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Cucurbit Genetics Cooperative Report 6:94-95 (article 48) 1983

Mentor Pollen in an Interspecific Cross in Cucumis: Effects of Irradiation Dose and of Order of Application of the Two Types of Pollen

J. B. M. Custers and A. P. M. den Nijs

Institute for Horticultural Plant Breeding, P. O. Box 16, 6700 AA Wageningen, The Netherlands

In recent years we regularly used irradiated mentor pollen (IMP) as a pollination aid in interspecific hybridization in Cucumis. When the control cross fully failed, the IMP-technique resulted in a high percentage of fruit set, but the number of hybrid embryos in those fruits was generally low (2, 3). Under conditions permitting regular fruit set and embryo development, the IMP-technique still increased fruit set, but strongly decreased the number of embryos (1). In Populus the low hybrid seed yield in IMP-aided crosses was interpreted as due to competition of the IMP with the fresh foreign pollen for the available ovules (4). This could also be the reason for our low hybrid embryo yield. We routinely used mentor pollen irradiated with 1 kGy (100 krad), and we always applied the IMP first and the foreign pollen directly thereafter. In an effort to decrease the competitive fertilizing ability of the IMP, we compared mentor pollen irradiated with different doses and we studies the effect of the order in which IMP and paternal pollen were applied.

The cross Cucumis metuliferus Naud. (Gene bank no. (Gbn) 0164) x C. zeyheri 2x Sond., a selected clone of Gbn 0181 (5) was chosen for the experiment. The same cross was used before in studies on the efficacy of pollination aids (1, 2). Plants were grown in 25 1 plastic containers with Trio peat soil in an insect-proof glasshouse in the summer of 1982. Irradiation of the pollen was conducted as described earlier (3) and this pollen was used within 3 hrs after irradiation. All pollinations were made during the period from June 22 till July 7. All treatments (see Table 1) were carried out on two dates, except those with 3 kGy pollen. Fruits were weighed and dissected to check for ovules with embryos, starting three months after harvest.

Table 1. Effects of irradiation does of mentor pollen and of the order of application of the two types of pollen in the cross Cucumis metuliferus Naud. (M) x C. zeyheri 2x Sond. (Z).


Treatment

No. of pollinations

No. of fruit set

Average fruit weight (g)

Percentage of fruits with embryos

No. of embryos in the fruits containing embryos


M x Z

10

 0

-

-

-

 

 

 

 

 

 

M x 0 kGy M

 3

 3

147

100

111, 335, 370

M x 1 kGy M

13

12

 86

  0

-

M x 2 kGy M

 9

 9

 70

  0

-

M x 3 kGy M

 5

 5

 46

  0

-

 

 

 

 

 

 

M x (0 kGy M + Z)

 3

 3

146

100

120, 325, 471

M x (1 kGy M + Z)

10

10

 99

 20

2, 2

M x (2 kGy M + Z)

 9

 7

 82

 71

1, 1, 1, 2, 19

M x (3 kGy M + Z)

 5

 4

 41

100

1, 1, 3, 8

 

 

 

 

 

 

M x (Z + 0 kGy M)

 6

 5

137

100

110, 216, 225, 299, 315

M x (Z + 1 kGy M)

11

11

 95

 36

1, 3, 3, 9

M x (Z + 2 kGy M)

10

 9

 76

 67

1, 4, 5, 8, 10, 17

M x (Z + 3 kGy M)

 5

 3

 33

 67

5, 30


The results are in Table 1. The failure of fruit to set in the control cross likely was caused by bright and warm weather during the pollination period. Earlier we found successful fruit set only under cool weather conditions (1). The average fruit weight decreased with increasing irradiation dose of the IMP. The same trend was observed for the number of developed ovules (260, 100, 70, and 40 ovules per fruit at 0, 1, 2, and 3 kGy, respectively). All fruits of the control self-pollination contained embryos, but no embryos developed after pollination with IMP only., With double pollinations, all fruits contained many embryos when fresh self pollen was used. All these embryos likely originated from selfing. The percentage of fruits with embryos was very low after double pollinations with 1 kGy IMP, but increased with higher irradiation dose. All these fruits contained few embryos, which should all be hybrids according to our earlier experience (2). The number of embryos after double pollination with IMP increased slightly with higher irradiation dose. Contrary to expected, the higher irradiation dose did not seem to decrease the competitive ability of the IMP. However, in view of the reduced fruit weight and number of ovules at higher irradiation dose, we believe that the competitive ability had in fact decreased but the effect of this decrease was counteracted by the reduced capacity of the IMP to perform certain functions in fruit growth. The number of embryos in the double pollination with IMP was slightly higher when the paternal pollen was applied prior to the IMP than in the reverse order. An explanation for this may be that pollen which is applied first very likely occupies the best sites of the stigmatic surface. This pollen may also have a numerical advantage over the pollen which is applied later. In the double pollinations with fresh self pollen, the reverse was found, supporting the conclusions that these embryos all resulted from selfing.

Literature Cited

  1. Custers, J.B.M., A.P.M. den Nijs and A.W. Riepma. 1981. Reciprocal crosses between Cucumis africanus L.f. and C. metuliferus Naud. III. Effects of pollination aids, physiological condition and genetic constitution of the maternal parent on crossability. Cucurbit Genetics Coop. Rpt. 4:50–53.
  2. Nijs, A.P.M. den, J.B.M. Custers and A.J. Kooistra. 1980. Reciprocal crosses between Cucumis africanus and C. metuliferus Naud. I. Overcoming barriers to fertilization by mentor pollen and AVG. Cucurbit Genetics Coop. Rpt. 3:60–62.
  3. Oost, E. and A.P.M. den Nijs. 1979. Mentor pollen as a tool in interspecific hybridization in Cucumis. Cucurbit Genetics Coop. Rpt. 2:43–44.
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