Hybrids of Solanum corymbiflorum and S. diploconos (Solanaceae) – a hope for new commercial varieties of edible fruits

Solanum diploconos (Mart.) Bohs („guava tamarillo”) and Solanum corymbiflorum (Sendtn.) Bohs („hardy tamarillo”) are wild relatives to subtropical, orchard, small tree – Solanum betaceum Cav. (called simply „tamarillo”). Both these species create edible fruits, but they have not been cultivated widely so far as fruit trees. However, each one of these species has valuable features. Solanum corymbiflorum is quite hardy to frost, deciduous and early blooming both in fenological year time as well as in its lifespan but has untasty fruits. Solanum diploconos has quite good tasting fruits and it is pest and disease resistant. This work has been an attempt to connect their best features in hybrids to create new, potentially commercial cultivars to grow as fruit plants. Three types of hybrids were created: F1 hybrid type – Solanum corymbiflorum × Solanum diploconos, BC1 backrossing type (Solanum corymbiflorum × Solanum diploconos) × Solanum corymbiflorum and the second BC1 backcrossing type – (Solanum corymbiflorum × Solanum diploconos) × Solanum diploconos. Out of the numerous variable individuals within each type, we selected three very valuable from an agronomic point of view cultivars (Solanum 'Lynn', 'Pinczow Springs' and 'Iwona'). The work with crossing and selecting the next cultivars will be continued.


PALAVRAS-CHAVE:
Both of the parent species have positive as well negative features from an agronomic point of view: -Solanum corymbiflorum -"hardy tamarillo": -it is deciduous and hardy to about (-8 to -10C degree) -starts blooming early in the year -starts blooming early when grown from seeds -has small seeds -the stone cell granules are present in fruits -fruits have mild taste but also unpleasant aftertaste, -Solanum diploconos -"guava tamarillo" -the common name has been created on New Zealand during the attempts to cultivating it -MEADOWS 2012 (www.naturalhub.com): -it is evergreen and hardy to only about (-3C degree), but good grows in cool, frost free climate -starts blooming late in the year -starts blooming late when grown from seeds -has small seeds -the stone cell granules are absent in fruits -it is pest and disease resistant (powdery mildew, whitefly) -fruits are aromatic but very acid (BOHS 1989, PRINGLE & MURRAY 1991a and our own insights).
There have been used crossing work methods described in breeding guides (BURBANK 1921, LAWRENCE 1957 andothers). The experiment has been placed in USDA zone 7a. The plants have been grown in large pots, and they were kept outside in summers and during winters stored in a cool cellar or greenhouse (protected against stronger frosts). The flowers were protected against aleatory pollination by organza-nettle bags. They were hand pollinated. The pollination was made in summer months. Each crossing combination has been tested.
The aim of work was to connect good features of these two species by hybridisation and selection. In addition, there has been a hope to obtain new, valuable fruit varieties to commercial growing.
It was necessary wait eight years to Solanum diploconos to start blooming from seeds, so the whole research took almost 12 years, although the time taken to cross and select was strictly four years so far and is currently being continued.
The first experiment was to obtain a hybrid of S. corymbiflorum × S. diploconos. The next step was to test backcrossing with both parents. Numerous hybrid individuals were obtained from these crossings with very variable appearances, especially within BC1 crossings due to the split of features. They were propagated easily by cuttings, so about 8-10 individuals of each variety exist now and may start further dissemination.
The combinations of BC1 crossings with each of parents were obtained. The F1 hybrid S. corymbiflorum x S. diloconos and BC1 hybrid (S. corymbiflorum × diploconos) × corymbiforum have been created for the first time by BOHS (1991), but BC1 (S. corymbiflorum × diploconos) × S. diploconos has been created as the first in the world by the authors.
About 60-70% pollinated flowers developed into fruits in the case crossing S. corymbiflorum × S. diploconos. In November and December, the fruits ripened about 4 months later (finally were harvested over 20 hybrid fruits). The seeds were only partially viable, and their germination rate was low (about 5%). About 9 seedlings for each 10 ones was chlorotic and died in young age. The seedlings started to bloom within a few months and next were created also BC1 hybrid (S. corymbiflorum x S. diploconos) × S. corymbiflorum.
Also backcrossing (S. corymbiflorum × diploconos) x S. diploconos was successful. Chlorotic seedlings were observed in the cases of both BC1 crosses but less frequent than in F1 combination. The chlorose in addition rarely deadly in these combinations. The seeds were also better developed and in a larger degree viable. The rate of viability of seeds in combination BC1 (S. corymbiflorum × S. diploconos) × S. corymbiflorum was about 20% and in the case BC1 (S. corymbiflorum × diploconos) × S. diploconosabout 10%.

