Vegetative propagation of Milicia excelsa by leafy stem cuttings: effects of auxin concentration, leaf area and rooting medium

Fores;;;ology Management Forest Ecology and Management 84 (1996) 39-48

Vegetative propagation of Militia excelsa by leafy stem cuttings: effects of auxin concentration, leaf area and rooting medium D.A. Ofori a, A.C. Newton b,*, R.R.B. Leakey b9’,J. Grace ’ ’ The Institute

of Ecology

a Forestry Research Institute. V.S.T. P.O. Box 63, Kumasi. Ghana b Institute of Terrestrial Ecology, Bush Estate. Penictdk, EH26 OQB, UK and Resource Management, University of Edinburgh, Kings Buildings, Mayfield Road, Edinburgh,

EH9 3JV, VK

Accepted 28 February 1996

Abstract

The effect of different auxin concentrations, leaf areasandpropagationmediaon the rootingability of leafy stemcuttings of Miliciu excelsawere investigatedusinga non-mistpropagationsystemin Ghana.Three separateexperimentstested(i) indole-3-butyric(IBA) concentrationsrangingfrom 0.0 to 1.6%, (ii) leaf areatreatmentsof 0, 10, 20, 40 and 60 cm* obtainedby trimming the leaves,and(iii) four propagationmedia,namelyfine sand(diameter2 mm or less),coarsesand (2-4 mm),decomposed sawdustanda 1:1 mixture of coarsesandandsawdust.In eachexperiment,cuttingsweretakento a standardlength of 6 cm from shootsof previouslyprunedpot-grownstockplantsgrown under 40% shade.IBA had no significant effect on final rooting percentage (P > 0.05, ANOVA), althoughvaluesdeclinedwith successiveincreasesin IBA concentration above0.2%. Addition of IBA increasedroot numberby approximately80%, but wasalso positively correlatedwith cutting mortality (r - 0.81, P < 0.05). Percentagerooting and root numberwere positively relatedto leaf area (r - 0.89 and 0.90, mspectively, P < O.OS),whereasshoot production, leaf abscissionand mortality decreased significantly (P < 0.05) with increasing leaf area.Highestrooting percentages androot numberswererecordedin sawdust. Overall, moisturecontentof the mediawaspositively relatedto root number(r - 0.98, P < 0.05), but negativelyrelatedto mortality andleaf abscission. Foliar relative watercontent(RWC) increasedin all experimentsduringthe fmt 2 weeksafter insertion by 1.3-15.38, higher values being recorded in media with a sawdust component. These results suggest that an application of not morethan0.2% IBA shouldbe usedfor masspropagationof Militia excelsacuttings,with a leaf areaof around 40 cm* and a medium with a high moisture holding capacity (24% or less water by volume). Keywords:

Vegetative propagation; Rooting physiology; IBA; Propagation medium; Leaf area

1. Introduction

Corresponding author at: The htstitute of Ecology and Resource Management, University of Edinburgh. Kings Buildings, Mayfield Road, Edinburgh, EH9 3JU. UK. Fax: 0131 662 0478. ’ Present address: International Centre for Research in Agroforestry, P.O. Box 30677, Nairobi, Kenya. l

Miliciu excelsa (Welw.) C.C. Berg (formerly Chlorophoru excelsa (Welw.) Benth. & Hook.,

Moraceae; trade name Iroko) is a commercially important timber species of West, Central and East

0378-l 127/%/$1X00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SO3781127(96)037371

