ABSTRACT

The drought resistance of Eruca sativa cultivars in Northwestern China was evaluated through the field experiment, and the pot experiment, and pot-experiment-related physiological and biochemical analysis. The present study indicates that materials of stronger drought resistance had a smaller increase in peroxidase activity and proline accumulation, and they also had a stronger ability in regulating stoma opening under water stress. The data show that E.sativa cultivars originating from arid and semi-arid regions were generally belonged to drought-resistant genotypes, whereas, those from humid and sub-humid regions could generally be classified as drought-sensitive genotypes.

1.Introduction

Eruca sativa belongs to Cruciferae family. It is grown as an oilseed crop in many regions

Of the world, including China, India, Pakistan, France, Turkey, and Span. In China, E. sativa is a traditional oilseed crop in Shanxi, Hebei, Gansu, Inner Mongolia, Ningxia, Shaanxi. It can also be found in Qinghai, Xinjiang, and Tibet.

E. sativa is well known for its outstanding drought tolerance. As a result . it plays an important role in oilseed production in arid and semi-arid regions around the world such as

Northwestern China where drought is by far the most threatening factor limiting crop yield. E.sativa is a traditional crop in Northwestern China. During hundreds - perhaps even thousands - of years of cultivation, many E. sativa lines in Northwestern China have developed exceptional adaptation to the arid and semi-arid ecosystems. This paper presents the results of an evaluation on drought resistance of local peasants' E. sativa cultivars in Northwestern China.

2.Materials and Methods

2.1 Plant materials

Eighteen genotypes were evaluated. They were grouped into two ecological types based on their places of origin (the names of the county where the line was originally collected is indicated in the parenthesis and are used as the names of the corresponding cultivars):

I. Cultivars that originated in arid and semi-arid places: E1 (Jingning), E2 (Dingxi), E3(Jingyuan), E4 (Huining), E5 (Lintao), E6 (Yuzhong), E7 (Weiyuan), E8 (Huachi), E9 (Huanxian), E10 (QingYang), and E11 (Yongjing).

Ⅱ.Cultivars that originated in relatively humid areas, including E12 (Longxi), E13(Tianshui), E14 (Guanghe), E15 (Hezheng), E16 (Kangle), E17 (Minxian), and E18 (Zhuoni).

2.2 Experiments

Field experiment: Field experiments were carried out on the experiment farm of Huining Agricultural Research Institute, Huining County, Gansu, China. A randomized complete block design was used, with three replicates and 3m×1m plots. The management practice was similar to that in local E. sativa production. Phenological phases and the performance related to drought tolerance was recorded during the growing season. Ten plants were sampled from each plot for morphological characterization and yield evaluation.

Pot experiment: The experiment was conducted from March to July in 1998 inside a net house at Gansu Agricultural University in Lanzhou, China. Pottery pots of 25 cm in diameter and 35 cm in depth were used. Each pot was filled with 8.5 kg of top soil. The moisture content was 12.8% for an pots at planting time. After emergence, eight seedlings were kept in each pot and the extra seedlings were removed. All plants in the same pot were of the same line, and each line was replicated twice. There were two soil moisture treatments: the normal (control) and the semi-arid. The control soil contains 15 to 25% of moisture, and the semi-arid soil, 10 to 18%. At maturity, five plants were collected from each pot for morphological characterization and yield evaluation.

2.3 Laboratorv examination

From the bud stage to the flowering stage, five physiological and biochemical parameters were determined. The activity of peroxidase were measured by using the callus phenolic spectrophotometry^[1]. Proline content was determined by using the toluene-extract spectrophotometry^[2].The cell membrane permeability was determined by using electric conductivity method^[3]. To examine the stomata density and opening degree, leaf epidermis were sampled at 2 p.m. on a clear day, fixed in glacial acetic acid, and observed and photographed under micro scope^[3].

2.4 Data manipulation:

The drought-induced increase in peroxidase and proline is expressed in percentages:

Value Under Drought-Value Under normal Soil Moisture

Increase(%)= ——————————————————————×100

Value Under normal Soil Moisture

The cell membrane permeability is expressed as the relative permeability:

Electric Conductivity of Osmotic Solution of The Living Leaf (L₁)

Relative Permeability (%)= ———————————————————————

Electric Conductivity of Osmotic Solution of The Living Leaf (L₂)

Stomata densities and their opening dimensions are expressed as the actual values determined.

