Background
Common head cabbage (Brassica oleracea var. capitata L.), also known as cabbage, is one of the important cruciferous vegetables, which is characterized by outstanding adaptability and stress tolerance [1], high in nutritional value, rich in dietary fiber, vitamins, VB1, VB2, calcium, and iron, lutein, β-carotene and even cancer-preventing substances [2,3,4,5]. So, it was recommended by the World Health Organization as one of the best vegetables. Now it is extensively planted in all parts of the world [6,7]. The planting area and yield of cabbage in China are on the rise year by year, taking first place in the world [8,9]. Although there have been many studies on cabbage yield and quality, head-splitting is still a crucial problem in the later growth period of cabbage [10]. The head-splitting of cabbage is a physiopathy that decreases the appearance quality and commodity value, thus decreasing farmers’ income to a substantial extent; furthermore, a more significant problem is the fact that it is also the door to bacteria, fungi and pests, and increases the cost of storage and transportation, and even cause serious economic losses [11].
In the case of sufficient moisture, the head-splitting of cabbage is mainly determined by the genetic property of the variety [10]. Screening germplasm and breeding tolerance varieties are the fundamental ways to solve the problem [12]. It is necessary to clarify the current screening and identification methods, anatomic properties, and the molecular progress of research related to them. But because the molecular mechanism of cabbage head-splitting is less studied, it is also closely related to the moisture condition [13]. Therefore, this paper will also involve the discussion of external factors that induce the expression of related genes to provide ideas for the study of molecular mechanisms and breeding of head-splitting tolerance.
Introduction
The head-splitting property of cabbage is the phenomenon of splitting of cabbage leaf head. In cultivation, it occurs mostly in the late stage of cabbage growth. When cabbages reach the heading stage, if they are not picked after the number, and weight of the heading leaves that make up the head reaches a certain level but continue to grow, it will cause the outermost heading leaves of cabbage leaf heads to split, which is called the head-splitting [14]. The top of the head is the common splitting part of the cabbage, and there are also lateral or ‘crossed’ dehiscences, which are mostly linear. At last, the inner tissues were exposed. The properties of its splits are transverse split, longitudinal split and mixed split [3,15]. The degree of splitting also gradually deepens, starting from the outermost leaves of the leaf head and gradually extending to the inner side. In severe cases, it can be split to the dwarf stem. The degree of splitting can be expressed by the length, width and depth of the split in the cabbage leaf head [10].
Although the physiological basis of splitting/cracking is not fully understood [16], there are main causes of head-splitting of cabbage, as follows. On the one hand, the cabbage head is prone to split when too much water is provided from outside during the late growth stage. It is easy to occur split in the waterlogged field after heavy rain or heavy irrigation. Especially when there is a sudden heavy rain or water flooding during drought, it is more likely to cause head-splitting. The main reason is due to the brittle tissue and poor cell flexibility; excessive water absorption leads to cell swelling and splitting.
On the other hand, When the cabbage harvest is delayed, the number of cabbage with head-splitting will increase. Sale experience has demonstrated that cabbage varieties in the same maturity stage are often put on the market in a concentrated way, resulting in a drop in the price of cabbage. In an attempt to coordinate the market demand and increase farmers’ income, the measure often taken is to delay picking, which will raise the risk of head-splitting of cabbage during the storage period in the field [17]. Although the cabbage has reached harvest and marketing conditions, it still continues to grow in the soil until head-splitting occurs, and this period is called the storage period in the field (called the ‘Cunpu’ period in China) [18]. The storage period in the field reflects the tolerance to head-splitting of the cabbage variety, and the stronger the tolerance to head-splitting, the longer the storage period in the field [19]. During large-scale production of cabbage, if the storage period in the field is too long, the cabbage put on the market will split in the field because it cannot be sold in due course, resulting in the loss of its commercial value [20]. Planting cabbage varieties with head-splitting tolerance can directly prolong the harvesting time of cabbage so that farmers can selectively harvest cabbage within a suitable range of selling price, increase the output value of cabbage and reap higher economic benefits.
