Reproductive biology of Melaleuca alternifolia (Myrtaceae) 2. Incompatibility and pollen transfer in relation to the breeding system

Abstract

The onset of stigma receptivity in Melaleuca alternifolia (Maiden & Betche) Cheel was evaluated by observing pollen-tube growth and seed set following controlled pollination. Pollen-tube numbers in the style, following controlled pollinations, increased from Day 1 to Day 6, then declining rapidly. The stigma was most receptive during Days 3–6, and still receptive at low levels as early as shortly after anthesis and as late as 10 days after pollination. The present study found that individuals of M. alternifolia differed in their degree of expression of self-incompatibility. Artificial self-pollination, with emasculation, in several families resulted in complete self-incompatibility, with no capsule retention. The microscopic observation of pollen-tube development revealed a mechanism of self-incompatibility in M. alternifolia. A self-incompatibility system operates in the style, although a few self-pollen grains are capable of germinating and producing pollen tubes. It also appears that late-acting self-incompatibility mechanisms discriminate against self-pollen tubes when they descend to the ovary. Artificial cross-pollination of selected parents produced seed with greater germination capacity and seedlings that grew faster than the corresponding open-pollinated seed and seedlings from the same parent. Freeze-dried pollen stored at −18°C maintained viability (22%) over 1 year of storage. This finding will allow greater flexibility in undertaking controlled pollinations, because stored pollen can be substituted for fresh pollen when insufficient quantities are available from new-season flowers. A wide variety of insects was observed visiting the flowers of M. alternifolia, and capsule set was high even in bags that excluded flower visitors greater than 2 mm. Thrips species seem likely to be important pollinators of this species because they are small and were abundant inside and outside of exclusion bags, although several other insect species such as bees, flies and wasps were also identified as frequent floral visitors.