Cucurbit Genetics Cooperative Report 12:11-14 (article 5) 1989

Source-Sink Relationships in Cucumber

Jack E. Staub

USDA, ARS, Department of Horticulture, University of Wisconsin, Madison, WI 53706

Average yield of cucumber (Cucumis sativus var. sativus L) in the United States has increased from approximately 65 (1920) to 200 (1980) bushels per acre (1). Much of that yield improvement was the result of improved cultural practices, gynoecious sex expression, and disease resistance ( 5,6). Knowledge of plant physiology will help in the direct improvement of yield. A fruit developing from the first pollinated flower on the cucumber plant inhibits the development of subsequent fruits. It is not known whether this inhibition is due to a substance which is translocated from the fruit, or to a substrate-limited source-sink relationship (2,4,7).

Unlike var. sativus, Cucumis sativus var. hardwickii (R.) Alef. possesses a sequential fruiting habit (3), and therefore has potential for increasing fruit yield in cucumber (9). Inbred lines derived from var. sativus x var. hardwickii matings have been developed in my program (10). Although the fruit quality of these lines is commercially unacceptable (11), their fruit yielding abilities are significantly higher than standard cultivars (10).

In order to gain more information concerning the fruit setting nature of var. hardwickii, an experiment was designed to compare the morphological and photosynthetic characteristics of a standard var. sativus inbred (WI 1606), a var. hardwickii accession (PI 215589), and a var. sativus x var. hardwickii derived inbred (WI 5551). It was thought that these comparisons would provide information concerning the role of source-sink relationships in cucumber.

Seeds of WI 1606, WI 5551, and PI 215589 were planted in 10 replications ( single plant), each equidistantly spaced 2.7 m apart (center to center) in a randomized complete block design. Fruit, seed, and plant (stem + leaf) dry weight, as well as fruit and seed number per plant were recorded at maturity (100 days after sowing). Harvested tissues were dried at 60°C for 7 days. The net CO2 assimilation rate of the 4th (leaf #1) from the terminal whorl was recorded 3 weeks after sowing on cloudless days using an LI 6000 portable gas analysis system (Li-Cor, Inc., Lincoln, Nebraska). Photosynthetic rates of the 4th and 6th (leaf #2) leaves were measured at 5 and 6 weeks, while rates of the 4th, 6th, 8th (leaf #3) and 10th leaf #4) leaves were measured 7, 8, 9 and 10 weeks after sowing. The LI 6000 consists of a battery powered nondispersive infra-red gas analyzer, a porometer, a communications divide, and a dedicated datalogger. When a leaf is placed into the monitoring chamber, CO2 concentration decreases as CO2 assimilation occurs. Net carbon assimilation rate is calculated based on leaf area, changes in CO2 concentration and air flow rate.

Stem weight per plant as well as fruit number per plant was significantly higher in PI 215589 when compared to the other inbred lines (Fig. 1). However, the seed number and weight per fruit of PI 215589 was significantly lower than for WI 1606. The means of WI 5551 for most characters were intermediate (seed weight per 500 seeds) to the parents, or closer to WI 1606 (stem and fruit weight, fruit number) than to PI 215589. There were no significant differences observed in the mean net CO2 assimilation rate (AR) among leaves or between inbred lines during the growing season. Mean AR fell dramatically in all lines when flowering (weeks 7 to 8) and fruit development began, but the magnitude of this decrease was similar in all three lines. Although this decrease may be associated with lower irradiance in weeks 9 to 10 (1017±431 mmols/m²/s) when compared to weeks 3 to 8 (1569±281 mmol/m²/s), irradiance was greater than light saturation (300-500 mmols/m²/s) for cucumber.

A significantly higher proportion of photosynthate was translocated to the fruit in WI 1606 when compared to the other lines (Table 1). In contrast, the percent of dry weight of leaf and stem tissue was higher, in PI 215589 (9 and 38% respectively) when compared to WI 1606. While the portion of assimilates in the leaf and stem in WI 5551 was similar to that of WI 1606, contribution to fruit development was 10% lower. PI 215589 typically flowers 2 weeks later than the other lines in days to anthesis (approx. 51 days in Wisconsin). The effect of this difference in maturity date on assimilate partitioning was minimized by delaying the harvest 100 days after sowing.

Consistent differences in the direction (+ or -) of phenotypic corrections in traits between lines may indicate dissimilarities in their physiologic nature. Different significant correlations in directions between lines were observed for fruit number and weight/500 seeds, weight/500 seeds and stem weight, and see weight/500 seeds and seed number (Table 2). Negative correlations in fruit number and weight/500 seeds and seed number were negatively correlated in PI 215589 and positively so in WI 1606.

