Growth variability in a Senegalese provenance of Acacia nilotica ssp. tomentosa

June 9, 2017 | Autor: E. Wolde-meskel | Categoria: Sample Size, Seed size, Agroforestry Systems, Leaf Area, Weeks After Planting, Seedling Growth
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Agroforestry Systems 48: 207–213, 2000.  2000 Kluwer Academic Publishers. Printed in the Netherlands.

Growth variability in a Senegalese provenance of Acacia nilotica ssp. tomentosa E. WOLDE-MESKEL1, * and F. L. SINCLAIR2 1 Department of Plant Production and Dryland Farming, Awassa College of Agriculture, P.O. Box 5, Awassa, Ethiopia; 2 School of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd, LL57 2UW, UK (*Author for correspondence: Agricultural University of Norway, Department of Biotechnology, P.O. Box 5040, 1432 Ås, Norway; E-mail: [email protected])

Key words: biomass, provenance, sample size, seed size Abstract. Growth variability and effect of seed size on growth of seedlings of Acacia nilotica ssp. tomentosa of a Senegalese provenance were studied in a greenhouse experiment. Seedlings were raised from a sample of 52 seeds with a seed weight range of 0.070 g to 0.258 g; and a range of growth variables were measured on seedlings harvested 11 weeks after planting. Based on the sample mean and variance, the sample size required to estimate the parameter (mean) of the growth variables was determined. Seedlings showed a large variability in growth. There were five-fold and three-fold differences between seedlings in leaf area and total dry weight, respectively. Seed weight has little effect on seedling growth. The number of replications required to estimate a parameter mean of the different growth variables, within ±20% margin of error at P = 0.05, ranged from nine to 24. The observed growth variability was indicated to be an important biological variable which could be used to improve growth and yield in A. nilotica.

Introduction Less variability in seedling growth parameters is desirable for establishment of managed plantations and when conducting experiments. In experiments, fewer replications will be required; and experimental differences can be more easily related to treatment effects if inherent differences in seedling growth are less. Conversely, trees useful in agroforestry are often wild and undomesticated, exhibiting high variability. Large within and between provenance variation in seed and seedling growth have been observed in different agroforestry tree species (Salazar, 1986; Ernst, 1988 Ngulube, 1989; Sniezko and Stewart, 1989; Joly et al., 1992; Wanyancha et al., 1994). Contrary to the need for uniformity in intensively managed plantations, there may be advantages in maintaining variability in agroforestry, especially in semi-arid areas where there is large and uncontrollable variation in environmental conditions. The extant variability provides ample biological resources to improve agroforestry trees with respect to growth and yield, nitrogen fixation, or canopy structure. Little experimental work has been done on wild species, such as A. nilotica, utilised in traditional agroforestry systems. It is desirable, therefore, to know the variability in growth so as to indicate their potential for future improve-

208 ment. It is also important to be able to calculate the number of replicates required for experiments investigating growth in relation to factors such as nitrogen fixation, drought, or other treatments. Homogeneity may be improved, and less labour and time may be required to collect data, by using uniform seed size if this is an important factor. The aims of the present study were: (i) to assess the variability in growth in A. nilotica ssp. tomentosa seedlings of a particular provenance and thereby to indicate the potential for improvement through selection, (ii) to examine the extent to which seedling growth in A. nilotica of this ssp. and provenance relates to seed size, and (iii) to determine the minimum sample size required to make acceptable estimates on a range of growth variables.

