Biological characteristic of ice plant

Ice plant has a strong fibrous root system and cylindrical stems. It is a semi-creeping plant, which grows erectly at the early stage and then creeps at the later stage. Ice plant has a strong branching capacity, with each leaf axil capable of germinating side shoots. The leaves on this plant are oval and succulent with entire leaf margins, and arranged in an opposite pattern. The leaf margins of mature leaves are undulate. A single f lower, white or light pink in color, is located at the axil. The f lower has a yellowish center, 5 f iliform styles, and 5 stigmas. Capsules come into being as the f lowers fade. Each ovary has 5 chambers, each chamber containing many seeds[5].

Ice plant is highly tolerant of drought and salt. It is reported that NaCl stress could influence the germination and survival rate of ice plant seeds. The germination and survival rate would peak at a NaCl concentration of 0.6%[6]. Ice plant can be irrigated by seawater. The optimal concentration of seawater for its growth is 20%, making it a typical saltsecreting plant[7]. The stems and leaves contain rich salt glands, through which extra salt contents could be transported to the crystalline water vesicles in a saline environment. Hence, ice plant is also very useful for improving saline environments. The plant does not have many crystalline particles in the seedling stage, but progressively, its stems, leaf edges, and leaf backs would be fully covered with crystalline particles. Ice plant enjoys sunlight and can grow at a light range of 3 000～14 000 lx[7]. It is cold-resistant but cannot tolerate heat. When the temperature exceeds 30℃, the plant clusters and produces thin and crinkled leaves.

4. Cultivation techniques

4.1. Selection and preparation of the planting field

The etiolation of leaves and the reduction of crystalline particles are two common problems in the cultivation of ice plants. In North Zhejiang, most ice plants are grown in greenhouses.

Well-drained and porous sandy soil is optimal for the growth of ice plants. Base fertilization: organic manure 14 992.50～22 488.76 kg/hm2and N-P-K compound fertilize (N ∶P ∶K=15 ∶15 ∶15) 224.89～299.85 kg/hm2. After fertilization, plough the field at a depth of 30～40 cm to fully mix the fertilizer with earth. Rake the soil smooth to make vegetable beds. Each bed should be 1.0 m in width and 25～30 cm in height. The ditch between adjacent beds should be 30～35 cm in width and 15～20 cm in depth. For the convenience of picking and adequate space for plant growth in the later stage, it’s better to grow ice plants in a “high-bed double-line” pattern. The line space should be 50～60 cm wide and the plant space 40～ 50 cm.

4.2. Cultivation time

The life cycle of ice plant is usually completed within six months. In North Zhejiang, ice plants can be grown for the whole year, except in the hot season which is from the last 10 d of June to August. To ensure the quality of the seedlings and the length of the picking period, it is suggested that the summerautumn batch, the autumn-winter batch, and winterspring batch are sowed in the first 10 d of August, the middle 10 d of September, and the last 10 d of December, respectively.

4.3. Seed propagation

Ice plants are usually grown from tiny seeds, and the seedlings are transplanted twice. First, sieve base materials into flat breeding trays. Wet the base material evenly with a watering can. The optimal moisture ratio is 60% (criterion: hold the watered base material tightly in your hand and no water could be squeezed out; loosen your grip and the material remains a cluster). Mix the seeds with the base material or vermiculite. Distribute the mixture evenly into breeding trays. The density of the seeding should be 1 g/m2. Add a layer of sieved base material until the seeds are covered. Gently spray the trays with water to prevent the seeds from washing away. For the winterspring batch, a strip heater should be utilized to keep the seeds warm. The seeding should be done in a “three tunnels-one f ilm” pattern, namely, “greenhouse-walkin tunnel-low tunnel-plastic film”. The seedlings would f inish emerging in around 7 d. For the summerautumn batch, the breeding of seedlings should avoid the hot days. It would be good if the planter has an artif icial climate box to breed the seedlings at 17～20℃.

Move the seedlings to 50 or 72-cell plug trays when they send forth 2 pairs of leaves. The leaves of ice plants at this stage are very fragile. Great care is needed to protect the leaves and roots when transplanting. The seedlings should be taken out of the breeding tray with substrates. The optimal temperature for growth is 20～25℃. There are three matters that require attention: Mist spray irrigation should be used rather than heavy spray. For winter-spring breeding, the bedding temperature should not be too high and the plastic f ilm should be unveiled to increase sunlight and avoid overgrowing of stems. A shading net or grass curtain should be used to protect the plants from high temperature in summer.