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Africa which is primarily obtained from natural forest. Establishment of the species in plantations is severely restricted by its susceptibility to attacks by the gall forming insect Phytolyma lura Walker. Studies at the Forestry Research Institute of Ghana are aimed at the development of pest-resistant planting stock. Initial results from seedling screening trials indicate that pest-resistant genotypes may exist (Cobbinah, 1990). Additional research is aimed at developing vegetative propagation techniques which would enable pest-resistant genotypes to be multiplied, providing suitable material for reforestation. No information is currently available on the factors influencing adventitious root production in leafy cuttings of this species, although the species has successfully been propagated in Cameroon (Ladipo et al., 1994). A number of studies have demonstrated that exogenous application of auxin accelerates the rate of rooting, increases final rooting percentage and increases the number of roots of leafy cuttings (Leakey et al., 1982; Leakey, 1990). However, relatively high concentrations of auxins have been reported to be inhibitory to rooting, indicating that in many species, optimal concentrations for rooting may be defined (Leakey et al., 1982). The presence of leaves also has a considerable influence on rooting of cuttings, owing to their ability to produce auxins and carbohydrates (Hartmann et al., 1990), and also through their influence on cutting water status (Newton et al., 1992a). The size of leaf retained has also been found to be influential in species such as Triplochiton scleroxyion (Leakey and Coutts, 1989) and Khaya iuorensis (Tchoundjeu, 1989). The type of rooting medium used is also critical to the rooting process. Cuttings of many species root successfully in a variety of rooting media (Leakey et al., 19901, but the rooting performance, in terms of both number of roots and rooting percentage, may be greatly influenced by the kind of rooting medium used (Leakey et al., 1990). This paper describes three separate experiments designed to assess the effects of (i) auxin concentration, (ii) leaf area and (iii) rooting medium on rooting of leafy stem cuttings of Militia excelsa, in order to define the optimum treatments for consistent high rooting percentage. The experiments were also designed to test the suitability of a low technology

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non-mist propagation system (Leakey et al., 1990) for propagation of this species.

2. Materials

and methods

Four seedlots of Militia excelsa (EA 7B, AA 2, AB 9 and BK 3) were used for these experiments. Seeds were sown on a nursery bed at Mesawam Research Centre of the Forestry Research Institute of Ghana, Kumasi (annual rainfall, 1520 mm; altitude, 300 m; 6”30’N, l”5O’W) to produce stockplants. Four weeks after germination, the seedlings were potted into black polyethylene bags (10 cm height, 8 cm width) containing sandy loam collected from under a 20-year-old stand of Militia excelsa at the Mesawam Research Centre. The seedlings were cut back to stumps of 5 cm height at the age of 1 year, and repotted into larger black polyethylene bags (30 cm height, 25 cm width), containing sandy loam soil collected from the same site. The plants were then conveyed to a nursery area in Kumasi and raised under approximately 40% shading, provided by green palm fronds placed at a height of 2.5 m above the ground. The plants received natural rainfall, supplemented by daily watering to field capacity when there was no rain for 2 days, and received weekly applications of foliar fertiliser (11.25 g of ‘Grofol’. Agrofarma Mexicana, S. A. de C. V. Mexico; 8.1 N: 1.3P: 2.5K in 10 1 of water), applied in place of normal watering. The stockplants were repeatedly cut back to 5 cm stumps after reaching approximately 0.5 m in height, and a maximum of three shoots were allowed to develop on each stump. Low-technology non-mist propagators were constructed following Leakey et al. (1990). The propagators consisted of a wooden frame enclosed in clear polyethylene, and filled with water to a depth of 5 cm below the surface of the rooting medium. The propagators were positioned under a shade screen (85% light interception) of green palm fronds and placed at a height of 2.5 m above the ground. Whenever the propagators were opened, the cuttings were sprayed frequently with water from a knapsack sprayer. A mercury thermometer was inserted into the rooting medium to a depth of l-2 cm to measure bed temperature. Air temperature and humidity were measured with a portable thermohygrometer (HYT-

D.A. Ofori

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Ecology

705OlOG, Gallenkamp Express, Loughborough, UK). Measurements were taken daily between 12:30 and 1400 pm throughout the experiments. The experiments were carried out consecutively during 1992/1993. 2.1. Experiment 1: Effect of indole-3-butyric concentration on rooting

(IBA)

A total of 20 plants (five from each seedlot) were randomly allocated to each of the five treatments, obtained by dissolving IBA in industrial methylated spirit (IMS) at concentrations of 0.0 (IMS only), 0.2, 0.4, 0.8 and 1.6%. The IBA solutions were applied to the cutting base in a 10 ~1 droplet using a micropipette (giving 0, 20, 40, 80 and 160 kg IBA per cutting, respectively). The shoots were harvested early in the morning (between 06:OO and 07:OO h) 9 weeks after sprouting, from stockplants which had been watered to field capacity the previous evening. Once cut, the shoots were kept in polyethylene bags and sprayed with a fine mist of water, using a knapsack sprayer. They were then severed into cuttings of 6 cm length, discarding the apical 2-3 cm softwood portion of each shoot, as these were found to root unsatisfactorily in a preliminary study. All the cuttings were defoliated, leaving only one leaf at the top node, which was then trimmed to an area of approximately 20 cm* using a paper template measured using a leaf area meter (Delta-T Devices, Burwell, UK). After the appropriate IBA solution had been applied to the base of each cutting, the solvent was dried in a stream of cold air for 30-90 s (following Leakey et al., 1982) before inserting the cuttings in the propagator. A total of 360 cuttings (72 cuttings per treatment) were taken, and laid out in 12 randomised blocks with each block containing six cuttings per treatment. The rooting medium used was coarse sand (approximately 2-3 mm diameter), obtained by sieving river sand. The medium was treated 3 days prior to the beginning of the experiment with fungicide (Dithane M.45, Rohm and Haas, France S.A.) at a concentration of 25 g in 10 1 of water, and insecticide (Cymbush 10 EC, Imperial Chemical Industries Plc, Haslemere, UK), at a concentration of 5.79 ml in 10 I of water.