These reasons above are closely related to the genetic characteristics of cabbage, in which different cabbage varieties showed different performances in head-splitting properties. Although the genetic base and molecular mechanism of cabbage are still poorly understood compared with other vegetable crops [21], collecting, evaluating and acquiring the germplasm resources of cabbage with head-splitting tolerance serves as the basis for cultivating varieties of cabbage with head-splitting tolerance. At present there is currently no uniform standard for evaluating the head-splitting property of cabbage. A few days after the cabbage reaches the appropriate harvest stage, the head-splitting rate of cabbage will be investigated to measure the head-splitting property of cabbage. At first, the splitting time of the mature leaf heads is taken as the identification index [22]. Subsequently, the head-splitting rate of cabbage or head-splitting rate is used in combination with the head-splitting time. However, owing to the different research materials, there also exist differences in the time consumed for different scholars to determine the head-splitting rate. After identifying the head-splitting tolerance of 35 common head cabbage varieties in the alpine environment of northwest Hebei province in China, it was found the best time to identify the head-splitting tolerance was to harvest the leaves 15 days late after maturity in this area [23]. The head-splitting of nine cultivated varieties in spring (cabbage in spring) was studied, and it was found that the investigation time of splitting tolerance thereof was 10 days after the maturity of leaf heads [24]. Since there are differences in the location, degree (split size and depth), split speed and split properties of cabbage leaf heads, some scholars, with a view to better judge and measure, have introduced the number of split layers, split size [25] and others have used comprehensive indexes. For instance, Ma M. et al. [2] used the ratio of split arc length to the maximum circumference in the middle of leaf head + ratio of split width to head height + ratio of split depth to 1/2 head width as an index to comprehensively evaluate the degree of head-splitting of cabbage. Su Y.B. et al. [26] established the evaluation method and grading standard of splitting tolerance of cabbage in combination with the maximum number of split layers and split area.
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Journal: Horticulturae Authors: Wang et al
Background Common head cabbage (Brassica oleracea var. capitata L.), also known as cabbage, is one of the important cruciferous vegetables, which is characterized by outstanding adaptability and stress tolerance [1], high in nutritional value, rich in dietary fiber, vitamins, VB1, VB2, calcium, and iron, lutein, β-carotene and even cancer-preventing substances [2,3,4,5]. So, it was recommended by the World Health Organization as one of the best vegetables. Now it is extensively planted in all parts of the world [6,7]. The planting area and yield of cabbage in China are on the rise year by year, taking first place in the world [8,9]. Although there have been many studies on cabbage yield and quality, head-splitting is still a crucial problem in the later growth period of cabbage [10]. The head-splitting of cabbage is a physiopathy that decreases the appearance quality and commodity value, thus decreasing farmers’ income to a substantial extent; furthermore, a more significant problem is the fact that it is also the door to bacteria, fungi and pests, and increases the cost of storage and transportation, and even cause serious economic losses [11]. In the case of sufficient moisture, the head-splitting of cabbage is mainly determined by the genetic property of the variety [10]. Screening germplasm and breeding tolerance varieties are the fundamental ways to solve the problem [12]. It is necessary to clarify the current screening and identification methods, anatomic properties, and the molecular progress of research related to them. But because the molecular mechanism of cabbage head-splitting is less studied, it is also closely related to the moisture condition [13]. Therefore, this paper will also involve the discussion of external factors that induce the expression of related genes to provide ideas for the study of molecular mechanisms and breeding of head-splitting tolerance.
