Abstract
Crosses among single-, double- and multi-petal jasmine cultivars (Jasminum sambac Aiton) are unable to easily generate hybrids. To identify the reproductive barriers restricting hybrid set, dynamic changes in jasmine pollen viability and pistil receptivity were compared at different flowering stages. Pollen-pistil interactions in six reciprocal crosses were also investigated to characterize pollen-stigma compatibility. Additionally, paraffin sections of pollinated embryo sacs were prepared for subsequent analyses of developmental status. Furthermore, pistil cell ultrastructural characteristics were observed to reveal cytological mechanism regulating pistil receptivity and the pollen-pistil interactions. We observed that pollen viability and stigma receptivity varied depending on petal phenotype and flowering stage and were easily lost during flowering. Different reciprocal crosses exhibited varied pollen-stigma compatibilities according to the pollen germination rates. Although some pollen grains germinated normally on maternal stigmas, the pollen tubes were arrested in the pistils and were unable to reach the ovaries. Additionally, the embryo sacs remained unfertilized until degenerating. Therefore, jasmine crosses are affected by pre-fertilization reproductive barriers. Low pollen fertility and poor stigma receptivity are detrimental to pollen germination and pollen-pistil compatibility, indicating they are two factors affecting hybrid set. Ultrastructural observation of the pistil cells revealed that cell death occurred during flowering. Thus, the early and rapid senescence of pistils is likely responsible for the decreased pistil receptivity and inhibited pollen tube growth. These findings may be relevant for future jasmine hybridizations. They provide new insights for the development of methods to overcome reproductive barriers and may also be useful for clarifying the phylogenetic relationships among jasmine cultivars with differing petal phenotypes.
Highlights
Jasmine (Jasminum sambac Aiton, Oleaceae) is an important commercial ornament plant that has been used in floral bouquets in many countries
In the following pollination experiments, the SP and DP jasmine pollen grains were collected in Stage 1, whereas the MP jasmine pollen grains were harvested during Stage 2
Anatomical observations of the pollinated ovaries indicated that the embryo sacs were unfertilized and pollen tubes were unable to reach the ovules (Fig 6). These results indicate that the pollen tubes were arrested or grew very slowly in the pistils after they entered the stigma
Summary
Jasmine (Jasminum sambac Aiton, Oleaceae) is an important commercial ornament plant that has been used in floral bouquets in many countries. Each jasmine phenotype is associated with specific characteristics related to growth, anthesis, stress tolerance, ecological adaptation and economic value [1,2,3,4,5]. The DP jasmine plants produce the most flowers, and are the most tolerant to abiotic stresses such as low temperatures, water deficiency and limited irradiance. The SP flower is considered to be the most fragrant, while the MP jasmine plants have the highest ornamental value because of their supernumerary petals [5,6,7]. Sexual hybridization is well known as one of the most efficient methods for breeding horticultural cultivars, which suggests it offers the best chance of creating novel jasmine cultivars using the currently available jasmine lines with differing petal phenotypes. Characterizing the nature of these particular barriers will be important for developing effective mitigating strategies
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