EFFECTS OF HAND POLLINATION ON FRUIT AND SEED SET IN THE ENDANGERED PLANT DISANTHUS CERCIDIFOLIUS VAR. LONGIPES

XIAO Yi-an, LI Xiao-hong, ZENG Jian-jun, HU Wen-hai, HU Xue-hua, ZHOU Bing



EFFECTS OF HAND POLLINATION ON FRUIT AND SEED SET IN THE ENDANGERED PLANT

*XIAO Yi-an1, 2, LI Xiao-hong1, 2, ZENG Jian-jun1, 2, HU Wen-hai1, 2, HU Xue-hua1, 2, ZHOU Bing1, 2

(1. School of Life Sciences, Jinggangshan University, Ji’an, Jiangxi 343009, China;2. The Key Laboratory for Biodiversity Science and Ecological Engineering of Jiangxi Province, Ji’an, Jiangxi 343009, China)

Pollen and resource availability are main factors that affect the reproductive success of flowering plants. Hand pollination experiment was conducted to investigate the patterns of female reproduction in the endangered plantMaxim. var.H.T.Chang (Hamamelidaceae). The results showed across-pollination induced significant differences between early-blooming flowers and late-blooming flowers so far as fruit-set and seed-set per plant concerned, and significantly increased the seed weight per plant and per fruit respectively. But self-pollination significantly decreased the fruit-set and seed-set. A trade-off between the number of seeds and weight per seed exists under hand-pollination but not under nature pollination.

pollen specificity; fruit set; seed set; trade-off;Maxim. var.H.T.Chang

Plants show great diversity in their patterns of sexual expression[1], and over the past decades many effort has been devoted to understanding of this diversity[1-2]. On the other hand, often only a small proportion of flowers become mature fruits, and in which only an average of one to five flowers can produce seeds. For instance, 50~85% of ovules in mature fruits can produce seeds, such as plants ofand[3-4]. The proportion of flowers that produce fruits, namely fruit level, is highly relevant to a plant’s reproductive success[5]. “Excess” flowers seem to have little contribution to fruit and seed-set[6-7]. This phenomenum appears usually because of initial overproduction of flowers and later reduction of the fruit yield due to limited resource capacity for fruit maturation[6]. Many ecological studies suggested that, in flowering plants, low seed-set can result from limitation of resource availability[8-10], low pollen source[11-12], pollen and/or pollinator limitation[13-16], low flower or seed predation[17], adverse climatic conditions, and/or genetic factors[18-19]. During last two decades, a surge of studies has been devoted to assessing the relative importance of those factors. However, most of the previous discussions of pollen and resource limitations have focused mainly on fruit set, and little attention has been paid to seed set, seed biomass per plant[15, 20].

The small treeMaxim. var.H.T.Chang (Hamamelidaceae), like many other plant species[5], produce many flowers, but only few flowers can produce fruits. It is hypothesized here that pollen and resource affect the ratio of flower to fruit, and a trade-off between the number of seeds and weight per seed exists. We tested this hypothesis in a hand pollination experiments.

1 Materials and Methods

1.1 Study area

This study was done on the south-facing slope of Caijiatian (26°46′N, 113°53′E) at 810 m elevation in Jinggangshan of Jiangxi province of China, which is in the middle of Luoxiao Mountains region where vegetation is the natural secondary forest or shrub after the subtropical evergreen broad-leaved forest was destroyed[21]. The plots were dominated byMaxim. var.H.T.Chang (Hamamelidaceae),,, etc. And D.lies in the canopy layer.

The soil in this area is mainly made of red brown soil. Its average annual temperature is 15.1℃, the average temperature is 3.9℃ in January, the mean temperature of July is 23.7℃, the most extreme low-temperature is -13.4℃, and the most extreme high temperature is 35.2℃. The annual precipitation is 1, 872 mm, the frost-free period is 247 to 263 days. It is cool, rainy, and is of heavy clouds and high humidity.

1.2 Study species

Maxim. var.H.T.Chang (Hamamelidaceae) (hereafter, it is referred to as its genus name,), a plant species that only occurs in a few counties in Hunan, Jiangxi and Zhejiang Provinces of China and with relatively small numbers of plants. Therefore it has been ranked as a 2nd Class endangered species for conservation in China[22-23].is a small tree (up to 3m in height), has two inflorescences growing in opposite directions in the axil at the same node. Each inflorescence has two opposite bisexual flowers without pedicels. The flower is of 1.0cm to 2.0cm in diameter and sub rotund capsule of about 1.5cm in diameter and lasts 6 to 7 days[24]. The average total ouvals are 12 per flower. In this area, the flowering period extends from September to November, and the flowering span of the individuals is about 49~55 days. The mature fruits appear in the July and August in the next year.