Drought resistance of a genotype is reflected by many characteristics of the plant. We have attempted to evaluate drought resistance by integrating all parameters tested. Since different characteristics are measured with different units at different scales, and the numeric values of a parameter may be positive or negatively correlated to drought resistance, it is impossible to compare these parameters directly. Therefore, the values of different parameters were standardized using the following formula:

∧ X_ij-X_jmin ∧ X_imax-X_ij

Y_ij=———— Y_ij=————

X_jmax-X_jmin X_jmax-X_jmin

where Xij is variable j of line i, Xjmax (Xjmin) is the maximum (minimum) value of variable j, Yij is the standardized value for Xij. Using the standardized values, the degree of the drought resistance of line i, known as the drought-resistance index, is expressed as

m Y_ij

Y_i=∑ —

j=1 m

A larger Yi value indicates a greater drought resistance.

3. Results and Discussions

3.1 Field evaluation

Huining was selected as the location for field evaluation of drought resistance because crops in this place encounters drought conditions in 9 out of 10 years. In 1998 severe drought occurred during E. sativa plants' budding, bolting, and the entire flowering period. The precipitation in June was only 9.9 mm, which was 15.6% of the average June precipitation in Huining. Therefore, the field performance of each line directly reflected its drought resistance. Among all lines tested, Huanxian, Huachi, Jingyuan, Dingxi and Lintao showed strongest drought resistance. During early stages of plant development, the above-gound parts of these materials grew very slow, but the roots grew very fast, forming a deep and wen developed root system which laid a good foundation for drought resistance at later stages. The results of field observation corresponded well to seed yield. Generally speaking, higher drought resistance resulted in higher seed yield. There were, however, exceptions. Hezheng, for example, demonstrated relatively weak drought resistance, and yet it had the higher seed yield. This line originated in a relatively humid area. The drought conditions in Huining had forced it to mature early whereby the plant partially escaped drought and produced high seed yield. Some of the materials that originated from arid areas virtually stopped growth after early flowering stage due to severe drought. Meanwhile, the 10-day continuous raining and cloudiness in the late growing season significantly reduced the seed yields of these materials. Some material, such as Yuzhong and Yongjing, demonstrated both good drought resi8ance and higher seed yields. Zhuoni and Minxian were among these materials that showed the weakest drought resistance. Comparing to "Longya 7" (the flax control), E. sativa was more resistant to drought. Twelve out of the 18 E.sativa lines tested had significantly higher yield than "Longya 7" (Table l ).

Table 1 The yield of oilseed in the experiment

Lines	Yield (kg/ha)	Compared with ck(%)	Lines	Yield (kg/ha)	Compared with ck(%)
E1	200.00	100.00	E9	180.00	90.00
E2	316.67	158.34	E10	270.00	135.00
E3	200.00	100.00	E11	350.00	175.00
E4	220.00	110.00	E12	220.00	110.00
E5	330.00	165.00	E13	225.00	112.50
E6	299.89	149.95	E14	270.00	135.00
E7	246.67	121.34	E15	550.00	175.00
E8	180.00	90.00	E16	190.00	95.00
Longya7	200.00	100.00	E17	200.00	100.00
(Flax as ck)			E18	280.00	140.00

3.2 Pot experiment

Changes in peroxidase activity: Water stress increased peroxidase activity in all lines tested. The increase ranged from 33. 8% to 497%. Generally speaking, drought causes disorder of the plants' enzyme systems. Under the water stress activities of peroxidases and other hydrolytic enzymes increase, inhibiting normal reductive synthesis^[4] Drought-resistant genotypes have less increase in peroxidase activity under drought, which is beneficial to maintaining normal metabolism in plants^[4]. E. sativa lines coming from arid and semi-arid regions, such as Jingning, Dingxi, Yuzhong, Lintao, had relatively high peroxidase activity under normal soil moisture conditions. Under water stress, the increase of peroxidase activity of these lines was much less than that of the control, i.e. "Longya 7" and "Ganjie 1 (Table 2). Water stress-induced increase in peroxidase of E. sativa was similar in quantity to flax ("Longya 7"), but significantly less than that of B. juncea ("Ganjie 1"). Other arid region-originated E. sativa lines, such as Yongjing and Jingyuan, also had relatively small increase in peroxidase activity. E. sativa lines coming from humid areas had dramatic increase in peroxidase activity under water stress, with the two maximum increases being 496% (Zhuoni) and 449% (Tianshui).

