Cucurbit Genetics Cooperative Report 21:11-13 (article 4) 1998
Jianguo Li
Tropical Horticulture Institute, Chinese Academy of Tropical Agriculture Science, Danzhou, Hainan, 571737, P.R. China
Hongwen Cui and Meng Zhang
Horticultural Department, Northwestern Agricultural University, Yangling, Shaanxi, 712100, P.R. China
Introduction. It is very important to improve chilling tolerance in cucumber (Cucumis sativus L.). In China, chilling tolerant varieties will not only decrease the production cost of winter-grown greenhouse cucumbers and raise their competitiveness in the market, but also save energy and reduce air-pollution (1). One of the main difficulties during cold-tolerant breeding is that selection requires large plant populations for screening. This is often expensive. In addition, the accuracy of selection is lowered when large populations are used. Thus, breeders have been searching for a convenient and cost-effective indirect selection method to identify cold tolerant cucumber varieties. In this paper, the relationship between low-temperature germination and chilling tolerance of cucumber seedlings is reported, and the possibility of indirect selection in cucumber chilling tolerance is discussed.
Materials and Methods. Fifteen cucumber lines with differing chilling sensitivity were used. The seeds of all lines were produced at the Vegetable Experiment Station of Northwestern Agricultural University in July 1993.
I. Germination test. Seed germination research was conducted in growth chambers with one chamber set at 15 C and another at 25 C (both without light). The experimental design was a randomized complete block with three replications (Test I). Seeds of each line were tested in 100 mm diameter petri plates into which tow pieces of filter paper, 10 ml of distilled water, and 100 seeds were added. Three replications of 100 seeds each were used for testing in each of the two test temperatures.
After the second day of experiment initiation, the number of germinating seeds were counted daily. Seeds were considered germinated when a radicle reached half of the seed in length. The end of germination was taken when seeds no longer germinated. The mean germination days (MGD) and germination index (GI) were calculated as follows:
MGD = ∑ (Gt * Dt ) / ∑ Gt
GI = ∑ (Gt / Dt)
Where Gt = number of seed germinating at time t, and Dt = corresponding days of germination.
II. Test of seedling cold tolerance. The germinated seeds in Test I were sown in plastic pots filled with manure and soil (manure:soil = 1:1: v:v). Seedlings at the third-leaf stage were moved to the two controlled environment (chambers). Plants in both experimental chambers were arranged in a completely randomized block design with three replications, with 12 plants per replication. In order to allow for seedling acclimation to the chamber environment, both chambers were kept in 25 c /15 C (day/night temperature) for 24 hour before the testing commenced. The temperature in one chamber was normal (25 C/15 C for 5 days, and 3 C for 1 day). In another chamber, the temperature was lower (20 /10 C for 5 days, and 3 C for 1 day). All other conditions in both chambers were similar (irradiance was 33.7 w/m, 10 hours per day. RH was between 80 to 90%).
When the three treatments ended, temperature in both chambers were returned to 25 C/15 C. Two days later, chilling injury of seedlings was investigated. Injury rankings (data not shown) were according to Li (3). The formula of chilling index (CI) was as follows:
CI = ∑ (r*n) / rmax* N)
where r = rank of injury, n = number of plants, rmax = the largest rank, and N = number of total plants examined.
Results. A variance analysis showed that no differences in MGD and GI existed at 25C, but differences in MGD (X1) and GI (X2) at 15 C were detected (C1:F + 4.59**; X2:F = 6.71**). This indicated that there was no difference in germinating ability among the various cucumber lines at 25 C, but that significant differences in low-temperature germination among cucumber lines existed at 15 C. The variances of chilling index at 20 C/10 C (X3) and that at 25 C /15 C (X4) were both significant (X3:F = 10.29*; X4:F = 8/84**) Data show that differences in chilling tolerance existed among the 15 cucumber lines tested. The results of genetic correlation analysis are presented in tables 1-3.
Table 1. Phenotypic correlation matrix.
X1 |
1.00000 | |||
X2 |
-0.635** | 1.000 | ||
X3 |
-0.652** | -0.184 | 1.000 | |
X4 |
0.480** | -0.043 | 0.729** | 1.000 |
X1 |
X2 |
X3 |
X4 |
|
Table 2. Genetic correlation matrix.
X1 |
1.000 | |||
X2 |
-0.738** | 1.000 | ||
X3 |
0.883** | -0.288 | 1.000 | |
X4 |
0.285 | -0.092 | 0.361 | 1.000 |
X1 |
X2 |
X3 |
X4 |
|
Table 3. Environmental correlation matrix
X1 |
1.000 | |||
X2 |
0.066 | 1.000 | ||
X3 |
-0.335 | 0.299 | 1.000 | |
X4 |
0.091 | 0.168 | 0.355 | 1.000 |
X1 |
X2 |
X3 |
X4 |
|
The phenotypic correlation matrix (Table 1) showed that strong correlations existed among 15 mean germinating days (X1), 20/10 CI (X3), and 25/15 CI (X4). The correlations between 15 MGD (X1), and 15 GI (X2), and between 20/10 CI (X3) and 25/15 CI (X4) were significant. The correlations between 15 GI (X2) and 20 C/10 C CI (X3) or 25/15 CI (X4) were, however, not significant.
The genetic correlation matrix showed that genetic correlations existed between 15 MGD (X1) and 15 CGI (X2) or 20/10 CI (X3). The genetic correlation coefficients between 25 C/15 CI (X4) and 15 MGD (X1) or 20/10 CI (X3), however, were not significant. Because the degree of chilling injury of cucumber seedlings under low temperature stress can be measured by CI, CI can be used to determine the chilling tolerance of cucumber seedlings. Accordingly, CI with lower values indicate better chilling tolerance. It is evident that although chilling tolerance of cucumber seedlings growing under 20 C/10 C correlated with that of seedlings grown under 25 C/15 C, they did not correlate with each other. The same situation also existed between X1 and X4.
Discussion. =Although it would be important to have the ability to select for chilling tolerance of cucumber indirectly, the present study does not completely clarify such selection. The research at the former IVT (now CPRO) showed that the proportion of leaf area of cold-tolerant cucumber lines was higher than that of controls (cold-sensitive lines) under 20 C/10 C, so t is possible to increase the ability of cold-tolerance through selecting fast-growing plants in seedlings stage under low temperature (2). The results of another study in former Soviet Union showed that cold-tolerance in cucumber could be increased significantly during selection by using dry matter content and leaf-area after chilling treatment as selection parameters. In this study, no difference in germination ability among 15 cucumber lines at 25 C existed, but significant differences in germination ability were detected at 15 C. Genetic correlation analysis indicated that MGD at 15 C positively correlated with CI at 20/10 C. This suggests that the slower cucumber seeds germinate under low temperature (15 C), the more severe cucumber seedlings injured by chilling. With regards to the genetic correlation, the association between CI at 20 C/10 C and CI at 25 C/15 C was not significant. A possible reason for this observation was that chilling acclimation at 25 C/15C was ineffective, and thus the genetic potential of chilling tolerance of cucumber seedlings was not expressed.
The initial results of this study were: 1) that 15 C could be used to distinguish the low-temperature germinating ability of cucumber lines, 2) the mean germination days of cucumber and the cold tolerance of seedlings was negatively correlated, and; 3) it is possible to screen the cold-tolerant cucumber germplasms and to select cold-tolerance of cucumber lines indirectly by testing their low-temperature germinating ability at 15 C. These results, however, require further investigation.
Literature Cited
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