1.3 Hand-pollination experiment

We employed hand pollination experiments in addition to assessing whether the amounts and source of pollen reaching stigmas constrain fruit and seed set.

In September, both in 2002 and 2003, we randomly selected 90 flowering plants ofwith similar initiative situation in the wild population in the study area. About 520 inflorescences (1040 flowers) on 30 reproductive branches were marked on each plant, respectively. 30 plants were hand pollinated early-blooming flowers on 16~18 September, and half of these plants were pollinated with cross-pollen, and the others were pollinated with self-pollen. 30 plants were hand pollinated late-blooming flowers on 6~8 October, and half of these plants were pollinated with cross-pollen, and the others were pollinated with self-pollen. The other 30 plants were natural pollination as control. In this study, the first 50% of flowers to open on a plant were identified as the “early blooming flowers”, and the late 50% as “late blooming flowers”.

We collected all of mature fruits on each marked branch and counted the number of seeds in each fruit. The dry weight of each fruit and seed was weighted respectively. And the parameters in each treatment as follows were calculated: fruit set, seed set, seed dry weight per plant, number of fruit per plant (only the marked branches), number of seed per plant and per fruit. The data were analyzed using one-way ANOVA followed by Tukey’s HSD tests to investigate the differences among the pollen pollination treatments.

1.4 Data analysis

The data were analyzed using two one-way ANOVA followed by Tukey’s HSD tests to investigate the differences among the flower removal treatments.

2 Results

2.1 Effects of pollen source on fruit-set and seed-set

In the year 2002, cross-pollination increased fruit-set (Fig.1) and seed-set per plant not only in early-blooming flowers (hand pollinated flowers were (40.75±3.269)% and (37.08±1.313)% respectively, controls were(5.73±0.528)%and (13.6±0.605)%, but also in late-blooming flowers(hand pollinated flowers are(15.44±3.119)%and(20.5±0.798) %respectively, controls were (2.58±0.531)% and(6.70±0.425)%. In the 2ndyear, the average fruit-set from cross- pollination was 53.44%, which was found to be significantly higher than that of control. There were significant differences between early-blooming flowers and late-blooming flowers so far as fruit-set and seed-set per plant concerned. But the self-pollination significantly decreased the fruit-set and seed-set (see Fig.1), and there was no significant difference between early-blooming flowers and late-blooming flowers.

Fig.1 Effects of pollen source on fruit-set and seed-set per plant in 2002 and 2003. (A, B) Fruit set per plant. Data shows means ± SE for all plants in each treatment. (C, D) Seed set per fruit, data show means ± SE for all fruits in each treatment. Symbols above each subfigure show significance levels of the F test among levels of each factor in an ANOVA. N.S. = not significant.

2.2 Effects of pollen source on weight of mature seeds

Cross-pollination also significantly increased the seed weight per plant and per fruit respectively (Fig.2). There were significant differences between early-blooming and late-blooming flowers in seed weight per plant and seed weight per fruit. Self-pollination significantly decreased the seed weight per plant and per fruit. However, there was no significant difference between early-blooming and late-blooming flowers.

Fig.2 Effects of pollen source on weight of mature seeds per plant and per fruit in 2002 and 2003. (A, B) Weight of mature seeds per plant, data show means ± SE for all plants in each treatment. (C, D) Weight of mature seeds per fruit, data show means ± SE for all fruits in each treatment. Symbols above each subfigure show significance levels of the F test among levels of each factor in an ANOVA. N.S. = not significant.

*< 0.05,**< 0.01.

3 Discussion

Willson and Rathcke[25]argued that the excess hermaphroditic flower production usually attributed to selection, acting through male function[27](Burd, 1998). Most of the previous researches on pollen and resource limitations have focused mainly on fruit set; little attention was paid to seed set, biomass of seed per plant. It was interpreted as that the levels of pollination were not affected by the number of flowers produced within the same season, although elevated seed production can cost a plant or population in terms of flower production in future years. However this argument depends on the types of inflorescence[15, 20].

3.1 Pollen specificity and resource limitation

Data collected from present investigation showed a considerable significant effect of pollen specificity on overall fruit set and seed set. It was also observed in the present study that the source of pollen significantly affected the fruit-set[26].The selfing lowered fruit-set but crossing did not. This indicates the existence of strong self-incompatibility mechanisms in. Many studies also have showed the existence of strong self-incompatibility[27-29],and if there is a complicated self-i may even become a stronger constraint for fruit production[30]compatibility system, self-pollination would have lead to pollen specificity[27, 31], and pollen specificity. The fruit and seed-set ofobserved in the current study was very low[26], which suggesting that the self-incompatibility mechanism is functional, but of course not absolute. In, after flowering, the selfed-pollen may cover the stigmas, so the cross-pollen cannot disperse on it. So most proportion of stigma would not have received enough crossed pollen to have fruit-set and seed-set.

