Heredity Analysis of Photosynthetic Rate and Chilling Tolerance of Cucumber Seedlings Under Low Temperature

Cucurbit Genetics Cooperative Report 18:23-25 (article 11) 1995

Janguo Li, Hongwen Cui and Meng Zhang
Horticulture Department, Northwestern Agricultural University, Yangling, Shaanxi, 712100, P.R. China

Increased attention is now being given to cold resistance in cucumber (Cucumis sativus L.). Cold resistance in cucumber includes two aspects: low temperature tolerance (10-15C) and chilling tolerance (0-5C). Although considerable attention has been focused on chilling tolerance, the reports on low temperature tolerance of cucumber are sparse.

Low temperature affects photosynthesis (1). Varieties whose net photosynthetic rates under low temperature are higher can grow better than sensitive varieties under low temperature. Thus, net photosynthetic rte (PR) under low temperature can bemused as an index of low temperature tolerance in cucumber. The chilling index (CI) indicates the extent of plant injury incurred after chilling. Thus, CI can provide an indication of chilling tolerance in plants. This report examines PR and CI as potential selection indices for developing chilling tolerance in cucumber. Six cucumber inbreds were used in a 3×3 incomplete diallel crossing scheme. The PR and CI of nine hybrids were analyzed to estimate genetic parameters.

Materials and Methods. Six cucumber inbreds were chosen as parents based on differences in of their chilling sensitivity. The chilling susceptible female parents were: ‘Jin-4’ (No. 1), ‘Jin-6’ (No. 2), and 3511 (No. 3). The chilling tolerance male parents were: ‘Ping li’ (No. 4), “Erzhaozi’ (No. 5), and ‘Xixiabai’ (No. 6). The 3×3 incomplete diallel crosses produced 9 hybrids.

Seeds were sown in plastic pots filled with manure and soil (manure:soil = 1:1). The diameter of the pots was 10 cm and the height was 9 cm. Seedlings in each pot were thinned to two per pot after the seedlings emerged. Plants in both experimental chambers were arranged in a completely randomized block design with 3 replications. Twenty days later, at the third leaf stage, uniform seedlings were chosen and moved to two control environments. The temperature in one chamber (N) was normal (25/15C, day/night for 5 days and 3C for 1 day). In another chamber (L), the temperature was lower (20/10C for 5 days, and 3C for 1 day)_. All other conditions in the chambers were similar (intensity of illumination was 33.78 w/m2 , 10 h per day; RH = 80=90%). The PR of the second leaf was estimated on the fifth day using an LI-6200 photosynthesis analysis system.

When the 3C treatment ended, the temperature in both chambers was returned to ambient temperature. Three days later, the injury of seedlings was recorded. Referring to the methods of Wang (1985) and Semeniuk (1986), the ranks if injury were divided as follows:

0 – no visible injury

1 – slight injury in edge of leaf

3 – visible injury in leaf, no visible injury in apical point

5 – slight injury in apical point or plant withered

7 – dead

Chilling index was calc8ulated using the following formula:

(sigma) r x n

CI =

rmax x N

Where r = rank of injury, n = number of plants, rmax = the largest rank, and N = number of total plants investigated.

An analysis of variance was performed using treatment means, and estimates of variance components, and broad (B) and narrow (N) sense heritability estimates were made.

Results. At variance analysis showed that difference in PR and CI exist among hybrids at both temperatures (N:PR, F = 2.722*/CL, F = 5.406** L:PR, F = 6.839**/CL, F = 5.344**). Further analysis showed that, PR under normal temperature was significantly higher than under low temperature. CI under low temperature was significantly smaller than under normal temperature. Under low temperature, the covariance between PR and CI was significant (Cov (PR,CI) = 11.7**). This indicates that highly chilling tolerant varieties have lower photosynthetic rates under low temperatures when compared to chilling sensitive hybrids.

Variance analysis of combining ability reveals that the general combining ability of female parents for PR and CI under low temperature was significant (ά = 0.05). The effect of specific combining ability was not significant.