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41

Cuttings were assessed weekly for the presence of roots (2 mm or larger), number of roots (2 mm or larger), cutting mortality, leaf shedding and shoot sprouting. Five cuttings per treatment were randomly sampled at 0 and 14 days after propagation for the determination of foliar relative water content (RWC), calculated as RWC = (F,- D)/(T - 0) X lOO%, where F is fresh mass, D is dry mass (taken after drying in an oven at 80°C for 48 h), T is turgid mass (taken after floating the leaf on water for 24 h). The mass of the samples was determined using an electronic balance. 2.2. Experiment 2: EfSect of leaf area on rooting The shoots of seedling stockplants were severed into 6 cm cuttings, which were randomly allocated to five different leaf area treatments (0, 10, 20, 40 and 60 cm2). As described in Experiment 1, 10 ~1 of 0.2% IBA solution were applied to the bases of the cuttings. The cuttings were then inserted in 12 randomized blocks, with each block containing 25 cuttings (5 treatments X 5 cuttings). The rooting medium used was sawdust (composted for 6 years) treated as described in Experiment 1. In addition to the 300 cuttings used for the rooting assessments, five cuttings per treatment were randomly sampled at time of insertion and after 2 weeks, for the determination of foliar RWC as described earlier. Cuttings were assessed weekly as described in Experiment 1. 2.3. Experiment 3: EfSect of propagation rooting

media on

Experimental treatments consisted of four rooting media: fine sand (2 mm or less), coarse sand (2-4 mm, obtained by sieving river sand), pure sawdust (well decomposed, 6 years old) and a mixture of coarse sand and sawdust (1:l by volume). Three non-mist propagators were used, and each was divided into four equal compartments. Each section was then filled with one of the media, which were pretreated 3 days prior to the beginning of the experiment with fungicide and insecticide as described above. To determine the relative proportion of air and water contents by volume in the rooting media, three 60 cm3 samples of each of the four media were taken. The volume of air was determined

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by measuring the amount of added water required to saturate the media. For the determination of water content, another three 60 cm3 samples of each medium were taken and dried in an oven at 100°C for 48 h. The results were expressed as percentage of each component by volume as described by Kohnke (1%8) using the formulae Volumetric water content = (Volume of water/volume of sample) X 100% Air content by volume = (Volume of air/volume

of sample) X 100%

pH was determined on three samples from each medium taken 1 week after insertion of the cuttings, using a pH meter (model 7020, Fisons Scientific Equipment, Loughborough, UK). Cuttings were prepared as described above, to a standard length of 6 cm and a leaf area of approximately 20 cm2. Twenty-four cuttings were randomly inserted in each medium (giving 72 cuttings per treatment) after 10 p,l of IBA solution had been applied to the bases of the cuttings. Twelve cuttings were sampled at the time of insertion and ten from each treatment were sampled after 2 weeks for the determination of foliar RWC, as described earlier. Cuttings were assessed weekly for the presence of roots and dry mass was determined at the end of the experiment, as described above. Data in each experiment were analysed by calculating standard errors and confidence limits following Snedecor and Cochran (1980). Rooting percentTable 1 Microclimate of non-mist propagation system, during tie Experiment 1 Relative humidity (‘96) Inside propagator Outside propagator Air temperature (“C) Inside propagator Outside propagator Temperature PC) of medium Irradiance (Ix) Inside propagator Outside propagator

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ages were arcsin transformed prior to analysis of variance using SAS Statistical Analysis Systems Institute Inc. (1980). Normally distributed data were analysed using t-tests or analysis of variance where appropriate (Snedecor and Cochran, 1980).