Proline accumulation: Water stress resulted in an increase in proline content in all materials tested (Table 2). E. sativa lines that had lower proline accumulation than the control "Longya 7" and "Ganjie l " under drought included Yuzhong, Weiyuan, Jingning, Dingxi, Jingyuan, Guanghe, Yongjing, and Lintao. On the other hand, Zhuoni, Minxian, and Kangle had very high proline content under drought. Physiologically, a increased proline content can make the .protoplasm more hydrophilic and reduce the damage caused by high ammonia accumulation^[4,5]. In this study, E. sativa lines from humid areas had higher proline accumulation under water stress than those from dry areas. This may be because the former was more susceptible to drought stress than the later, Here we observed a negative correlation between proline content and drought resistance.

Permeability of cell membranes: Data in Table 2 show that different lines differed greatly in their cell membrane permeability, which ranged from 5.2% to 84.2%. E. sativa lines that had significantly lower membrane permeability than the two control genotypes "Longya 7" and "Ganjie l " included Lintao (5.2%), Jingning (11.7%), Yuzhong ( 12.5%), Guanghe ( 17.5%), Weiyuan ( 18.4%), Jingyuan (19.1%), Dingxi (21.0%), and Yongjing (22.8%). This suggests that the cell membranes of these materials were more stable under water stress.

Stoma density and opening: Number of stomata per unit area was related to plant species (Table 2). Flax ("Longya 7") had a low stoma density of 91/mm². All E. sativa lines had higher stoma density than flax. Among E. sativa lines, Zhuoni had the highest stoma density (286/mm²) whereas Huachi had the lowest ( 122/mm²). The general trend was that materials from arid areas had lower stoma density and those from humid areas had higher stoma density. Similar trend also existed with stoma opening. The degrees of stoma opening of Yuzhong, Jingning,Jingyuan, and Yongjing were relatively small at 2 p.m. when it was dry and hot, indicating a greater ability in regulating stoma opening. Under drought and heat stress, these lines may be able to effectively reduce water loss by lessening stoma opening, by which the plants become more resistant to drought. Stomata of Zhuoni, Kangle, QingYang, Weiyuan, and Minxian were less sensitive to Drought and heat. Under the high temperature and low humidity conditions at 2 p.m., these materials could not effectively close their stomata. This suggests that these lines can not effectively reduce transpiration under drought, making them less resistant to drought.

Seed yield: Seed yield is still the ultimate and most reliable indicator for evaluating the overall drought resistance of a crop variety^[5]. Breeders have found that genotypes of different yields tend to have consistent performance under different environment^[5]. Generally those that have high yields under normal conditions also have high yields under drought. From drought-resistance point of view, some traditional varieties have the smallest yield reduction under water stress comparing to yield under normal conditions. That is, drought-resistant local varieties have high stability in their seed yield production. Our study revealed the same trend. The materials tested differed greatly in their seed yield and biomass per plant. The yield reduction rate of a genotype under drought in reference to its yield under normal soil moisture is a good parameter reflecting its drought resistance. Cultivars of low yield reduction rates included Jingyuan ( 14.3%), "Longya 7" (21.1%), Yongjing (21.4%), "Ganjie 1" (25.0%) Guanghe (27.3%), Dingxi (27.3%), and Lintao (28.6%). E. sativa lines that had lower biomass reduction rates than "Longya 7" (23.9%) included Jingyuan (16.7%), Yuzhong (19.3%), Jingning (2l. l%), Yongjing (2l.3%), and Lintao (22.4%). Low reduction in seed yield and biomass under water stress suggests that these varieties had strong torlerance to drought.

Drought-resistance index: Of all parameters determined, stoma density had an uncertain relationship with drought resistance. It was excluded when the integrated parameter, i.e. drought-resistance index, was developed. Materials that had higher index values than "Longya 7" included Jingyuan, Yuzhong, Jingning, Yongjing, and Lintao. The remaining E. sativa lines had higher index values than the other control "Ganjie 1 " except Huanxian, QingYang, and Tianshui.Zhuoni, Minxian, and Longxi were also among the varieties that had weak drought resistance (Table 3).

