Abstract:

Objective: This study aims to explore the impact of flower traits and mating system on sexual reproduction of Acer griseum, an endangered species endemic in China, by investigating the possible causes of low seed setting rate. This study would lay a theoretical foundation for formulating effective measures to improve sexual reproduction, in order to alleviate the current contradiction between supply and demand of seedlings. Method: In this study, the cultivated A. griseum was targeted, and the flowering dynamics, floral traits and mating system were investigated through phenological observation, flower morphological characteristics determination, calculation of outcrossing index (OCI) and pollen-ovule ratio (P/O), and determination of pollen vitality and stigma receptivity. An artificial pollination and bagging experiment was carried out to explore the mating system. Result: 1) The sex expression of A. griseum florets was complicated and the florets were divided into two types based on their functions: male floret and female floret. Male florets were divided into male floret type Ⅰ and male floret type Ⅱ according to the degree of pistil degeneration. The male floret I only had stamens, while male floret Ⅱ had stamens and vestigial pistil. The length of stamens was much longer than that of the pistils. They were both male due to lack of ovary. Female florets were pseudohermaphrodite. The stigmas were longer than those of the anthers. Throughout the opening process, the stigmas were able to make curving movement, while the anthers were not dehiscent and did not release pollens. 2) The flowering process for one floret could be divided into five periods: bud stage, budding, anther or stigma appearing, blooming and withering. It took an averaged time of 7.6 days for male florets from opening to shedding, while the averaged time for female flowers from opening to fruiting was 11.6 days. The cyme usually consisted of three florets. In the inflorescences that were composed of male and female florets, the female florets usually bloomed earlier than male florets. 3) A. griseum bloomed once a year in April. Generally, the flowering lasted about 19.5 days. Protandry and protogyny coexisted for an individual tree and there was a certain overlapping time for the opening of male florets and female florets. The number of female florets was more than that of male flowers on an individual tree with the mean ratio of about 3.71∶1. 4) The duration of pollen viability was relatively short. When the pollen was stored at room temperature and dry, the maximum viability of pollens reached 63.86%±5.35% within 24 hours and then decreased to 1.03%±0.13% after 24 hours, almost losing viability. It indicated that pollens could keep their viability for a short time at room temperature. The stigma of female florets became receptive when it was out of the corolla before florets blooming. The receptivity of stigma reached its highest at full open and maintained throughout the entire opening process of female floret. Male flower pollens had viability during stigma fertile period of female florets. 5) There were 45 900-58 900 pollens for each male floret and 4 ovules of each female floret. The P/O was 3 093-3 969 based on the ratio of male and female floret per plant, and the OCI was 4. Combining the results of artificial pollination, mating system of A. griseum was facultative outcrossing and partially self-compatible, thus pollinators were necessary for the pollination process. Acer griseum showed a strong parthenocarpic tendency. Conclusion: The flower of A. griseum is functionally unisexual, monoecious and incomplete dichogamous. Its mating system is facultative outcrossing with low pollination efficiency. It shows strong parthenocarpy tendency. All the above traits are the important reasons for its low seed setting rate.

Key words: Acer griseum, floral traits, mating system, sexual expression, parthenocarpy