Materials and methods The experiment was conducted in a heated greenhouse facilities at the University of Wales, Bangor, UK with natural light supplemented from 06:00 to 22:00 by an array of 400 W high pressure sodium vapour lamps 2 m above bench height. Mean day and night temperatures during the experimental period were maintained at 26 and 20 °C, respectively and relative humidity at around 70% (Brinkman Glasshouse Computing Systems, Hull, UK). A 16 hour day length was used and photon flux densities (PFD) were measured across the bench at plant height using a systematically arranged array of six laboratory built quantum sensors, scanned at ten second intervals with hourly means stored by an electronic data logger (21x, Campbell Scientific Ltd, Nottingham, UK). Mean daily PFD ranged from 20 mol m2 day–1 on a cloudy day (with a peak of 750 mol m2 s–1) to 30 mol m2 day–1 on a sunny day (with a peak of 1300 mol m2 s–1). Seeds of Acacia nilotica (L) Willd. ex Del. subsp tomentosa collected from 22 mother trees growing about 50 m apart, in a densely populated natural stand, in the ‘Forêt classée de Richard-Toll’ at ‘Region du fleuve’, Senegal were obtained from the DANIDA Forest Seed Centre, Humlebaek, Denmark, and used for the experiment. The latitude, longitude and altitude of the location of seed collection were 16°28′N, 15°42′W and 4 m a.s.l., respectively. The seeds were collected from young and old trees. The height and DBH measurement of the trees fell between 7–17 m and 23–63 cm, respectively. Trees were also variable in the amount of fruit they bore, length and straightness of their boles. Random samples of 500 seeds from the seedlot were weighed individually and six seed weight classes were established (Table 1). From these, a total of 52 seeds were selected with each seed weight class represented in the same proportion as in the 500 seeds. The seeds were planted in 9 cm (top diameter) pots filled with John Innes compost after surface sterilising with alcohol and scarifying with sand paper. The pots were placed randomly on a bench in the

209 Table 1. Frequency table of seed weight of a Senegalese provenance of A. nilotica ssp. tomentosa. Class limit (g)

Frequency

% of total

Seedlings studied

< 0.1 0.101–0.125 0.126–0.150 0.151–0.175 0.176–0.200 > 0.201

013 069 138 128 097 055

02.6 13.8 27.6 25.6 19.4 11.0

02 07 14 13 10 06

Range = 0.0703–0.2584 g; Mean = 0.1575 g; SD = 0.0325

greenhouse and seedlings were kept well watered throughout the growth period. Seedlings were harvested 11 weeks after planting. A week after germination, the following measurements were made on each seedling: (i) cotyledon size (mm2) (the sum of the product of the length and width of cotyledons) and (ii) seedling height (cm) (from the point of cotyledon attachment to the tip of the shoot). Measurements made at harvest from each seedling were: (i) seedling height (cm), (ii) leaf area (cm2) (measured directly from the detached pinnae using a leaf area meter (Model delta T Devices, Cambridge, UK), and (iii) leaf, stem and root dry mass (g) after oven-drying at 80 °C for 48 hours. Shoot and total dry weight and the root shoot ratio were subsequently calculated. The variation in each growth variable was assessed using the MINITAB statistical package. Correlation analyses were carried out on seed weight in relation to different growth variables, and between pairs of growth variables. The sample variance and the sample mean of each growth variable were used to estimate the minimum sample size (n) required to estimate the mean, within ±20% accuracy at P = 0.05, following Gomez and Gomez (1984). Result and discussion Seedlings showed high variability in growth under uniform conditions in the greenhouse. The coefficient of variation (CV) of the different growth variables ranged from 30% to 50% (Table 2). There was a five-fold difference in leaf area and leaf dry weight, and a three-fold difference in total dry weight between seedlings. There was also high correlation between the different growth variables measured on seedlings (Table 3). Thus, seedlings with larger biomass were taller and had higher leaf and stem biomass whereas smaller seedlings were shorter and had smaller magnitude of leaf and stem biomass. The seed sample used in this study included seeds with two-fold difference in weight (Table 1). Cotyledon size was strongly related to seed weight,

210 Table 2. Variability in the growth variables of a Senegalese provenance of A. nilotica ssp. tomentosa grown under greenhouse conditions in Bangor, UK. Parameters

Mean (± SE)

Seed weight (g) Cotyledon size (mm2) Leaf area (cm2) Leaf dry weight (g) Stem dry weight (g) Shoot dry weight (g) Root dry weight (g) Total dry weight (g) Height, initial (cm) Height, final (cm) Root/shoot ratio

0.159 297 81.3 0.368 0.577 0.945 0.165 1.108 2.49 22.4 0.193

n = 52. CV = coefficient of variation (%). CI = Confidence interval (95%). N = Estimated sample size.

