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
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
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
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
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
Fruit no. vs. seed wt./500 seeds
Seed wt./500 seeds vs. stem. wt.
Seed wt./500 seeds vs. seed no.
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.
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