U. 5. Depr t mw t of Agricul t we

Forest Seruice Research Note S E - 294

Direction of Pollination Affects Seed Productivity

in (Shortleaf x Loblolly) x Loblolly Hybrids

by Timothy La Farge and Davie L. Hunt’

ABSTRACT- In reciprocal crosses between shortleaf X loblolly pine hybrids and loblolly pines, seed

production per pollinated flower was higher when the loblolly pine was the mother tree. We therefore

recommend that loblolly pines be favored as mother trees in such work, even though storage of pollen for

1 year is required.

Keywords: Interspecific hybridization, Pinus rue& P. echinofa, seed yields.

Hybrids of loblolly (Pinus taedu L.) with

shortleaf pine (P. echinata Mill.) show promise of

combining the superior growth of former species

with the latter’s resistance to fusiform rust caused by

Cronurrium quercuum (Berk.) Miyabe and Shirai

f. sp. jiisiforme. Backcrosses of the hybrid with

loblolly pine appear particularly promising in these

respects. Although we have not yet progeny tested

reciprocal crosses, we believe that selection of the

hybrid or the loblolly as the female parent in the

backcross yields identical genetic results, but it is of

considerable practical significance. In a large

breeding program, it is important to make

backcrosses in the direction that will yield the most

seeds for a given amount of bagging and pollination

effort.

This Note describes the results of pairs of

reciprocal backcrosses. Seed yields and efforts to

obtain these yields are described. In each pair of

backcrosses, one member is loblolly X (shortleaf X

loblolly) and the second is (shortleaf X loblolly) X

loblolly. In this standard notation, the female

parent is always listed first.

METHODS

From 1972 through 1977 eight (shortleaf X

loblolly) X loblolly hybrid crosses and their reciprocals were made and their seeds collected. Data

*Plant Geneticist and Forestry Technician, respectively.

Macon, Georgia 3 1208.

1980

collected included: (1) the number of flowers in each

bag at the time of pollination, (2) the number of

sound cones collected for each bag, and (3) the total

number of sound seeds extracted from each cross.

Variables for analysis were: (1) the percentage

of pollinated flowers which developed into sound

cones, (2) the number of sound seeds per cone, and

(3) the number of sound seeds per pollinated flower.

Trait 1 was transformed to arcsin dw.

Values for traits 2 and 3 were transformed to

dxs because they are small whole numbers

which tend to conform to the Poisson distribution.’

The study design contained randomized

complete blocks in which the two reciprocal backcross types in the paired comparisons constituted

the randomly chosen blocks (from Sokal and Rohlf

1969). The F-ratio so produced equals the square of

the t-value obtained in the traditional f-test of

paired comparisons.

The loblolly parents for the study were in the

Arrowhead Seed Orchard of the Georgia Forestry

Commission in Pulaski County, Georgia.3 The

hybrid parents were 45 miles to the north on the

Hitchiti Experimental Forest in Jones County. It

would have been best to make each pair of

‘Sokal, Robert R., and F. James Rohlf. 1969. Biometry.

776 p. W. H. Freeman and Co., San Francisco.

‘Land and facilities donated by the Georgia Forestry

Commission and the Georgia Forest Research Council are gratefully acknowledged.

Southeastern Forest Experiment Station - Asheuille, North Carolina
I

reciprocal crosses in the same year, but logistical RESULTS AND DISCUSSION

problems precluded this approach. As a result,

results may be confounded somewhat by year

effects. Also, because the hybrids flower later than

loblolly pine, all hybrid pollen used on loblolly trees

had to be stored for I year. We used fresh loblolly

pollen on the hybrids. Despite these possible

sources of bias, the data presented are useful to tree

breeders interested in hybridization.

Breeding efficiency is best measured by

determining the number of seeds recovered per

pollinated flower (Bayne Snyder, personal communication). Although number of sound seeds per

pollinated flower may be somewhat confounded

with year of pollination, a breakdown of these

results indicates a degree of consistency. Results of

eight reciprocal crosses are shown in table 1. In

Table I.-Summary of pollination. cone-yield and seed-yield data collected from 1972 through 1977