4.4. Cuttage breeding

Seeds for experiments were purchased at 0.4～0.5 yuan/seed, much higher than normal vegetable seeds. According to our experience, cuttage breeding of side branches is feasible. The resulting plants showed no difference in flavor or taste compared with seed propagation plants. Ice plants flower very early. In order to ensure a good yield, one should choose fresh and healthy side branches for cuttage breeding at the early stage, instead of at the late stage. Currently, there are no reports of ice plants bred by cuttage. Cuttage breeding is a good supplement to remedy the low survival rate of transplanting.

4.5. Transplanting and field planting

Ice plants with 5～6 leaves are ready for field planting. Acclimatization measures should be taken one week before field planting. 15～20 g phosphatic fertilizer should be added to each planting hole in the field. Healthy, regular, pest-free and diseasefree seedlings with fully-developed roots could be transplanted to the field. The transplanted seedlings should be irrigated thoroughly. The fields should be covered with plastic film to reduce the contact of plants with soil and to suppress weeds, thus protecting the commercial nature of the ice plants.

4.6. Field management

4.6.1. Temperature control

The temperature in the growth stage should be controlled to 15～30℃. For winter nights with temper- atures below 10℃, low tents should be constructed to keep the plants warm at night and the tents should be removed in the daytime. Shading nets are needed to reduce the heat in summer.

4.6.2. Watering and fertilization

The principle of irrigation should be “water the fields thoroughly when the soil surface turns white”. Remove the leaves and stems that are resting on the plastic film to avoid the decay of the leaves and stems touching the f ilm. Spray 0.2% monopotassium phosphate on the leaves once in the early stage of field planting. When the plants have survived the field planting for two weeks, apply urea fertilizer at 149.93～224.89 kg/hm2according to the growth of the plants; irrigate the field to stimulate absorption. No further fertilization will be needed as there are suff icient base fertilizers.

4.6.3. Salt management

Irrigate the field with NaCl solution once a month to improve the taste and yield of the ice plants. The optimal NaCl concentration for ice plants should be 0.9%, thus producing maximal crystalline particles and f lourishing plants[6]. 4.6.4. Plant management

Prune the whole plant to maintain proper branch density, so that the plant could grow better and stronger. Strengthen air circulation to avoid pests and diseases.

4.7. Pest and disease control

According to existing statistics, pests and diseases are rarely seen among ice plants. Dampingoff is the most prevalent disease. Snails, aphids, and bemisia tabacis are the major pests.

4.7.1. Damping-off

Damping-off mainly exists in the breeding stage and the seedling stage，and it is caused by the overcrowding of plants, excessive moisture, and bad air circulation. Water stains on the base of the seedling are common symptoms of this disease. These water stains spread quickly, cause excessive shrinkage of the seedling base, and eventually weaken or kill the plant. Damping-off can be prevented or controlled in several ways: By sterilizing the growing medium with carbendazim before sowing the seeds; by spraying the seedlings with sterilizing agents (e.g. metalaxyl); by preventing the water accumulation on the plastic f ilm or in the ditches after field planting; by maintaining drier conditions with better ventilation and avoiding excessive moisture; by removing infected leaves to prevent the spread of the disease.

4.7.2. Snails

In June, the north part of Zhejiang Province is in the wet plum rain season, providing a favorable environment for snails. Application of 6% metaldehyde granules (Woda) around the greenhouse could help prevent the invasion of snails.

4.7.3. Aphids and bemisia tabacis

Ice plants are suitable for eating without being cooked. The minimum amount of pesticide should be used. Pest-control nets and yellow boards are strongly recommended to prevent aphids and bemisia tabacis.

4.8. Harvest and seed preservation

4.8.1. Harvest

The plants enter the fast-growing stage one month after field planting. During this stage, the side branches grow at high speed and are ready for harvest. Fresh branches about 8 cm in length can be gently sheared off at the second node for eating. A pair of functional leaves at the first node should be reserved with axillary buds. The sheared branches should be handled with care to avoid damage to the plants or crystalline particles. The harvest should be accompanied by pruning. The management of overgrown plants or branches are top priorities during harvest. Old, weak and sick leaves should also be pruned. Overcrowding and bad air circulation could cause the branches to become very thin. The harvest should be carried out in the morning or at dusk when the temperature is low, thus protecting the taste and flavor of the ice plants. Precooling and cold storage can help extend the supply period. The tips of the leaves of ice plant could turn purple-red because of excessive light, which is a natural phenomenon and plants in this case are still edible[9].

4.8.2. Seed preservation

There is no cultivar of ice plants at present. The existing species are wild, and seeds could be preserved accordingly[1]. A single plant with excellent characteristics could be chosen for artif icial pollination. Excessive flowers should be removed to save nutrients for the growth of seeds[8]. The entire seed capsule should be picked when it turns purplered. Smooth, rounded and dark seeds should be selected. The seeds should be dried in the sunshine and preserved for seeding.