Introduction The head-splitting property of cabbage is the phenomenon of splitting of cabbage leaf head. In cultivation, it occurs mostly in the late stage of cabbage growth. When cabbages reach the heading stage, if they are not picked after the number, and weight of the heading leaves that make up the head reaches a certain level but continue to grow, it will cause the outermost heading leaves of cabbage leaf heads to split, which is called the head-splitting [14]. The top of the head is the common splitting part of the cabbage, and there are also lateral or ‘crossed’ dehiscences, which are mostly linear. At last, the inner tissues were exposed. The properties of its splits are transverse split, longitudinal split and mixed split [3,15]. The degree of splitting also gradually deepens, starting from the outermost leaves of the leaf head and gradually extending to the inner side. In severe cases, it can be split to the dwarf stem. The degree of splitting can be expressed by the length, width and depth of the split in the cabbage leaf head [10]. Although the physiological basis of splitting/cracking is not fully understood [16], there are main causes of head-splitting of cabbage, as follows. On the one hand, the cabbage head is prone to split when too much water is provided from outside during the late growth stage. It is easy to occur split in the waterlogged field after heavy rain or heavy irrigation. Especially when there is a sudden heavy rain or water flooding during drought, it is more likely to cause head-splitting. The main reason is due to the brittle tissue and poor cell flexibility; excessive water absorption leads to cell swelling and splitting. On the other hand, When the cabbage harvest is delayed, the number of cabbage with head-splitting will increase. Sale experience has demonstrated that cabbage varieties in the same maturity stage are often put on the market in a concentrated way, resulting in a drop in the price of cabbage. In an attempt to coordinate the market demand and increase farmers’ income, the measure often taken is to delay picking, which will raise the risk of head-splitting of cabbage during the storage period in the field [17]. Although the cabbage has reached harvest and marketing conditions, it still continues to grow in the soil until head-splitting occurs, and this period is called the storage period in the field (called the ‘Cunpu’ period in China) [18]. The storage period in the field reflects the tolerance to head-splitting of the cabbage variety, and the stronger the tolerance to head-splitting, the longer the storage period in the field [19]. During large-scale production of cabbage, if the storage period in the field is too long, the cabbage put on the market will split in the field because it cannot be sold in due course, resulting in the loss of its commercial value [20]. Planting cabbage varieties with head-splitting tolerance can directly prolong the harvesting time of cabbage so that farmers can selectively harvest cabbage within a suitable range of selling price, increase the output value of cabbage and reap higher economic benefits. These reasons above are closely related to the genetic characteristics of cabbage, in which different cabbage varieties showed different performances in head-splitting properties. Although the genetic base and molecular mechanism of cabbage are still poorly understood compared with other vegetable crops [21], collecting, evaluating and acquiring the germplasm resources of cabbage with head-splitting tolerance serves as the basis for cultivating varieties of cabbage with head-splitting tolerance. At present there is currently no uniform standard for evaluating the head-splitting property of cabbage. A few days after the cabbage reaches the appropriate harvest stage, the head-splitting rate of cabbage will be investigated to measure the head-splitting property of cabbage. At first, the splitting time of the mature leaf heads is taken as the identification index [22]. Subsequently, the head-splitting rate of cabbage or head-splitting rate is used in combination with the head-splitting time. However, owing to the different research materials, there also exist differences in the time consumed for different scholars to determine the head-splitting rate. After identifying the head-splitting tolerance of 35 common head cabbage varieties in the alpine environment of northwest Hebei province in China, it was found the best time to identify the head-splitting tolerance was to harvest the leaves 15 days late after maturity in this area [23]. The head-splitting of nine cultivated varieties in spring (cabbage in spring) was studied, and it was found that the investigation time of splitting tolerance thereof was 10 days after the maturity of leaf heads [24]. Since there are differences in the location, degree (split size and depth), split speed and split properties of cabbage leaf heads, some scholars, with a view to better judge and measure, have introduced the number of split layers, split size [25] and others have used comprehensive indexes. For instance, Ma M. et al. [2] used the ratio of split arc length to the maximum circumference in the middle of leaf head + ratio of split width to head height + ratio of split depth to 1/2 head width as an index to comprehensively evaluate the degree of head-splitting of cabbage. Su Y.B. et al. [26] established the evaluation method and grading standard of splitting tolerance of cabbage in combination with the maximum number of split layers and split area.
References