Pollen tube growth and fruit-set success in Hevea brasiliensis hand-pollination influenced by the choice of clone and female flower

Differences in fruit-set success reputed to exist between Hevea brasiliensis genetic clones were verified by an analysis of breeding records for the clones PB 5/51, RRIM 600 and PR 107. Although the best fruit-set success was obtained with PB 5/51 following artificial (hand) pollination, this clonal trait was not reflected in terms of greater numbers of pollen tubes developing it the styles of hand-pollinated PB 5/51 female flowers. It was observed, however, that more pollen tubes reached the ovules in PB 5/51 than in RRIM 600 or PR 107. Significantly, PB 5/51 female flowers required fewer pollen tubes to effect penetration of all three of its ovules. As fruit formation in Hevea is dependent on all three ovules of the flower being successfully fertilised, PB 5/51 female flowers have hence a greater propensity for successful fruit-set. In all the three clones studied, the frequency distribution of female flowers with 0, 1, 2 or 3 of the ovules penetrated by pollen tubes did not conform to binomial expectations. Flowers with no ovule penetrated and flowers with all three ovules penetrated were greatly over-represented. One explanation for this non-random distribution is the existence of 'receptive' female flowers that favour successful fertilisation whereas 'non-receptive' flowers tend to remain unfertilised even when hand-pollinated.