on (shortleaf X loblolly) X fobfolly hybrids and their reciprocals

Backcross Year Flowers

1.D.’ pollinated pollinated

Sound Sound Sound

cones cone seed i

collected survival* cone

Sound

seed /

pollinated

flower

HHSX

TRO 2

TRO 2

X HH 5

HHSX

CHA 3

CHA 3

XHHS

HH6X

CHA 3

CHA 3

X HH 6

HH 13X

CHA 3

CHA3X

HH I3

HHSX

FL0 32

FL0 32

X HH 5

HH 13X

HEA I5

HEA I5

X HH I3

HHGX

HEA 15

HEA I5

X HH 6

HH39X

MER 8

MER 8

X HH 39

1972

1975

1974

1975

1972 & 74

1975

1974

1975

1974

1975

1974

1975

1974

1977

1975

I977

Number Percenr Num her

62 22

57 35

15 5

38 32

102 30

40 38

20 I

22 19

I6 II

45 42

I8 6

22 I3

26 II

24 I4

42 22

42 34

35.5 26.3 9.3

61.4 38.5 23.6

33.3 18.4 6.1

84.2 36.8 31.0

29.4 20.9 6.1

95.0 35.9 34. I

5.0 74.0 3.7

96.4 35.8 31 .o

68.8 18.8 12.9

93.3 64.6 60.3

33.3 60.3 20.1

59. I 19.6 I I.6

42.3 19.0 8.0

58.3 2.4 I .4

52.4 1.X

81.0 42.6

I .o

34.5

I Parents designated by 34etter abbreviations for their Georgia counties of origin are loblolly; those with “HH” designations are

hybrids.

‘(Sound cones collected/flowers pollinated) x 100.
terms of sound seed per pollinated flower (last

column), use of the loblolly as female parent was

more efficient in six of the eight comparisons. Both

comparisons in which this result did not hold

involved HEA 15 as the loblolly mother tree. This

tree may simply be a poor seed producer. In an

overall comparison, loblolly female parents

produced significantly more sound seeds per

pollinated flower (table 2).

the detrimental effect of storage was more than

offset by some unknown detrimental effect

associated with the hybrid mother trees.

The differences in seed yields between

reciprocal backcross types may be attributed partly

to difference in seed potential, defined by Bramlett’

as two times the number of fertile cone scales, or the

maximum number of fertile seeds a cone is capable

of producing. Bramlett found that the seed potential

Table 2.-Mean squares comparing reciprocal backcross types for three cone and

seed-yield traits

Source

Degrees

of

freedom

Sound

cone

survival

Sound

Sound seed/ seed/

cone pollinated

flower

Backcross

Reciprocal

backcross type

Error

7 140.74 3.54 1.91

1 2,678.06** 1.16 20.09*

7 133.48 4.87 2.76

*Significant at the 5 percent level.

**Significant at the I percent level.

Also, the flowers of the hybrids, which

resemble the flowers of shortleaf pine more closely

than those of loblolly pine, aborted between

pollination and cone collection much more frequently than did those of loblolly pine; this difference is significant at the 1 -percent level. These abortions probably were, in turn, a major cause of the

relatively low number of sound seeds per pollinated

flower of the hybrid mother trees. Supporting this

conclusion is the lack of a significant difference in

the number of sound seeds per cone between the

reciprocal backcross types (table 2).

for loblolly pine cones is 155 seeds, while that for

shortleaf is 90 seeds. He therefore hypothesizes (personal communication) that shortleaf X loblolly

hybrids might have a seed potential of about 120

seeds. Between reciprocal backcross types, however, differences in number of sound seeds per pollinated flower were much larger, for the most part,

than seed potential would seem to account for.

Hence, some external factor in addition to seed

potential seems to be operating to produce the conelet losses in the hybrid mother trees.

Although the data are confounded by the use of

stored versus fresh pollen, conelet losses of the

hybrids pollinated with fresh loblolly pollen

exceeded those of the loblolly mother trees

pollinated with stored hybrid pollen, These results

are opposite to those of Snyder and Squillace: who

reported considerable reductions in cone survival,

seeds per flower, and seeds per cone when stored

pollen was used, in comparison with fresh pollen.

This is not to say that using fresh pollen in this study

had no effect on seed or cone yield or that stored

pollen was better than fresh pollen, but rather that

CONCLUSIONS

In hybrid breeding work, it is convenient to use

shortleaf or shortleaf X loblolly trees as mother trees

because fresh pollen is available from the earlier

flowering loblolly trees. To use loblolly as the

mother tree, it is necessary to store the shortleaf or

hybrid pollen until the following year; consequently, there is some loss in pollen viability. However, the lesson from these reciprocal crosses seems

to be that it would be most efficient, for many crossing schemes, to use loblolly trees as mother trees

4 Snyder E. B., and A. E. Squillace. 1966. Cone and seed ‘Bramlett, David L. 1974. Seed potential and seed

yields from controlled breeding of southern pines, USDA For. efficiency. In Proceedings of a colloquium: Seed yield from

Serv. Res. Pap. SO-22,7 p. South For. Exp. Stn.. New Orleans, southern pine seed orchards. p. 1-7. John F. Kraus, ed. Ga. For.

La. Res. Count., Macon, Georgia.
whenever possible. When shortleaf or hybrid trees

must be used, as in diallel mating schemes, it

probably would be best to bag more flowers to

obtain the required amount of seed rather than

using loblolly as the female parent.

U.S. Go”*rnne,nt Printlna OffIce: 1981- 735~056/4023 Reqlo” NO. 3-11