Pollination Biology of Lespedeza davurica

Lespedeza davurica is a perennial herbaceous semi-shrub of the genus Legume, widely distributed in northeast China, North China, northwest China, southwest China, central China, south China, Inner Mongolia and Taiwan. It has good stress resistance and environmental adaptability and can be used for soil improvement, wind prevention and sand fixation. The stems and leaves are rich in protein, fat and amino acids, which are of high feeding value. The abundant aboveground biomass in florescence can be used as silage. The flavonoids contained in the stems and leaves of L.davurica have significant antioxidant activity, which can promote the production performance of broilers and improve their antioxidant capacity (Xu et al., 2010). Some compounds such as Kaempferol, rutin and vanillic acid contained in the aboveground parts have high medicinal value (Weng et al., 2011). As a local species in China, the growing and adaptive abilities of L.davurica are strong. And it is easy to be planted and managed. It can be planted in a large area in some regions. It has strong economic benefits and high application value.
       
The abundant wild resources of L.davurica in Shanxi Province provide favorable conditions for the study of L.davurica in China, but the studies on L.davurica mainly focus on stress resistance, nutritional components, and medicinal value, while the studies on the pollination characteristics and pollination mechanism of L.davurica have not been reported yet. In recent years, studies on pollination biology mainly focus on the following points: the relationship between floral characteristic structure and pollination medium (Wilcock and Neiland 2002; James, 2003), the competition of reproductive organs in flowers (Harder 2006), effects of pollination biology on genetic diversity (Fenster et al., 2004; Rio 2010), the transmission route and pollination efficiency of pollen flow, chemical composition of pollination system, etc. In this paper, we took wild L.davurica from different parts of Shanxi Province as the experimental material to study the Floral dynamic, pollen viability, Stigma receptivity, etc. The study is aiming to clarify the biological characteristics of pollination of L.davurica, provide a reference for carrying out sexual hybridization, making effective breeding programs and promoting and applying excellent varieties.

Floral dynamic
 
The flowering time of L. davurica is from July to August. As for a single flower, it’s flowering period is about 3 days. Flowers open from base to top in sequence, with distinct different synchronicity. The florets are slow-growing when the buds first appear, then their growth rate gradually increases. The flowers are large, light yellow and lightly fragrant when they open. Usually, the petals open during the day and close at night. After the florets have fully bloomed for about 4 days, they gradually lose moisture, wilt, dry out, and finally fall off completely, leaving the stigma behind (Fig 1 and 2). Flowers open from 7:00 am on sunny days, and the number of blooms peaks at 12:00 to 14:00 and almost all close at about 18:00 (Fig 3).
 

 

 

 
Pollen storage
 
The pollen viability of the L. davurica was relatively short, about half a month. When the pollen was stored for 12 days, its viability was less than 10%. After it had been stored for 23 days, only the -20°C storage condition could keep the pollen viability above 5%. On the 30^th day, the pollen viability measured under these storage conditions was extremely low. At the storage condition of 25°C, the pollen viability was completely lost. In terms of pollen storage conditions for L. davurica, -20°C is the best and in terms of wet and dry conditions, the dried one is better than the undried one. Therefore, it is best to pollinate as soon as they are harvested to ensure safe pollination (Table 2).
 

 
Pollen viability and stigma receptivity
 
The pollen viability of single flowers of L. davurica was basically zero in phases I and II and reached its highest value up to about 80 % in the phase V. The pollen basically lost vitality in Phase VIII (Fig 4). Daily variation of pollen viability of L. davurica was positively correlated with air temperature and negatively correlated with humidity. The pollen viability reached its highest value at 12:00-14:00 (Fig 5).
 

 

       
In Phase I, L. davurica stigmas have no receptivity. In Phase II, III and VII, a smaller number of stigmas have receptivity at noon. In Phase IV, V and VI, the stigmas have receptivity, which reached the highest level of all in Phase V. With the increase of temperature and the decrease of humidity, the receptivity of stigma continued to improve and reached its peak at 12:00 ~ 14:00 (Table 3).
 

The onset of pollen viability is an important period in the process of morphological changes in flowers, and pollen viability is not only determined by genes but also influenced by environmental differences (Steven and Kim 2000; Lin et al., 2013). It was found in this study that when the pollen grains first cracked, their pollen viability was about 40%. Then their vitality increased rapidly in a period that followed, until it reached a peak of about 80% when the flowers bloomed. After that the pollen viability decreased rapidly. Pollen viability of L.davurica reached its peak between 12:00 and 14:00 in a day. To a large extent, the receptivity of stigma influences pollination rate, outcrossing rate and competition relationship among different pollens, which is an important sign in the process of flowers entering the maturity stage (Dafni, 1994; Arroyo and Arroyo 1981). Studies have shown that the stigma of L. davurica has a certain degree of receptivity before the flower opens and its receptivity is higher from the time the flowers opened to a later period. The receptivity of stigma reaches its highest value at 12:00 ~ 14:00 in the day. There is a distance of 0.8 mm between the stigma and the anthers of L. davurica. The separation of stamens and pistils in time and space to some extent reduces the probability of self-pollination and increases the possibility of cross-pollination. (Barrett et al., 1999; Róisín and Beverley 2020).

This research was financially supported by the key research and development program of Shanxi (201703D221009-4) and Province Prize and Supplement National Grass Variety Regional Test Station Fund of Shanxi.