3. Results Mean relative humidity inside the propagators differed only slightly between the three experiments, values ranging from 90.2 to 94.8% (Experiments 2 and 1, respectively). Air and propagation medium temperatures also displayed relatively little variation between experiments, values tending to be lower in Experiment 1 (Table 1). However, mean irradi&ce values were approximately twice as high during Experiment 1 than the other two, as a consequence of a sunnier period of weather. 3.1. Experiment I

The 0.2% IBA treatment increased the final rooting percentage by 9% above that of the control. Rooting percentage in the other IBA treatments declined with successive increase in IBA concentration, such that the percentage rooting of cuttings which received 0.2% IBA was significantly (P < 0.05) higher than those which received 1.6% IBA (Fig. l(a)>. However, the effect of IBA on final rooting percentage was not significant overall (P > 0.05,

propagation experiments with Militia

excelsa

Mean

Range

Experiment 2 Mean Range

Experiment 3 Mean Range

94.8 76.3

90.2-100 66.1-79.2

90.2 67.4

81.6-100 61.8-97.5

91.9 61.5

87.3-98.3 56.2-69.5

27.7 27.5 25.7

26.0-29.4 25.3-28.9 24.0-28.0

29.2 27.4 26.4

27.5-31.0 25.5-30.5 24.5-28.0

29.9 28.0 26.0 a

25.5-30.4 25.7-31.5 23.5-29.0 a

1096 5600

550-1800 4589-5850

530 5400

360-900 3800-5580

553 5483

360-900 3mo-5650

a Measurements in coarse sand; temperatures of the other media were not significantly different (mean values less than 0.3”C different from that of coarse sand).

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4

Time (weeks)

L

r 0.0

0.2

0.4

IBA concentration

0.8

1

1.6

(%)

Fig. 1. Effect of IBA concentration on (a) percentage rooting, (b) number of roots of Militia excelsa cuttings in a non-mist propagation system in Ghana (e, 0.0% IBA; A, 0.2% IBA; 0, 0.4% IBA; A, 0.8% IBA; 0, 1.6% IBA). Values presented are means kSE(n=62).

ANOVA). Application of 0.8% and 1.6% IBA generally inhibited rooting from the base of the cuttings; roots tended to emerge from a mean of 4 mm above

Table 2 Effect of propagation Experiment

treatment

on foliar

relative

water content

Experiment

1

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43

the cutting base. A strong negative correlation (r = 0.90, P < 0.05) was observed between rooting percentage and auxin concentration. The mean number of roots per rooted cutting was increased by approximately 80% when auxin was applied, but no significant differences between the four IBA treatments were observed (Fig. l(b)). Final percentage mortality in the 0.2% IBA treatment was 7.5% lower than in the control treatment, but overall, mortality was positively correlated (r = 0.81, P < 0.05) with increasing IBA concentration. The highest percentage mortality (34.6%) was recorded in the 1.6% IBA treatment, which was 15.4% higher than the control value. Production of new shoots by the cuttings during propagation was also significantly (P < 0.05) reduced by auxin application. The control treatment (IMS only) displayed the highest percentage shoot production (72.4%), which decreased significantly (P < 0.05) with successive increases in auxin concentration, reaching a minimum of 5.7% in the 1.6% IBA treatment. Similarly, application of 0.2% IBA caused a 7.6% reduction in leaf abscission below that of the control; further increases in IBA concentration were accompanied by increases in percentage leaf abscission to values up to 17.6% higher than controls (1.6% IBA treatment). The RWC of cuttings measured in week 2 ranged from 86.1 to 99.0% depending on treatment, representing increases of 1.3-15.3% over values recorded at time of insertion (Table 2). However, no significant differences were recorded between treatments at either time of measurement.

(RWC)

2

Experiment

3

IBA cont. (%)

RWC (%o)

Leaf area (cm)

RWC (o/o)

Propagation medium

RWC (%o)

0.0 0.2 0.4 0.8 1.6

94.4a 99.0a 94.6a 94.6a 86.la

10 20 40 60

94.5a 89.7a 87.8a 86.6a

SD SD+CS cs FS

95.2a 94.8a 84.2b 84.7b

Means in a column followed by the same letter are not significantly different (P > 0.05). Values obtained 14 days after insertion (n k- 5 in each case). Propagation media: SD, sawdust; mm or less).