Table 3 Drought-resistance index of the lines tested

Lines	IRP	IROP	PCM	SO	SYRR	BYRR	DRI
E1	1.00	0.98	0.92	0.84	0.47	0.88	0.85
E2	0.91	0.98	0.80	0.62	0.64	0.57	0.68
E3	0.68	0.98	0.82	0.74	1.00	1.00	0.87
E4
E5	0.89	0.87	1.00	0.53	0.60	0.84	0.80
E6	0.84	1.00	0.91	0.85	0.56	0.93	0.85
E7	0.64	0.98	0.83	0.46	0.00	0.80	0.62
E8	0.63	0.94	0.73	0.20	0.38	0.59	0.58
E9	0.33	0.94	0.22	0.21	0.28	0.15	0.36
E10	0.56	0.84	0.27	0.32	0.00	0.00	0.33
E11	0.73	0.95	0.78	0.71	0.80	0.87	0.81
E12	0.58	0.55	0.80	0.00	0.47	0.37	0.46
E13	0.10	0.56	0.65	0.47	0.56	0.61	0.41
E14	0.65	0.97	0.84	0.72	0.64	0.37	0.70
E15	0.58	0.95	0.73	0.69	0.35	0.58	0.65
E16	0.50	0.88	0.75	0.52	0.32	0.65	0.53
E17	0.40	0.87	0.71	0.48	0.11	0.44	0.50
E18	0.00	0.81	0.76	0.18	0.38	0.52	0.43
Ganjie1	0.70	0.00	0.12	0.46	0.70	0.66	0.44
Longya7	0.84	0.95	0.00	1.00	0.81	0.80	0.73

SO=Stoma opening SYRR=Seed yield reduction rate BYRR=Biomass yield reduction rate

DR=Drought-resistance index PCM=Increase rate of peroxidase IROP=Increase rate of proline

4.Summary

The present study indicates that E. sativa cultiva`rs in Northwestern China, are rich in drought-resistant genotypes. Some drought-resistant E. sativa cultivars were identified through the field evaluation under a natural drought-prevailing environment, and the pot experiment, and the pot-experiment-related physiological and biochemical analyses. Materials of strong drought resistance included Jingyuan, Yuzhong, Jingning, Yongjing, and Lintao. These lines have stronger drought resistance than both flax and B. juncea.

Materials of among drought resistance had a smaller increase in peroxidase activity and proline accumulation under water stress. Therefore the metabolic system inside the plants of these cultivars were more stable. Damages on cell membranes caused by the drought were smaller in these genotypes, which helps the plant cells to carry out normal metabolic processes. They also had a stronger ability in regulating stoma opening so that stoma opening was smaller under drought or dry, hot conditions, thus reducing transpiration, maintaining a proper water balance inside the plants, and becoming more resistant to drought.

Our data show that E. sativa cultivars originating from arid and semi-arid regions were generally belonged to drought-resistant genotypes, whereas those form humid and sub-humid regions could generally be classified as drought sensitive genotypes. This suggests that E. Sativa has enriched its genotypes and evolved into many different classes of varieties over a long history of cultivation and natural selection under diverse conditions in Northwestern China. E. sativa in Northwestern China contains many valuable genetic resources.

Presented here is the experimental result in 1998 only, and lack of certain equipment has prevent us from measuring a number of other drought-tolerance-related characteristics. Further investigations are needed on drought resistance of E. sativa.

Acknowledgement

The authors thank Dr. Pan Qiyuan, Zhang Renzhi and Niu Junyi for their technical assistance.

Reference

1. 1. Jin Jiaha, Zhong Rong, Analysis methods of plant biochemistry, Science Press, Beijin, 198l: 197-201

2. 2 Northwest Agricultural University, Experimental guides for basis biochemistry, Shannxi science and technology press, Xian, Shannxi, 1985: 55-56

3. 3 Northwest Agricultural University, Experimental guides for plant physiology, Shannxi science and technology press,Xian,Shanxi, I 987: 149-150

4. 4 Liu Zuqi, Zhang Shicheng, Plant resistance-physiology, China agricultural press, Beijin, 1990:84-159

5. 5 Zhao Kefu,Wang Zhaojun.Crops resistance-physiology, China agricultural press, Beijin,

6. 1990: 145-225