The hybrids with smaller CI were 1×6, 2×4, 3×5, (Table 1), and F1’s with higher PR values were the higher PR F1 were 1×6, 3×4, and 2×5. The values of general combining ability for CI of No. 3, No.5 and No.6 were negative, and the variance of special combining ability of No.3 and No.5 was comparatively large. Therefore, it could be predicted that more chilling tolerant progeny would be produced when using No. 3, No. 5 and No. 6 as parental stock. Progeny with higher PR generations would likely be produced when using No. 1, No. 3, and No. 4 as parents, because of their relatively high general combining ability values and the large specific combining ability recorded for PR under low temperature.

Estimates of genetic parameters are given in Table 2. Under low temperature (20.10C), the values of heritability of PR and CI were relatively high and genetic variances were conditioned by additive gene action. The heritability of CI under low temperature was higher than that under normal temperature (h2B-56.49%). Data suggest that, if there had been no cold acclimation, the genetic potential of plants could not have been fully expressed. The selection of chilling tolerant varieties is likely to be very difficult because of the difficulty of distinguishing genotypes based on their phenotypes.

Discussion. cold resistance is a trait in which genes are induced and expressed. These genes are only induced by certain environmental conditions (i.e. low temperature), and only after induction can the cold resistant genes be expressed. In this study, the chilling tolerance ability of cucumber was increased significantly after 5 days low temperature treatment (20/10C).

A study of cold resistance in tomato (3) showed that the effect of general combining ability, and its expression was mainly attributable to additive gene action. A study by Wehner (1984) showed that the heritability of cucumber germination percentage and germination speed under low temperature was high and was mainly controlled by additive genetic factors. The results of our study confirms the work of Wehner (4). Improvement of cold resistance in cucumber may be possible in the future. However, selection must be done in cross progeny which carefully induced by low temperature.

There is a negative covariance (correlation) between chilling tolerance and low-temperature tolerance in cucumber. In order to select cold-resistant cucumber varieties which are both tolerant to chilling and low temperature, the negative relationship among these traits must be broken. In this experiment, the hybrid 1×6 did not have acceptable commercial quality, but it endured exposure to low temperature and chilling. It will be difficult to select a chilling and low temperature tolerance cucumber variety. Nevertheless, we believe that the time and expense to do so is warranted.

Table 1. Combining ability estimates for response to low temperature in cucumber (Cucumis sativus L.)

	INBRED LINE  No. 4		INBRED LINE No. 5		INBRED LINE No. 6		INBRED LINE g1
Inbred line	PR	CI	PR	CI	PR	CI	PR	CI
No. 1	-0.21	-0.30	-0.06	1.85	0.28	-1.55	0.78 0.008*	3.66 1.145*
No. 2	-0.23	-1.90	0.21	-0.01	0.02	1.10	-0.17 -0.005*	0.09 -0.614*
No. 3	0.45	1.39	-0.15	-1.84	-0.30	0.45	-0.08 -0.103*	-3.75 0.947
g?	0.34 0.095*	2.11 -0.208*	-0.07 -0.019*	-0.98 1.591*	-0.26 0.030*	-1.13 0.095*

*Indicates specific combining ability estimates.

Table 2. Genetic estimates of low temperature response in cucumber (Cucumis sativus L.).

Character	-a	-b	-ab	-c	h² B(%)	h² N(%)
PR	0.514	0.049	0.052	0.244	71.62	65.53
CI	12.575	2.198	0.731	8.157	65.52	62.44

Literature Cited

1. He Jie, et al. 1986. Low temperature and photosynthesis of plants. Plant Phys. Comm. 2:1-6.
2. Liu Hogxian, et al. 1991. Alternation of cold-induced gene expression and cold tolerance in plants. Acta Botanica Austro Sinica. 7:54-61.
3. Van de Dijk, et al. 1985. Genotypes variation in chilling-induced leakage of electrolytes leaf tissue of tomato in relation to growth under low energy conditions. J. Plant Physiol. 120:39-42.
4. Wehner, T.C. 1984. Estimates of heritabilities and variance components for low-temperature germination ability in cucumber. J. Amer. Soc. Hort. Sci. 109:664-667.