(± (± (± (± (± (± (± (± (± (± (±

0.005) 7.8) 5.7) 0.023) 0.033) 0.053) 0.007) 0.058) 0.13) 0.97) 0.009)

Range (min.–max.)

CV (%)

CI 95%

N

0.070–0.258 162–415 34–201 0.128–0.807 0.265–1.157 0.419–1.965 0.069–0.272 0.536–2.236 0.8–4.4 12.5–40.2 0.093–0.414

22.5 19.0 50.4 45.1 40.8 40.6 30.2 37.7 37.0 30.8 34.0

0.149–0.169 281–313 69.9–92.7 0.322–0.414 0.510–0.641 0.836–1.050 0.152–0.179 0.992–2.225 2.2–2.8 20.8–24.7 0.174–0.211

24 19 16 16 09 14 14 09 11

211 Table 3. Coefficient of correlation (r) of a pair of growth variables of a Senegalese provenance of A. nilotica spp. tomentosa grown under greenhouse conditions in Bangor, UK.

LAR LWT SWT RWT TWT HWT CYZ HT1 HT2

SST

LAR

LWT

SWT

RWT

TWT

HWT

CYZ

HT1

0.273 0.253 0.433* 0.244 0.373* 0.375* 0.834* 0.282 0.435*

0.931* 0.817* 0.622* 0.903* 0.904* 0.207 0.176 0.834*

0.824* 0.655* 0.938* 0.938* 0.244 0.154 0.868*

0.617* 0.963* 0.969* 0.429* 0.416* 0.913*

0.727* 0.662* 0.305 0.311 0.728*

0.996* 0.374* 0.332 0.945*

0.368* 0.322 0.935*

0.391* 0.422*

0.349

SST = Seed weight. LAR = Leaf area. LWT = Leaf dry weight. SWT = Stem dry weight. RWT = Root dry weight. * Significant at P = 0.01.

TWT = Total dry weight. HWT = Shoot dry weight. CYZ = Cotyledon size. HT1 = Height, initial. HT2 = Height at harvest.

indicating that the amount of nutrient for the newly emerging seedling was proportional to the seed weight (Table 3). There was a correlation (P = 0.01) between seed weight and stem weight, seed weight and shoot weight, seed weight and total biomass, and seed weight and final height. However, the correlation was weak and indicated that only less than 18% of the observed differences has been associated to seed weight. Seedlings have been grown under uniform conditions and there was no disparity in germination which might cause physiological variation among seedling in growth. Hence, the observed variation may also be associated with genetic differences among seedlings. Large seed and seedling growth variation (not related to seed size) within provenance have been reported for a number of useful agroforestry tree species, including Acacia albida (Sniezko and Stewart, 1989), A. nilotica ssp. indica of the Indian provenance (Krishan and Toky, 1996), Gliricidia sepium (Salazar, 1986), Brachystegia spiciformis (Ernst, 1988) and for some Central American multipurpose trees (Ngulube, 1988), and these were attributed to genetic differences. Contrary to these, however, variation in seed weight, within the range of that produced by a single parent, was reported to result in proportionate variation in seedling growth in A. nilotica (ssp. and provenance not specified) (Oboho and Ali, 1985) and in the temperate tree Pseudotsuga menziesii (provenance not specified) (Sorensen and Campbell, 1985). The large growth differences observed amongst A. nilotica ssp. tomentosa seedlings of this particular provenance indicate an ample biological resource that is available to improve growth and yield of the tree through selection. This could make significant contribution to easing the prevailing acute fuelwood shortage in sub-Saharan Africa where fuelwood is the main source