In the 2ndyears where it was observed thatplants were frequently visited by few insects, and in fact the efficiency of wind pollination was very low. Pollen quantity may be a limiting factor if pollinators are scarce or inefficient[32-34]. The results of hand-pollination indicate the evidence of pollen specificity and; pollen reaches stigmas, germinates, and pollen tube growth occurs. It was found to be the source and not the quantity of pollen grains that limits the fruit-set and seed-set in our study in the 2ndyears.

Some studies showed that the pollen limitation is irrelevant to overall reproductive allocation and sex allocation in density-dependent populations. Increasing the extent of pollen limitation will lead to more resource allocated to ovule production, amplifying the effect of pollen limitation[35].According to Haig and Westoby’s[36]model, a chronic pollen limitation situation should favor traits that either increase pollen receipt or decrease the dependence on pollinators for pollen receipt. However, some data also showed that pollen limitation may not be spatially constant, even within the same population[37]. In the present study, hand-pollination increased the fruit-set in, and the validity of pollen would strictly limit the fruit-set. That indicated the excess flowers would act as male organs and pollen donors[7, 38].

3.2 Trade-offs between weight per seed and number of seed per fruit

There did not exist trade-off between the weight per seed and number of seed per fruit with nature pollination (Fig.3A), but after hand-pollination with across pollen, the trade-offs between them existed (Fig.3B). After hand-pollinating, the fruit-set and seed-set were significantly increasing (Fig.1A, B). But the resources which the plant can provide to the fruits or seeds in it are limited, so that, with the seeds increasing, the trade-off would exist between the weight per seed and the number of seed per fruit. However, the resource which the plant provide were enough to the seeds need under nature pollination, so that the weight per seed may correspondingly stabilization, and there was no correlation between the weight per seed and number of seed per fruit.

Fig.3 Effects of pollen source on the trade-off between number of per fruit and weight per seed. A and B, are natural pollination and hand-pollination with across pollen, respectively.

Acknowledgements: We express our appreciation to the National Natural Science Foundation of China (No 30560025, 31060069) and the Program for New Century Excellent Talents in University of China (No NCET-07-0385).

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人工授粉对濒危植物长柄双花木结果率和结籽率的影响

*肖宜安1, 2，李晓红1, 2，曾建军1, 2，胡文海1, 2，胡雪华1, 2，周 兵1, 2

(1.井冈山大学生命科学学院，江西，吉安 343009; 2. 江西省生物多样性与生态工程重点实验室，江西，吉安 343009)

花粉和资源有效性是影响有花植物生殖成功的关键因素。本研究探讨了人工授粉对濒危植物长柄双花木雌性生殖模式，尤其是对其结果率、结籽率以及果实和种子重量的影响。研究结果表明：异花授粉提高了长柄双花木的结果率和结籽率，且前期花的结果率、结籽率均显著高于后期花的；而自花授粉不但显著降低了其结果率和结籽率，同时在前期和后期花的结果率、结籽率之间均无显著差异。异花授粉显著提高了单株植物种子重量及单果种子重量，而自花授粉则显著降低了它们的重量。异花授粉后前期花的果实重量及单果种子重量均显著高于后期花的。同时，异花人工授粉后种子数量和单粒种子重量之间存在权衡，而自然授粉条件下则不存在这种权衡。

花粉特异性；结果率；结籽率；权衡；长柄双花木

1674-8085(2012)03-0096-06

O945.6+5

A

10.3969/j.issn.1674-8085.2012.03.022

2011-03-18;

2011-04-11

Natural Science Foundation of China (30560025);Program for New Century Excellent Talents in University in China (No.NCET-07-0385)

XIAO Yi-an (1968-)，Male，Yongfeng, Jiangxi, PhD, Professor, Master Instructor,Research interesting is plant ecology and conservation biology (Email: iyanxiao@yahoo.com.cn)

LI Xiao-hong (1970-)，Female，Jishui, Jiangxi, Bachelor, Senior Experimentalist,Research interesting is plant ecology and plant physiology (Email: lxhxya@yahoo.com.cn)

ZENG Jian-jun (1974-)，Female，Wanan, Jiangxi,PhD, Associate Professor, Research interesting is plant ecology and plant physiology (Email: jianjunzeng@139.com)

HU Wen-hai (1973-)，Male，Jian, Jiangxi,PhD, Professor, Master Instructor, Research interesting is plant ecology and plant physiology (Email: huwenhaicy@yahoo.com.cn)

HU Xue-hua (1977-)，Female，Xiajiang, Jiangxi, Master, Experimentalist,Research interesting is plant ecology(Email: huxuehua1210@126.com)

ZHOU Bing (1977-)，Male，Huangmei, Hubei,PhD, Associate Professor,Research interesting is plant ecology and conservation biology (Email: zhoubing113@126.com.)