Breeding

by Donald Smith
January 1995

Introduction: Over the past several decades many hybridizers have reported various degrees of success with the intersectional cross (Paeonia lactiflora cv. x lutea hybrids). This cross was first accomplished in Japan by Toichi Itoh in 1948 or 49 and is often still referred to as the Itoh cross. In the U.S., this cross has been successfully repeated many times by a number of hybridizers resulting in at least several hundred hybrid progeny. To date a total of only 28-30 intersectional hybrids have been named and / or introduced by several different breeders including the 4 original Itoh hybrids.

The first report of success with the reciprocal intersectional cross (tree peony x herbaceous peony) came from Bill Seidl in 1973 (Paeonia, Vol. 4, No. 1, March 1973). He reported seeds from two F1, lutea hybrids (Paeonia 'Age of Gold' [lutea group] and Paeonia 'Thunderbolt' [lutea group]) by using pollen from fertile tetraploid herbaceous hybrids. These seeds were described as large, black, and almost-firm, but apparently none of these seeds ever germinated. Later (APS Bulletin No. 216, Dec. 1975), Bill Seidl suggested using Paeonia 'Alice Harding' (lutea group) as the seed parent in the intersectional cross (i.e. the reverse Itoh cross). Recently, Roger Anderson stated that he has made the reverse cross repeatedly for more than ten years but has yet to produce a single hybrid plant.

I have experimented with the reciprocal intersectional cross for the last three years with results similar to those of Seidl and Anderson. This cross has proved to be one of the most interesting and frustrating crosses which I have ever encountered. It consistently produces a large number of big, shiny black, slightly under-filled seeds most of which float when placed in water. Some of these seeds were simply huge, but only a rare few seemed fully plump and firm. To date however, only two seeds have germinated and thus far no plants have been produced. When Paeonia lactiflora 'Martha Washington' pollen was used on Paeonia 'Golden Era' (lutea group) it was extraordinarily effektive with an average seed production rate of over 8 seeds/cross (or 2.3 seeds/pod). I know of no other cross which "takes" better. When less fertile parents are used the results are considerably more modest, but in general, seeds are never difficult to produce. Getting them to germinate and grow, however, is quite another matter. Maybe one seed in 30 is plump and firm enough to merit an attempt at germination, and yet many of these are also "floaters". I have examined the endosperms of a number of these seeds by cutting them open with a knife. Most of these seeds contain a small void (hole) in the center about the size of a BB-shot. The results of numerous reverse "I" crosses made over the last 3 years are summarized in Table 1. 

Table 1
Results of Reverse Intersectional Cross Summary of 3 Years (1992-94)

L. H.(F1) = Paeonia 'Age of Gold' (lutea group), Paeonia 'Banquet' (lutea group)
L. H.(F2) = Paeonia 'Golden Era' (lutea group), Paeonia 'Exotic Era' (lutea group)
Tet. Herb. Hybrid = Paeonia 'Requiem' (herbaceous hybrid),
Paeonia 'Paula Fay' (herbaceous hybrids)

From these results, a number of general conclusions concerning the reverse intersectional cross can be drawn. Some of these conclusions may apply more generally to other crosses as well.

These are:

• F1 lutea hybrids are about 14 x less effective (fertile) as pod parents than fertile F2 lutea hybrids (such as Paeonia 'Golden Era' (lutea group), etc.).

• Fresh pollen is about 7.5 x more effective than stored (frozen) pollen.

• Pollen of tetraploid herbaceous peonies is 3 x less effective than pollen of Paeonia lactiflora (for example 'Martha Washington').

It is interesting to compare these results with other crosses which we might expect to be far more successful. For example, when pollen of Paeonia suffruticosa (Moutan) is used on Paeonia 'Golden Era' (or other fertile F2s), the cross takes rather poorly (~0.3 seeds/cross). It is also important to note that all of the lutea hybrids (F1s, F2s, and advanced generation hybrids) appear to be self sterile (at least, the ones which I have tried so far). This fact simplifies breeding with these hybrids considerably, making it unnecessary to protect these crosses against self-contamination.

One of the more curious aspects about this cross relates to the pod parents of the few "good" seeds thus far produced. To date, the only seeds to "germinate" have come from a single fairly infertile F1 parent (Paeonia 'Age of Gold' [lutea group]). Although these plants (F1s) produce far fewer seeds overall (by a factor of ~15), those which are produced are much more likely to germinate. Despite an overall germination rate of only 1.5 %, seeds of Paeonia 'Age of Gold' (lutea group) have germinated at a 40 % rate (2 of 5). Unfortunately, both of these seeds have failed to grow normally after initial germination which occurred in approximately 6-7 weeks. At first, growth appeared normal and a short normallooking root about 2-3 mm long was produced. Then something seemed to go wrong and these roots simply "refused" to grow any longer even after a month or more. One seed, held longer at room temperature began to rot, the other was transferred to the refrigerator, but I am doubtful that it will grow after the winter dormancy period. And so it goes.

Despite the lack of success to date, I will continue to pursue this cross for a few more years as I am convinced that this effort will eventually result in at least a few hybrid plants. I would encourage others to try this cross as well. The more total seeds produced, the sooner we will achieve some degree of success.

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