212 of household energy and where vegetation growth is relatively slow (FAO, 1983; EDIWB, 1989). The present study indicated that uniform seedling growth may not be achieved by selective use of a specified seed weight class; and randomly taken seeds may be used for experiments involving A. nilotica ssp. tomentosa from this provenance. The sample size required to estimate the mean of the different growth variables within ±20% margin of error ranged from nine to 24 (Table 2). Estimated sample size showed differences depending on the growth variable. A margin of error less than 20% would be desirable for increased precision. However, this is only at the expense of increased replications. Knowledge of the variability of seedlings is, therefore, advisable when studying growth in relation to some factors, such as drought, nutrition or nitrogen fixation, in unimproved wild agroforestry species such as used in the present study. This may improve the quality of information gained from experiments. In conclusion, the study has demonstrated a large variability in growth of A. nilotica ssp. tomentosa of a Senegalese provenance seedlings which was less related to seed weight in a uniform growing conditions. Post nursery field evaluation and further genetic studies will be necessary to elucidate the pattern of variation in this particular provenance. There is a need to use more than 20 seedlings to detect treatment differences when experimenting with A. nilotica seedlings of this particular provenance, which indicated the need to assess variability in growth before experimenting with undomesticated agroforestry species.

Acknowledgements Greenhouse facilities were made available for the experiment under the auspices of project R4181, funded by the UK Overseas Development Administration's Forestry Research Programme. We also thank the DANIDA Forest Seed Centre, Denmark, for supplying the seeds for the experiment.

References EDIWB (Economic Development Institute of the World Bank) (1989) People and Trees: The role of social forestry in sustainable development, pp 39–55. The world Bank, Washington DC Ernst WHO (1988) Seed and seedling ecology of Brachystegia spiciformis, a predominant tree component in miombo woodlands in south central Africa. Forest Ecology and Management 25: 195–210 FAO (1983) Fuelwood supplies in the developing countries. Forestry paper No. 42, FAO, Rome Gomez AK and Gomez AA (1984) Statistical procedures for agricultural research, 2nd edition, John Wiley & Sons, New York, 532 pp Joly HI, Zehnlo M, Danthu P and Aygalent C (1992) Population Genetics of an African acacia,

213 Acacia albida. 1. Genetic diversity of populations from west Africa. Australian Journal of Botany 40: 59–73 Krishan B and Toky OP (1996) Provenance variation in seed germination and seedling growth of Acacia nilotica ssp. indica in India. Genetic Resources and Crop Evolution 43: 79–101 Ngulube RM (1989) Seed germination, seedling growth and biomass production of eight central American multipurpose trees under nursery condition in Zomba, Malawi. Forest Ecology and Management 27: 21–27 Oboho EEG and Ali JY (1985) Preliminary investigation of the effect of seed weight on early growth characters of some savannah species. In: Okojie JA and Okoro OO (eds) Proceedings of 15th annual conference of the forestry association of Nigeria, Yola, 25–29 November, 1985, pp 144–156. University of Sokoto, Nigeria Salazar R (1986) Genetic variation in seeds and seedlings of ten provenances of Gliricidia sepium (Jacq.) Steud. Forest Ecology and Management 16: 391–401 Sniezko RA and Stewart HTL (1989) Range-wide provenance variation in growth and nutrition of Acacia albida seedlings propagated in Zimbabwe. Forest Ecology and Management 27: 179–197 Sorensen FC and Campbell RK (1985) Effect of seed weight on height growth of Douglas-fir (Pseudotsuga menziesii (Mirb.) France var. menziesii) seedlings in a nursery. Canadian Journal of Forest Research 15(6): 1109–1115 Wanyancha JM, Mills WR and Gwaze DP (1994) Genetic variation in Acacia albida (Faidherbia albida) and its agroforestry potential in Zimbabwe. Forest Ecology and Management 64: 127–134

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