Sexing Pawpaws: what have I got in the backyard?
Domestic and commercial pawpaw growers world-wide almost universally use seedlings for pawpaw propagation, mainly because seed supply is plentiful and cheap, germination is easy and the juvenility period is so short - 9 months under ideal conditions. But with seedlings you have to wait till flowering time to know which of the three main gender types a given plant will be ie male (staminate), hermaphrodite (bisexual) or female (pistillate). Modern genetic techniques can answer this question much sooner, but without research lab facilities the rest of us have to wait for events to unfold. The dioecious types perform better in sub-tropical climates like ours, and only one male is required to pollinate several females; fruit can be formed without pollination (parthenocarpy) but quality is compromised. So given you’ve probably sourced your seed from open-pollinated dioecious or self-pollinating hermaphrodite plants, what proportion of each gender have you grown up and which ones are either in excess or unwanted and can be culled? Obviously the sooner you’re able to answer this question following flowering and then continue with only the desired types, the better. First you should read the companion article in this series, 'The mysterious antics of pawpaws', to allow some understanding of the markedly variable behaviour and sexual properties of the species. Then when you have some feel for what you’re dealing with, the intention of this article is to move on to practical identification of the various types. Of the three main genders, hermaphrodites show the greatest variation in features, and their behaviour is also the most influenced by cultural and environmental factors.
Check out the following features:
1. Peduncle length
Left: Male inflorescence; Right: Male flower minus petals showing 2 whorls of different length
For female and hermaphrodite plants this is short, usually with a single axial flower but sometimes up to 2-3, occurring as short internode cymes. Ideally the peduncle should be 4-6cm long so that when fruit mature they’re not all tightly jammed up against one another on the trunk, as this leads to misshapen fruit and difficulty in harvesting. In males, there may be many dozens of flowers on drooping peduncles 60-90cm long with flowers arranged as long internode cymes (raceme-like).
2. Flower morphology - exterior
- Staminate. The small narrow flowers have a fused tubular 5-lobed corolla that extends almost the whole flower length. There are 5 separate petals at the throat of the corolla, with stamens in the centre partly visible. They are the smallest of the 3 main types.
- Pistillate. Globular-obovoid with separate twisted petals inserted at the base of the slightly inferior ovary, with 5 apical stigmatic lobes protruding. Largest of the three types.
- Hermaphrodite types. These are intermediate in size between the other 2 types, and the apical stigma is apparent in all cases.
- Elongata. This is the normal hermaphrodite, so named because of the elongated pistil that leads to more elongated cylindrical fruit. Petals are not twisted and are fused together for a proportion of their length to form a corolla tube free from the pistil. In the unopened bud, there is usually a slight restriction around the throat region.
- Pentandria. Globular-elongated. Petals are free for most of their length but more fused to each other and to the pistil base than in pistillate flowers.
- Carpelloid forms. The least satisfactory of the hermaphrodites and the most variable. Petals can be inserted at the pistil base or fused for up to 70% of their length.
Left: Pistillate inflorescence; Centre: Pistillate stigmatic rqys; Right: Pistillate petal insertion.
Left: Pistillate fruit, smooth ovoid; Right: Pistillate scars surrounding the stem end of the fruit. Sepals are not prominent in pawpaws but here the 5 separate outer and alternately-placed scars from them can also be seen.
3. Flower morphology – interior
Gently peel off the petals one at a time as needed. You might find a hand lens helps you see some of the following details.
- Staminate. The 10 stamens can now be seen more clearly in 2 separate alternating whorls of 5, with one series being sub-sessile anti-petalous and the other anti-sepalous on short filaments. There is only a rudimentary and non-functional central pistil (pistillode).
- Pistillate. There are no stamens. The 5-carpellate pistil is usually smoothly circular or slightly undulate, and the apical stag-like stigma is now clearly visible.
- Hermaphrodite types.
- Elongata. This is the preferred type of hermaphrodite. There are 10 stamens at the throat of the corolla tube. The stamens are arranged similarly to staminate flowers. In some plants the anthers and stigmas are at the same level and in others different, with consequent effects on ease of pollination. The pistil is 5-capellate but can vary from 1-10.
- Pentandria. So-called because it has 5 fairly short anti-sepalous stamens. These are fused to both the pistil and the petals at the base, which is just below the throat of the short corolla tube. The pistil is 5-carpellate and deeply furrowed with the stamens lying along these grooves.
- Carpelloid forms. Carpellody is where stamens lose their pollen forming ability and become carpel-like by fusing with the pistil. There is great variation in these forms. Petals may be almost free from one another or fused with others for up to 70% of their length, even within a single flower. There may be 2-10 stamens in different stages of transition from anthers to carpels, and their fusion with petals and pistil can also vary. Pistils are often distorted with 5-10 carpels. Sometimes there may be faulty fusion leaving ovules exposed and in others the original 5 carpels may abort. Many of these unsatisfactory variants are driven not only by the genetic make-up of the plant but by unfavourable cultural and environmental factors.
You can firm up your diagnosis when the plant begins to fruit.
Left to right: Elongata fruit; Pentandria fruit; Carpelloid pentandria fruit.
- Pistillate. Globose-obovoid without furrowing. Surrounding the stem at the base of the fruit, there are 5 separate scars where the 5 individual petals have abscised. This type is the least subject to cultural and environmental influences.
- Elongata. The best fruit of the 3 hermaphrodite types. The most common form is 5-carpellate and pyriform or cylindrical. Flowers with only a few carpels form narrow fruits similar to bananas.
- Pentandria. Sightly oblong-cylindrical with 5 longitudinal furrows. The fruit stem end shows a continuous circular scar which comes from the remnants of the abscised corolla tube.
- Carpelloid forms. The least satisfactory of the 3 basic hermaphrodite types. They are usually misshapen, sometimes grotesquely, and are not commercially marketable. When you cut the fruit you will sometimes see more than a single, central seed cavity. Even for the home grower it’s probably not worth keeping the tree.
Left: Male and recently-fertilized carpelloid pentandria flowers in a single cyme; Right: Carpelloid pentandria flower de-petalled showing incomplete fusion and exposed ovules.
With adverse cultural and environmental conditions (eg temperatures below 12°C through our winters) these trees which normally produce male flowers can become hermaphrodite. Incidentally, although commonly used in the literature, the term ‘sex-reversal’ is somewhat misleading. True reversal would take male through to female, but in these plants their gender change only goes half-way, having both male and female organs The flowers are larger and more bulbous than pure male types. They usually occur terminally on the peduncle and have an elongated pistil with a variable number of surrounding stamens. The fruit are narrow, elongated, furrowed and sometimes quite distorted. Certainly not commercially marketable and even for the home grower, not worth producing. If you decide to keep the tree, at least remove the fruit to prevent waste of plant resources.
Left: Normal inflorescence in a sex-reversing male; Centre: Sex-reversed hermaphrodite de-petalled flower showing elongata-like features with a substantial fused corolla tube length and stamen insertion at the corolla throat; Right: Elongated sex-reversed male fruit showing deep pentandria-like furrows.