Solanum 'Pinczow Springs' -(S. corymbiflorum × S. diploconos) × S. corymbiflorum (BC1 hybrid)
Deciduous (or half-evergreen if grown in warm climate), shrub or small tree about 1,5-3 m tall with irregular shape. Leaf blades medium-green, simple, chartaceous to subcoriaceous. Trunk leaves simple, 3lobed. Crown leaves simple, unlobed, smaller. Inflorescence branched, ca. 30-50-flowered. Flower buds tubular-ellipsoidal. Calyx chartaceous, the radius 6-8 mm. Corolla lavender-purple when young, fading to lavender-white with age ( Figure 3A), chartaceous to membranaceous, stellate, the radius ca. 15-25 mm. Anther thecae yellow, oblong-elliptic. Connective yellow. Stigma narrow, concave, about 1 mm, 1,5-2 mm exserted beyond stamens. Fruit ovate, ca. 2-3 cm × 1,5-2 cm in, shortly, shiny pubescent, pale-greenishwhite, with a few, darker, longitudinal stripes. The flesh of ripe fruits is pale creamy-white ( Figure 3B). The seed are usually not fully developed The reason we selected this cultivar was very valuable features in horticultural point of view. The fruit ripes very early (in our climate it can fruit even before first frosts in the end of September and in October). It also creates the heaviest crop. The wall of fruit is thick and firm by long time after harvest, so it can be stored by long time, even in room temperature. The flesh is pale creamy-white, not acid, but has distinct "tropical", "bubble-gum like" taste. Fully ripe fruits of this variety have not the astringent aftertaste (which is characteristic to Solanum corymbiflorum).

Solanum 'Iwona' -(S. corymbiflorum × diploconos) × S. diploconos (BC1 hybrid)
Deciduous (or half-evergreen if grown in warm climate), small tree about 0,5-2,5 m tall with regular shape. Leaf blades pale-green, simple, coriaceous. Trunk leaves simple, unlobed. Crown leaves simple, unlobed ( Figure 4A). Inflorescence unbranched, ca. 5-20-flowered. Flower buds ovate, obtuse at apex. Calyx waxy, glabrous, the radius 8-10 mm. Corolla first adaxially lavender, lavender-white adaxially, later changing color to pale aquamarine ( Figure 4B). Shape of corolla first campanulate, next campanulatestellate ( Figure 4C). The radius of corolla 8-15 mm. Anther thecae pale lavender first, next pale lavenderyellowish, elliptic. Connective lavender-yellowish fading to bright yellowish. Stigma concave with two apical glands, ca. 3 mm in diam, ca. 1 mm exserted beyond stamens. Fruit ovate to roundish-ovate, ca. 2-2,5 × 1,5 cm, shortly pubescent, blurred fine, whitish papillary-spotted ( Figure 4D). The color of overripe fruits, its flesh and taste are unknown because fruits have not ripped yet. This cultivar was selected due to its very valuable features in a horticultural viewpoint The above taxon ripes quite early, creates large crop, is deciduous and day-length neutral. The leaves of this cultivar are always unlobed, entire and pale green, but completely free of chlorose. The colour and organoleptic value of the fruits is unknown, due to the fruits still being unripe. Due to domination of S. diploconos genes there is suspected high organoleptic value of the fruits. This cultivar individuals bloom and fruit even when they are very small (about 0,5m tall) and barely branched.
Eponym: Iwona -is the first name of one of the cultivar breeders and co-author of this paper. Suggested common name: Tamarillo 'Iwona` Solanum `Iwona`. The cultivar standard: S Poland. Pińczów. In a greenhouse. September 2021, coll. Wojciech M. Szymański, Iwona Ziółkowska (KOR 55914) In the botanic viewpoint the variety Solanum 'Lynn' stands out from others mainly by the color of their flowers (young-purple, olderalmost pure white), 3-lobed trunk leaves and aquamarine color of overripe fruits with orange flesh with good taste. Solanum 'Pinczow Springs' individuals are very similar to S. corymbiflorum, but usually have 3-lobed trunk leaves, dirty purple-yellow anthers, larger flowers and inflorescences and has tasty fruits without an unpleasant aftertaste. Solanum 'Iwona' is similar to S. diploconos but differs mainly by the flowers' color (young-lavender, olderaquamarine), seasonal pale green, never lobed leaves (deciduous) and is day-length neutral.
The combinations of features within the chosen hybrids was noted and illustrated in Table 1. However, the hardiness remains untested. The research with crossing and selecting of Tamarillo species has been not finished yet. The results of further research and descriptions of the new cultivars will be presented soon.