For. Stud. China, 2008, 10(3): 149–152 DOI 10.1007/s11632-008-0029-3

RESEARCH ARTICLE

Seedling test and genetic analysis of white poplar hybrid clones LI Bo1, JIANG Xi-bing1, ZHANG You-hui2, ZHANG Zhi-yi1*, LI Shan-wen1, AN Xin-min1 1

Key Laboratory for Genetics and Breeding in Forest Trees and Ornamental Plants of Ministry of Education, Beijing Forestry University, Beijing 100083, P. R. China 2 Guanxian Forestry Nursery of Shandong Province, Guanxian 252500, P. R. China Abstract Cross breeding strategies are very efficient for gaining new and superior genotypes. Ninety-eight new white poplar hybrid clones produced from 12 cross combinations within the Section Leuce Duby were studied using genetic analysis and seedling tests. We exploited the wide variation that exists in this population and found that the differences among diameter at breast height (DBH), root collar diameter (RCD) and height (H) were statistically extremely significant. The repeatability of clones of these measured traits ranged from 0.947–0.967, which indicated that these traits were strongly controlled by genetic factors. Based on multiple comparisons, a total of 25 clones showed better performance in growth than the control cultivar. These 25 clones were from six different cross combinations, which can guarantee a larger genetic background for future new clone promotion projects. This study provides a simple overview on these clones and can guide us to carry out subsequent selection plans. Key words white poplar, multi-clonal plantation, seedling test, genetic analysis

1

Introduction

Populus L. is a pioneering tree species of the forest industry, which has been planted widely in China because of its excellent properties of fast growth, adaptability and ease of propagation (Zhang et al., 1992, Li et al., 2005b). In China, there are about 6670000 hm2 of poplar plantations, four times as many as the total number in the rest of the world. This area accounts for 1/5 of all tree plantations in China (Wang, 1995). Among the cultivars of this genus, most clones were produced by cross breeding (Zhu et al., 1995; Wang and Li, 2001). Crossbreeding strategy is very efficient for producing new and superior genotypes which can pass the excellent traits of two parents onto progenies and provide the progenies with the hybrid vigor, called as “heterosis”. Over the long term, field tests are viewed as the best way to select clones with good performance. Although marker assisted selection developed rapidly in recent years, field tests are still the most accurate and direct way to estimate the performance of clones. No doubt, field tests need long term observations, which have become the largest drawback of this method, require large areas of land, are labor intensive and directly or indirectly increase the cost of breeding. Early-stage selection has been used to solve the problem for some time; however, the results have not been satisfactory by a long shot. In most countries, some other potential questions still exist, including monoclonal plantations, aging of promoted clones and so on, which will destroy genetic diversity and threaten the stability of the bio-system. In order to overcome these problems, it is urgent that new clones are promoted and to enhance multi-clonal *

Author for correspondence. E-mail: [email protected]

plantations. Therefore, our laboratory began collecting white poplar breeding material which has largely been found in northern China since the 1990s. In 2001, a series of crossbreeding experiments were carried out and 12 promising combinations were chosen from more than 30 controlled pollinations and over 3000 progenies have been harvested (Li, 2004). In 2002 and 2003, 98 elite individuals were obtained under rigid selection standards, including consideration for growth, leaf trait, stem form, etc (Li et al., 2004, 2005a). In the spring of 2004, these 98 “super” individuals had been propagated by grafting and then all 98 clones were arranged in a completely randomized block design in Guanxian County, Shandong Province. In this study, the growth traits, including height, diameter of basal area and diameter at breast height were investigated during the first growing season. This first year performance provided a simple overview of these clones and guided us to carry out the following selection plans.

