What causes mustard tuber (zha cai) to bolt?

Zha cai (pickled mustard tuber), in the phylum Angiospermae, a family within the class Dicotyledoneae. Mostly herbaceous plants. Zha cai is a type of mustard, generally referring to the leafy mustard group, such as nine-head mustard, snow mustard (potherb mustard), pig-blood mustard, tofu-skin mustard, etc. Zha cai is a semi-dry, non-fermented pickled vegetable made by pickling stem mustard as the raw material. It is one of China's famous specialty products and, together with French pickled cucumber and German sweet-and-sour cabbage, is known as one of the world's three great pickled vegetables.

Zha cai is a variety of mustard. Mustard is often used to pickle salted and sour vegetables; its fleshy stem pith is the raw material for Chongqing zha cai, and it belongs to the family Brassicaceae and the genus Brassica. The outstanding features of Brassicaceae are: herbaceous habit, cruciform corolla, tetradynamous stamens, and silique fruit. There are many economic plants, including a variety of everyday edible vegetables. The genus Brassica is herbaceous, bears long siliques, and has a yellow corolla. Chinese cabbage (B. pekinensis) originated in North China and is widely cultivated in various places; it is a major vegetable for the winter and spring seasons in Northeast and North China.

What is pickled for zha cai is the superior stem mustard, also called fresh vegetable head. The fresh vegetable head can also be made into a side dish, stir-fried with meat or made into soup, but it is more often used for pickling. It has a crisp and tender texture, a delicious flavor, and rich nutrition. It has a special flavor, with a distinctive sour taste and a savory-umami taste, crisp, tender and refreshing. It contains abundant protein, carotene, dietary fiber, minerals and other substances essential to the human body, as well as 17 free amino acids including glutamic acid, aspartic acid, and alanine. Zha cai can be used to accompany meals, in stir-fries, and in soups.

Why does zha cai bolt?

Zha cai is less adaptable to climate and less disease-resistant than leafy mustard. Its growing period is long, its temperature requirements are relatively strict, and its cold tolerance is relatively poor. Its stems and leaves are very closely related to the sowing date and environmental conditions. Climate conditions differ from place to place, and yields and quality differ in various respects from place to place; different sowing dates also produce very different results. The entire growing period of zha cai is about 150 days, of which the fleshy stem swelling stage requires an optimum growing temperature of 8-13°C; below 16°C is favorable for the swelling of the fleshy stem. Precisely because of zha cai's special requirements for climate and environment, it has become a specialty vegetable of many regions in China. With the development of the vegetable industry, various places have introduced and trial-planted it on different scales. At present, the main production areas for zha cai cultivation are along the two banks of the Yangtze River in Sichuan, Chongqing, Hubei, Hunan and other places. Jiangsu, Shanghai, Zhejiang and other provinces located in the middle and lower reaches of the Yangtze River have successively introduced and cultivated it in recent years. Because climates differ from place to place, sowing dates and harvest periods also differ accordingly. In central Sichuan, sowing is generally in early September and harvest in February and March of the following year, with relatively high yields; sowing later than early October noticeably reduces the yield. In Wuhan, Yichang and other places in the middle reaches of the Yangtze River, sowing is in early September and harvest at the end of December, or sowing from late September to early October, planting out to survive the winter, and harvesting from late March to early April. In Shanghai, Nanjing, Hangzhou and other places in the lower reaches of the Yangtze River, sowing is from September to October, and by the following spring there are both successes and failures; in some areas, zha cai is small in size and low in yield year after year; in a few areas, premature bolting and similar phenomena also occur, causing certain losses. The reasons for such phenomena mainly involve the following factors: 1. Climatic factors. Because zha cai has strict temperature requirements and poor cold tolerance, and its growing period is long, the winter climates of different places differ greatly, bringing certain difficulties to cultivation. During the swelling stage of its fleshy stem, when the temperature is within the range of 8-13°C, the longer this lasts, the longer the fleshy stem swells and the higher the yield; conversely, the yield decreases and the zha cai is small. When the winter temperature drops to 7-8°C, it will differentiate flower buds and switch to reproductive growth; when the temperature rises the following year, the flower stem also elongates rapidly. Long daylight promotes bolting after flower bud differentiation, and combined with relatively high temperatures it rapidly promotes bolting; the longer the day length, the greater the degree of promotion. The Jiangsu and Zhejiang areas are affected relatively strongly in this respect. When the winter temperature is at a level to which zha cai can relatively well adapt, the zha cai is large and the yield is high. When the temperature changes greatly, zha cai is prone to freezing injury and frost injury, which obstructs its normal vegetative growth and thus shifts it to reproductive growth; the yield decreases noticeably, and bolting is also likely to occur. When the climate changes greatly in a given year, the entire growth of zha cai will not adapt to the climate change, which can also cause bolting. 2. Cultivation conditions. In arranging the sowing date, if sowing is relatively late, the winter seedlings undergo vernalization and flower bud differentiation; after the temperature rises the following year, the plant mainly undergoes reproductive growth, the fleshy stem swelling stage is short and swelling is slow, and when the temperature is above 10°C, it quickly bolts. During its early growth process, zha cai is harmed by diseases, insect pests, pesticide injury, fertilizer injury and other aspects, which affect its growth and also easily cause bolting. During seedling raising, if the seedling density is high, the seedlings become leggy or weak, and their growth is affected, the yield decreases and bolting occurs earlier. In addition, soil fertility, the level of fertilization, and the proportions of nitrogen, phosphorus, potassium and other nutrient elements all indirectly affect flower bud differentiation and bolting by affecting the plant's vegetative growth. Generally, fertile soil and a relatively high nitrogen level will delay flower bud differentiation and bolting in the plant; on the contrary, bolting occurs earlier. In cultivation techniques, because more organic fertilizer is applied and phosphorus and potassium fertilizers are added as base fertilizer, appropriate topdressing is also needed during the peak swelling stage; harvest must be timely. When flower buds appear, the fleshy stem has already fully grown and should be harvested in time to avoid bolting affecting quality. 3. Influence of growth-regulating substances. Auxin-type substances such as indoleacetic acid (IAA), naphthaleneacetic acid (NAA), 2,4-D, and 2,4,5-T can promote bolting, and gibberellin can also promote bolting in zha cai. During its growth stage, if pesticides, micro-fertilizers, or other preparations containing the above substances are sprayed, premature bolting may result, thereby affecting yield. Because of the special characteristics of zha cai, its cultivation is also limited to certain regions. Because the requirements for cultivation techniques and climatic conditions are very high, during its cultivation and growth stage, climatic factors and cultivation level can easily trigger bolting. To avoid this bolting phenomenon, sowing must be carried out according to the locally appropriate sowing date, and cultivation techniques must be continuously improved to reduce the occurrence of this phenomenon. However, in abnormal years it is very difficult to determine the best sowing date for zha cai, and bolting is unavoidable.