The Nelumbonaceae is a family of perennial, aquatic, herbaceous angiosperms, consisting of the two species Nelumbo nucifera Gaertn
. and N. lutea
(Willd.) Pers. Nelumbo nucifera
, also called the Indian or sacred lotus, is found throughout Asia and Australia, whereas N. lutea
, also known as the American lotus or water chinquapin, occurs in eastern and southern North America [1
]. Fossilized leaves of N. nucifera
show that wild populations were distributed previously in eastern Asia including China, India and Japan about 50 million years ago. Wild populations of N. nucifera
in China are now mainly distributed in the provinces of Helongjiang, Jilin, and Sandong. Nelumbo nucifera
was cultivated in China more than 2000
years ago. The current main cultivation areas in China comprise the provinces of Jiangsu, Zhejiang, Hubei, Anhui, Guangdong, Hunan, Fujian, and Jiangxi. This species is propagated vegetatively by rhizomes in commercial cultivation, and by seeds for breeding purposes [3
]. As an economically important aquatic plant in China, N. nucifera
cultivars are classified into three groups: rhizome lotus, seed lotus and water lotus or ornamental lotus according to their use and morphological features [4
]. Water lotus is among the 10 most popular traditional flowers in China, and is an important ornamental plant, widely cultivated in gardens prized for the beauty of its flowers as well as its ability to purify pond water [6
]. With the recent rapid growth in the Chinese economy and increased standard of living, the demand for new lotus cultivars with novel characteristics has also increased. Therefore, Chinese breeders are making great efforts to develop new cultivars using methods such as artificial interspecific hybridization, mutation techniques, and multiploid approaches [7
]. Hybridization is the most widely used breeding technique and the most efficient method by which to produce new water lotus cultivars. However, the existence of incongruity barriers often makes many water lotus crosses difficult, and thus raising hybrids may be hard to achieve, thereby seriously reducing the breeding efficiency [7
]. Few previous studies have been carried out to investigate this problem, and thus the factors that influence the breeding efficiency of water lotus remain unclear.
In general, the characteristics of parental reproductive systems and their interactions are closely related to the breeding efficiency of plant hybridization. Consequently, parental reproductive systems and post-pollination phenomena have been examined in many crops, including Chrysanthemum grandiflorumLeymus chinensisPhaseolus vulgaris
, and Fragaria ananassa
]. Most of these studies have successfully unraveled the factors that influence seed production and breeding efficiency. Following on from these previous studies, we set out to investigate systematically the reproductive processes following artificial pollination in two reciprocal crosses between water lotus cultivars, including pollen viability of the male parents, germination behavior of pollen grains on the stigma, and embryo development. The aim of the present study was to reveal the main causes that lead to low seed set in water lotus crosses, to overcome reproductive barriers and improve the efficiency of water lotus cross-breeding in the near future.
The cultivars used in the present study were developed about 10
years ago through cross-breeding. Anthesis of lotus flowers is very specific and complex. The pistil of N. nucifera
flowers is shaped like a flat-topped cone and contains 20–35 ovules when fully developed. Surrounding the pistil are 200–300 yellow stamens, and both the male and female reproductive organs are protected by 10–35 petals and sepals. Anthesis of individual flowers usually lasts about three days on sunny days. On the first day, the flower begins to open at about 03:00 in the morning and attains its maximum diameter (about 6
cm) at about 05:30. The flower remains at this diameter until about 09:00, after which the flower gradually closes and is completely closed at about 15:00. Substantial fluid is secreted on the stigma surface from 04:00 to 06:00 and the stigma is a bright yellow color, which indicates that the stigma receptivity is high during this period. The anthers do not dehisce on the first day, therefore the pistils mature before the stamens in an individual flower (termed ‘protogyny’). On the second day of anthesis, the process of flower opening is very similar to that on the first day and a maximum diameter of about 10
cm is attained. The anthers dehisce to release quantities of mature pollen from about 04:00 to 05:30. The stigma surface becomes dry and brown, which indicates the loss of stigma receptivity. On the third day of anthesis, the flower begins to open at about 05:00 and is fully open with a maximum diameter of more than 12
cm in diameter at about 10:00. Subsequently, the petals and stamens abscise. The stigma becomes severely dry and brown, and its receptivity is completely lost. An individual flower usually produces about one million pollen grains. Thus the pollen:ovule ratio (P/O) is 30,000-50,000.
The study site is about 33 hectares in area and contains many pools. Many lotus cultivars, including the four included in the present study, were planted in the pools. Therefore, all of the lotus cultivars growing in at study site were possible sources of pollen for “naturally pollinated flowers” in the present study. Bees, flies and beetles may be effective pollinators at the study location.