These calculated associations along with observed differences in carbohydrate partitioning between lines suggest that they are physiologically different. It appears that PI 215589 has the ability to set large number of fruits containing small but numerous seeds. On the other hand, WI 1606 does not. Although AR among inbred lines is similar, PI 215589 partitions more of its photosynthate to leaves and stems when compared to the other inbred lines examined, suggesting that sinks and/or their strengths are dissimilar. A similar finding was reported by Ramirez and Wehner (8). The fact that WI 5551 is higher yielding than WI 1606, but partitions significantly more assimilates to seeds than to fruit suggest that: i) Seeds may be a significant sink; and ii) Seed maturation may be related to the observed reductions in fruit size. One could hypothesize that selection for fewer seeds per fruit in populations having high fruit number per plant may result in derived inbreds partitioning more assimilates to the mesocarp of the fruit, thereby resulting in larger length/diameter ratios.

Table 1. Dry weight percentage of plant tissue of a C. sativus var. sativus (WI 1606), a C. sativus var hardwickii ( PI 215589) and a derived var. sativus x var. hardwickii ( WI 5551) inbred line grown at Hancock, WI.^z

	Proportion of plant by weight (%)y
Plant part	WI 1606	WI 5551	PI 215589
Fruit	50a	40b	14c
Leaf	22b	22b	31a
Stem	16c	18b	54a
Seed	12d	20a	1c

^z Different letters within a row indicate that mean percent values are significantly different (5%) using LSD test.
^y WI 1606 = C. sativus var. sativus inbred; PI 215589 = C. sativus var hardwickii; WI 5551 = var. sativus x var. hardwickii derived inbred.

Table 2. Phenotypic correlations between dry weights of tissue of a C. sativus var. sativus (WI 1606), a C. sativus var. hardwickii (PI 215589) and a derived var. sativus x var. hardwickii (WI 5551) inbred line grown at Hancock.

	Inbred line or accession z
Parameters correlated	WI 1606	WI 5551	PI 215589
Fruit no. vs. seed wt./500 seeds	-0.56*	0.33	0.67*
Seed wt./500 seeds vs. stem. wt.	0.62*	-0.01	0.83**
Seed wt./500 seeds vs. seed no.	0.60*	0.38	-0.63*

^z WI 1606=C. sativus var. sativus inbred; PI 215589=C. sativus var. hardwickii; WI 5551=var. sativus x var. hardwickii derived inbred.
*, ** Indicates that correlation coefficients are significant at 5 and 0.1%, respectively.

Literature Cited

1. Agricultural Statistics. 1940, 1980. United States Departments of Agriculture. United States Government Printing Office, Washington D.C.
2. Fuller, G.L and C.A. Leopold. 1977. The rose of nucleic acid synthesis in cucumber fruit set. J. Amer. Soc. Hort. Sci. 102: 384-388.
3. Horst, E.K. and R.L. Lower. 1978. Cucumis hardwickii: A source of germplasm for the cucumber breeder. Cucurbit Genet. Coop. Rpt. 1:5.
4. Nienhuis, J. and R.L Lower. 1980. Influence of reciprocal donor scions on fruit setting characteristics of recipient scions of Cucumis sativus and C. hardwickii (R.) Alef. Cucurbit Genet. Coop. Rpt. 3; 17-19.
5. Peterson, C.E. and D.J. DeZeeuw. 1963. The hybrid pickling cucumber, Spartan Dawn. Mich. Agr. Expt. Stat. Quart. Bul. 46: 267-273.
6. Peterson, C.E. 1975. Plant introductions in the improvement of vegetable cultivars. HortScience 10: 575-579.
7. Pharr, D.M., S.C. Huber and H.N. Sox. 1984. Leaf carbohydrates status and enzymes of translocate synthesis in fruiting and vegetative plants of Cucumis sativus var. hardwickii (R.) Kitamura. Cucurbit Genet Coop. Rpt. 11: 25-28.
8. Ramirez, D.R. and T.C. Wehner. 1984. Growth analysis of three cucumber lines differing in plant habit and yield. Cucurbit Genet. Coop. Rpt. 11: 25-28.
9. Smith, O.S., R.L Lower and R.H. Moll. 1978. Estimates of heritabilities and variance components in pickling cucumber. J. Amer. Soc. Hort. Sci. 103: 222-225.
10. Staub, J.E. 1985. Preliminary yield evaluation of inbred lines derived from Cucumis sativus var. hardwickii (R.) Kitamura. Cucurbit Genet. Coop. Rpt. 8: 18-21.
11. Staub, J.E. and L.R. Fredrick. 1988. Evaluation of fruit quality in Cucumis sativus var. hardwickii (R.) Alef.-derived lines. Cucurbit Genet. Coop. Rpt. 11: 25-28.