Floral biology and late-acting self-incompatibility in Jacaranda racemosa (Bignoniaceae)

Breeding-system studies have been conducted with 38 of the approximately800 species of Bignoniaceae, and self-incompatibility was found in 31 of these. In species for which the site of self-incompatibility barrier was studied, self-pollinated flowers consistently failed to develop into fruits, even though pollen tubes grew down to the ovary and penetrated most of the ovules. In this study, we have investigated the floral biology and the breeding system in Jacaranda racemosa Chamisso, with hand-pollination experiments and the histology of post-pollination events. Flower anthesis lasted 1–3 days, and although the frequency of flower visitation was extremely low, natural pollination seemed to be effected mainly by medium-sized bees. Because the conspicuous staminodium favours eventual pollination by small bees, a possible role of the staminodium in the increase of potential pollinators is suggested. Hand-pollinations indicated that J. racemosa is a self-sterile species. Histological analysis of post-pollination events indicated the occurrence of a kind of late-acting self-incompatibility in which the processes of ovule penetration, fertilisation and endosperm initiation were slower in selfed than in crossed pistils. Until the time of self-pollinated pistil abscission, no signs of endosperm malfunction or proembryo development were observed in selfed pistils. Therefore, inbreeding depression is an unlikely explanation for self-sterility in J. racemosa.

Reproductive-Biology of the Australian Species of the Genus Pandorea (Bignoniaceae)

We studied the reproductive biology of Australian species of Pandorea to facilitate a breeding program designed to develop elite cultivars for the horticultural industry. P. pandorana is protogynous whereas anther dehiscence and stigma receptivity occur at the same time in P. jasminoides and P. baileyana. The stigmas of all species are receptive prior to anthesis and can be artificially pollinated at this stage provided that the stigma lobes can be separated. Pollen viability, tested for P. pandorana and P. jasminoides, deteriorated during the flower life although pollen samples with a low Fluorescein diacetate response (5-14%) still produced many pollen tubes which grew to the base of the style. For all species tested, pollen tubes grew into the ovary in both outcross pollinations and in self pollinations which are known to be incompatible indicating that the incompatibility barrier is within the ovary. Pollen-ovule ratios, determined for P. pandorana and P. jasminoides, were low compared with other published pollen-ovules ratios for taxa with breeding systems based on outcrossing.

Incompatibility in Petunia

Petunia has a monofactorial gametophytic self-incompatibility system which is characterized by a mechanism for pollen-tube rejection after incompatible pollination about half-way between stigma and ovule. From in vitro experiments it is known that pollen tube extension requires de novo synthesis of proteins on 80 S ribosomes. The synthesis of these structural proteins has no need of new mRNA formation. The situation in the style is quite different: both RNA and protein synthesis are positively affected by pollination. There is a qualitative difference in synthesis between self- and cross-pollinated styles. These findings suggest that inhibition or rejection of pollen tubes is ‘the normal reaction’, whereas unaffected penetration is based on the synthesis of special enzymes after compatible pollination which break down the existing incompatibility barrier.

RELATIONSHIPS OF TAXA IN THE GENUS MEDICAGO AS REVEALED BY HYBRIDIZATION. VIL M. TORNATA COMPLEX

Different taxa of Medicago tornata Mill, were investigated on their relationships using hybridization and inheritance studies. It was found that taxa with lenticular pods, spiny and spineless, could be intercrossed and the inheritance of spininess in F2 was 3 spiny and intermediate to 1 spineless (35:1s). Hybrids between lenticular and cylindric pods segregated 3:1 spiny: spineless; 3:1 anticlockwise pod coiling: clockwise coiling (3C:1c), and 1:4:6:4:1 for number of coils per pod (1-2:2-3:3-4:4-5:5-6) indicating that two genes acted additively. Spininess and pod coiling direction segregated independently. M. striata was recognized as a variety of M. tornata because no serious impairment in vitality of hybrid progenies was observed and the inheritance of pod coiling direction in F2 followed closely the normal 3:1 segregation ratio. M. tornata var. shepardii has a complete incompatibility barrier with the M. tornata tested and should not be considered as belonging to M. tornata.