2 2.1

Materials and methods Experimental site

This clone selection trial was carried out in forestry nurseries of Guanxian County, Shandong Province. 2.2

Materials and design

A total of 98 white poplar hybrid elites were chosen from 12 cross combinations after a two-year family

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test. During the following spring, up to 120 individuals per elite clone were obtained by grafting. A completely randomized block design was adopted for clonal tests, with four replications, 30 seedlings per block and a plant spacing of 30 cm × 100 cm. The details of the total cross combinations and the number of clones are presented in Table 1. 2.3

Data analysis

Growth traits including height (H), root collar diameter (RCD) and diameter at breast height (DBH) were investigated. The mean values and standard deviations of H, RCD and DBH were obtained, followed by an analysis of variance and multiple comparisons of these three traits. The data were analyzed using statistical methods from the SAS 8.0 software package (SAS Institute Inc.).

3

Results

3.1 Mean values and standard deviations of RCD, DBH and H Mean values, standard deviations (SD), coefficients of variation (CV) of RCD, DBH and H are listed in Table 2. The mean values of RCD, DBH and H were 1.9 cm, 1.2 cm and 291 cm respectively. The RCD of this population varied from 1.2 to 2.7 cm and DBH ranged from 0.7 to 2.1 among the 98 clones. The maximum value of H is more than twice the minimum. Standard deviations of RCD, DBH and H were 0.331 cm, 0.287 cm and 53.1 cm respectively. The coefficients of Table 1 No. 1 2 3 4 5 6 7 8 9 10 11 12 Total

Cross combinations and number of clones

Table 2

Population characteristics Mean value (cm) 1.9 1.2 291

RCD DBH H

variation of RCD, DBH and H were 17.85%, 22.25% and 18.28%. The results indicate that the growth traits i.e., RCD, DBH and H varied dramatically among these 98 clones during the first growing season. The large variation existing in this population made it feasible to select outstanding clones. On the one hand, the results reflect the great diversity in this population and this variation was found just during the first year; on the other hand, this diversity further indicated that this population was established on the basis of extensive genetic variation. It satisfied our breeding aims of multi-clone selection. 3.2 Analysis of variance and estimates of repeatability Analysis of variance and the estimation of repeatability of three traits are shown in Table 3. The variation in all three traits were statistically extremely significant among the 98 clones. The result also indicates that extensive variation exists in this population, which provides for a large room of clone selection. The repeatability of RCD, DBH and H were 0.962, 0.950 and 0.967 respectively, which indicates that these growth traits are strongly controlled by genetic factors. 3.3

Multiple comparisons of growth traits

Multiple comparisons of RCD, DBH and H were carried out among the means of the 98 clones and a control cultivar LM50. Part of the results is presented in Table 4. For RCD, the best 10 clones are listed in Ta-

Cross combinations (P. tomentosa × P. bolleana) × P. bolleana (P. tomentosa × P. bolleana) × P. tomentosa ‘LM50’ (P. tomentosa × P. bolleana) × (P. alba × P. glandulosa) (P. alba × P. glandulosa ‘No. 1’) × P. bolleana (P. alba × P. glandulosa ‘No. 2’) × P. tomentosa ‘LM50’ (P. alba × P. glandulosa ‘No. 2’) × P. bolleana (P. alba × P. tomentosa ) × (P. alba × P. glandulosa) (P. alba × P. tomentosa ) × P. bolleana (P. alba × P. tomentosa) × P. tomentosa ‘LM50’ P. alba × P. bolleana (P. alba × P. bolleana) × P. bolleana (P. alba × P. bolleana) × (P. alba × P. glandulosa ‘No. 2’)

Maximum (cm) 2.7 2.1 440

Minimum (cm) 1.2 0.7 190

Number of clones 8 6 14 12 8 6 5 16 7 12 2 2 98

SD (cm) 0.331 0.287 53.1

CV (%) 17.85 22.25 18.28

LI Bo et al.: Seedling test and genetic analysis of white poplar hybrid clones Analysis of variance and repeatability Variation resource RCD Clone DBH Clone H Clone Note: *** means significant at 1% level.

