Abnormalities in pistil development result in low seed set in Leymus chinensis (Poaceae)

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Abstract

Megasporogenesis and megagametogenesis in Leymus chinensis and the callose deposition during these developmental processes were investigated. In addition, morphological and histochemical studies of pistils at anthesis and pollen behavior on the stigmas after pollination were examined. The results indicate that embryo sac development and callose deposition pattern of this grass follow the archetypal Polygonum type. Nearly half of the pistils developed abnormally in megagametogenesis, while only 8.6% of abnormalities occurred in megasporogenesis. Over 47% of pistils at anthesis were abnormal in appearance. By 24 h after anthesis, many pollen grains had germinated on the stigmas of normal pistils. The high percentage of abnormal pistils and their low capacity to capture pollen grains may be the main factor in the low seed set of L. chinensis.

Introduction

Leymus chinensis (Trin.) Tzvel. (Poaceae) is a perennial rhizome grass that is widely distributed in the eastern end of the Eurasian steppe, from North Korea westward to Mongolia and northern China and north-westward to Siberia (Kuo, 1987). This grass is economically and ecologically important because it is rich in vitamins, high quality protein, minerals, and carbohydrates; moreover, it grows rapidly and is highly tolerant of arid conditions (Huang et al., 2002). However, its low rate of sexual reproduction drastically limits its propagation (Huang et al., 2002). The average rate of seed set in this species is about 56.7% in artificially constructed grasslands and less than 25% in natural grasslands (Wang, 1998).

Because L. chinensis is an economically and ecologically important grass, it has received considerable attention in recent decades (Pan and Sun, 1986; Wang, 2001; Song et al., 2003). However, most studies of this species have focused on the effects of ecological factors on its seed production, such as climate (Wang et al., 2003), water use (Song et al., 2003), and nutrient uptake (Wang, 2001). Only a few investigations of the sexual reproduction of this species have been reported. Pan and Sun (1986) and Ma et al. (1984) found that over 83.5% of the pollen grains were viable and concluded that pollen grain viability is not a factor in the low seed yield. We have previously investigated the pollination, microsporogenesis, and callose deposition during microsporogenesis in this species, and have found that pistil receptivity and pollen longevity may have slight effects on the low seed set but are not the main factors. We have also observed that microsporogenesis and callose deposition generally appear normal, and have thus speculated that male reproductive development is not a factor in the low seed output (Huang et al., 2002, Huang et al., 2004; Teng et al., 2005). Therefore, the causes of the low rate of sexual reproduction in this grass are still unclear, largely because many aspects of the reproductive processes, especially the female reproductive development, remain unknown. So far, only a few investigations on the female reproductive processes have been reported, and almost no reports have examined the relationship between these processes and the low seed production in this vital species (Wei and Shen, 2003).

As a continuation of our studies of the past several years (Huang et al., 2002, Huang et al., 2004; Teng et al., 2005; Yang et al., 2001), this study is part of a series examining sporogenesis and gametogenesis, callose deposition during these developmental processes, pollination and fertilization, and embryogenesis in L. chinensis. Our overall aim was to identify the main causes of the low seed yield and to provide knowledge that will be useful in increasing the seed production of this important grass.

Section snippets

Plant material and growth conditions

Plants of L. chinensis growing in the south of the Xilin River basin, Inner Mongolia, China (43° 32′ 58″ N, 116° 40′ 34″ E), at approximately 1265 m ASL (above sea level) were used in the study. The average rate of seed set is less than 30%. This site has a temperate, semi-arid climate with a mean annual precipitation of 350 mm and a mean annual temperature of 0.3 °C (Chen, 1988).

Examination of pistil development

Spikes were collected daily from June 8 to 28, 2002 and immediately immersed in FAA solution (formalin 5 mL, acetic acid 5 mL,

Flower morphology

L. chinensis flowers are hermaphroditic and arranged in compound spikes (Huang et al., 2004). The length of spikes ranges from 11.7 to 14.2 cm (average 13.4±0.9 cm). Each spike comprises 32.1±3.7 spikelets. There are approximately 6.1±1.6 flowers or florets per spikelet. Each flower has an outer bract (lemma) and an inner bract (palea) at its base, and contains two lodicules and one pistil surrounded by three anthers (Fig. 1). The mature pistil has two feathery stigmas and a hairy ovary that

Discussion

The embryo sac of L. chinensis follows the Polygonum type of development (Johri et al., 1992; Reiser and Fischer, 1993), as do those of all other Poaceae species that have been examined (Johri et al., 1992) and over 80% of angiosperms (Batygina, 2002). In most L. chinensis florets, microsporogenesis and megasporogenesis occur nearly simultaneously within the floret. In our study, microsporogenesis and pollen development took place from June 12 to 26 (Teng et al., 2005), and megasporogenesis and

Acknowledgments

This work was supported by the Key Project of the Chinese Academy of Sciences (KSCX1-08) and National Science Fund of China for Distinguished Young Scholars (30225005). We thank Dr. Arthur Benson and other anonymous botanists for their critical comments on the first draft of this manuscript. We also thank the Inner Mongolian Grassland Ecosystem Research Station for providing field and laboratory facilities.

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