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Table 3

Table 4 No. 51 45 33 74 46 17 44 25 41 27

Multiple comparisons: incomplete results RCD (cm) Mean 0.05 level No. 2.68 A 51 2.53 AB 44 2.50 AB 17 2.48 BC 27 2.43 BCD 41 2.40 BCD 33 2.40 BCD 74 2.38 BCDE 45 2.38 BCDE 10 2.38 BCDE 76

F value 26.10*** 20.18*** 29.11***

DBH (cm) Mean 0.05 level 2.13 A 1.88 B 1.83 BC 1.80 BCD 1.78 BCD 1.75 BCDE 1.75 BCDE 1.75 BCDE 1.73 BCDEF 1.70 BCDEF

ble 4, which indicated that the No. 51 clone was the best and was significantly different from all other clones. For H, No. 51 still had best performance among all clones and showed significant difference with others. Generally, No. 51 had the best performance on the three growth traits. According to the result of multiple comparisons of H with LM50, a total of 25 clone had better growth performance than LM50. Of these, 17 clones showed significant differences with LM50. The height growth of No.51 was 36.2% better than that of LM50.

4

Discussion

Crossbreeding within the Section Leuce Duby in China began in 1946 (Ye, 1955). In the following 50 years, many promising clones were selected through field testing. For example, by using P. alba as female parent and P. hopeiensis × P. davidiana as male parent, the superior hybrid of P. alba × (P. hopeiensis × P. davidiana) was bred by Wang et al. (1987). This hybrid had 9.8% more volume than P. hopeiensis and a better cutting survival rate and resistance to diseases and pests (Wang et al., 1987). Hybrid clone 1333, which grew twice as fast as P. davidiana when it was 14 years old, was screened out from the combination of P. alba × P. davidiana (Liu and Zhao, 1991). The magnitude of genetic gain through clone selection depends on variation and inheritance of desired traits (Foster et al., 1986). In our study, three traits, RCD, DBH and H, were chosen to reflect the growth performance of different clones, on the basis of which we carried out selection. The first growing season performance in growth showed significant differences

p>F 0.0001 0.0001 0.0001

No. 51 25 27 50 57 45 41 17 52 71

Repeatability 0.962 0.950 0.967

H (cm) Mean 442 402 397 393 390 385 380 367 362 355

0.05 level A B B BC BCD BCD BCDE CDEF DEFG EFGH

among the 98 clones. Analyses of variance of the variables RCD, DBH and H indicated that variation in these three variables was extremely significant among the 98 clones. Genetic analysis indicated that these growth traits were strongly controlled by heredity. Therefore, selecting clones by these traits in this population with great variation can guarantee successful and efficient seedling tests. In these three traits, seedling height (H) had the highest repeatability and was viewed as the most important trait to select elite clones. Multiple comparisons showed that 25 clones grew better than the local cultivar LM50, which was also the male tree for several cross combinations in our study (Table 1). Among the 25 clones, 17 were significantly different from LM50. More attention should be paid to these 25 clones in subsequent testing programs. Generally, about the best 10% of total clones are selected for the first phase in a clonal trail. In this study, more than 25% of the clones were selected because a conservative strategy was taken in our selection program for the first step. The selection intensity will be strengthened when field test are carried out. Single clones of poplars have been propagated extensively. There have been disease problems and the overuse of single clones and use of large monoclonal plantations has been questioned (Stelzer and Goldfarb, 1997). Multi-clone plantation is a more efficient way to change the situation of monoclonal promotion which specifically threatens biodiversity and helps the development of diseases, especially when the plantations need to be kept for a long time in order to play a role in maintaining the balance of the bio-system. In this study, the 25 genetic clones with potential for promotion have been selected from six different cross

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combinations, which enlarged the genetic basis of breeding population. Compared with the clones from the same cross combination, clones from different combinations showed larger variation not only in growth but in other phenotypes (data not shown). Compared with black poplars, white poplars have a better shape and a longer life cycle and can, therefore, better serve as a plantation for safeguarding the environment. We hope the clones selected can be used not only in pulp production, but in ecosystem conservation as well. Usually, poplars have a long rotation period and different growth rhythms, so a relative long period of observations is needed to judge the clones. Observations of only one year are insufficient to draw an accurate conclusion on the performance of these clones. However, it can provide us with an overview on this population and help make a scientific plan for field selection. Clone selection is a stepwise process of evaluation. It starts with a large number of genetically different progenies and ends up with a few clones with commercial utility (Fikret and Ferit, 2004). On one hand, this population with 98 clones has been maintained for future investigations. On the other hand, these 25 clones will be propagated again and this study will be repeated next year in various locations under different environmental conditions. This strategy can save the cost of selection by shrinking the size of the original population and at the same time avoid the risk of dropping other possible good clones. These 25 clones were chosen only on the basis of one year of observation, so a repetition of this study can ensure the accuracy of our results and produce more seedlings for following field tests in different locations. In addition, during subsequent years, besides the growth traits, other traits such as morphological and physiological traits and disease resistance should be investigated.

Acknowledgements This study is supported by National Natural Sciences Foundation of China (Grant No. 30571516) and National Project of Science and Technology for the 11th Five-Year Plan (Grant No. 2006BAD01A1502). The authors thank Dr. YAO Na, Dr. SUN Feng-bo and Mrs. PENG for their helps in field investigation and

Forestry Studies in China, Vol.10, No.3, 2008

we also thank Ms. FENG Xia-lian and QIAO Meng-ji and TAO Feng-jie for their kind help with data input and suggestions for data analysis.

References Fikret I, Ferit T. 2004. Variation in juvenile traits of natural black poplar (Populus nigra L.) clones in Turkey. New For, 27: 175–187 Foster G S. 1986. Provenance variation of western cottonwood in the lower Mississippi valley. Silva Genet, 35: 32–38 Li S W. 2004. Study on Genetic Variation of Parents and Progenies and Their Molecular Basis in Populus. PhD Dissertation of Beijing Forestry University (in Chinese) Li S W, Zhang Z Y, He C Z, An X M, Yu Z S, Li B L. 2005a. Variation analysis of seed and seedling traits of cross combination progenies in Populus. For Stud China. 7(3): 61-69 Li S W, Zhang Z Y, Luo J M, He C Z, Pu Y S, An X M. 2005b. Progress and strategies in cross breeding of poplar in China. For Stud China, 7(3): 54-60 Liu P L, Zhao J G. 1991 Selection and breeding studies of gene resources of aspen. In: Department of Science and Technology, Ministry of Forestry (ed.). Genetic Improvement on Broad-leaf Trees. Beijing: Scientific and Technical Documents Press. 116-141 (in Chinese) Stelzer H E, Goldfarb B. 1997. Implementing clonal forestry in south eastern United States. Can J For Res, 27: 442-446 Ye P Z. 1955. White poplar breeding. Sci Silv Sin, 1(1): 37-46 (in Chinese) Zhang Z Y, Zhu Z T, Mueller-Strarck G, Hattemer H H. 1992. Studying isozymes gene markers in clones of Populus tomentosa Carr. J Beijing For Univ, 14(3): 9-18 (in Chinese with English abstract) Zhu Z T, Lin H B, Kang X Y. 1995. Studies on allotriploid breeding of Populus tomentosa B301 clones. Sci Silv Sin, 31(6): 499-505 (in Chinese with English abstract) Wang S J. 1995. Advance in Poplar Research. Beijing: Chinese Forestry Publishing House (in Chinese) Wang S Y, Li G H, Qiang X Y. 1987. A characteristic analysis of new hybrid clones of the Leuce poplars. J Ningxia Agric For Sci Tech, 1: 19-27 (in Chinese) Wang Z R, Li Z Q. 2001. New progress in research of forest tree cross breeding. J Northwest For Univ, 16(4): 93-96 (in Chinese with English abstract) (Received April 2, 2008

Accepted June 10, 2008)