CS, coarse sand (2-4

mm); FS, fine sand (2

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3.2. Experiment 2

I ns?

90

Percentage rooting after 4 weeks increased significantly (P < 0.05) with each successive increase in leaf area, such that rooting percentage and leaf area were positively correlated at this time (r = 0.89, P < 0.05). The effect of leaf area on final rooting percentage was highly significant (P < 0.001, ANOVA). However, no statistical differences in rooting percentage were recorded between cuttings with leaf areas of 20, 40 and 60 cm2. Total defoliation of cuttings drastically reduced rooting percentage but did not prevent it altogether (Fig. 2(a)). The mean number of roots per rooted cutting and the mean root dry mass per rooted cutting assessed at week 4 increased with increasing leaf area to 40 cm2 leaf area (Fig. 2(b)), such that marked positive correlations were recorded between these two variables

8

80

.zSJ

70

2 5 5

60

r”

40

1

2 Time

25

03 L

20 9 8

15

B z

10

-E z’

5 0

3 (weeks)

11

1: 0

10

I

tl 20

Leaf area

l-

l-

40

60

(cm*)

Fig. 2. Effect of leaf area on (a) percefitage rooting, (b) number of roots of Militia excelsa cuttings in a non-mist propagation system inGhana(O,Ocm’;@, lOcm*; q ,20cm2; a,40cm2; n ,60 cm*). Values presented are means + SE (n = 55).

50

L

Shoot

production

(x)

Fig. 3. The relationship between rooting percentage and shoot production at the fourth week after insertion of Miliciu exceka cuttings in a non-mist propagation system in Ghana ( y = 89.5 1.24x, r = 0.99, P = 0.002) (Experiment 2: see text).

(r = 0.90, P < 0.05 and r = 0.98, P < 0.05, respectively). The mean root dry mass per rooted cutting varied between 0.2 and 8.4 mg, depending on treatment. Death of some cuttings commenced in the second week of propagation, with the highest percentage mortality recorded in the leafless cuttings. Mortabty decreased significantly (P < 0.05) with increasing leaf area, although the differences between cuttings with leaf areas of 20, 40 and 60 cm* were not significant. A strong negative correlation (r = 0.87, P < 0.05) was observed between leaf area and mortality at the end of the experiment (week 4). The production of new shoots on cuttings during propagation was stimulated significantly (P < 0.05$ by reductions in leaf area. The leafless cuttings displayed the highest percentage sprouting (49.0%). whereas cuttings with leaf areas of 40 and 60 cm2 displayed the lowest (3.6% in each case>. A strong negative correlation was observed between percentage sprouting and leaf area at week 4 (r = 0.99, P < 0.05; Fig. 3). The proportion of cuttings which abscised their leaves decreased successively with increasing leaf size, values ranging from 64.4% in the 10 cm* treatment to 22.0% in the 60 cm* treatment. Foliar RWC measured at week 2 ranged from 86.6 to 94.5%, showing an increase of 3.7-l 1.3% over initial values (Table 2). No significant difZerences were recorded between treatments at either

D.A. Ofori Table 3 Composition propagation

Ecology

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84 (1996) 100

of water, media

air, air/water

Water content (%I

(o/o)

SD CS+SD cs FS

40.0 24.2 14.1 Il.5

40.9 20.3 31.1 20.0

media:

ratio and pH of the different

Air content

Medium

Propagation

et al./ Forest

SD, sawdust;

Air/water ratio

pH

1.0 0.8 2.2 1.2

5.1 3.5 3.1 3.0

L‘i;

1(

39-48

45

a)

CS, coarse sand; FS, fme sand. Time (weeks)

4o 03 I

time of measurement, although a negative trend between RWC and leaf area was recorded at week 2. 3.3. Experiment 3

The four rooting media showed pronounced differences in the relative proportions of water and air, and in pH. In particular, water and air content, as well as pH, were higher in sawdust than in the other media (Table 3). Cuttings inserted in pure sawdust and in the mixture of coarse sand and sawdust displayed higher rooting percentages than those in coarse sand and fine sand. In addition, cuttings propagated in coarse sand had a higher rooting percentage than those in fine sand, although this difference was not statistically significant (Fig. 4(a)). The effect of propagation medium on final rooting percentage was highly significant overall (P = 0.004, ANOVA). The mean number of roots per rooted cutting was also higher in sawdust (33.0) than in coarse and fine sand (16.2 and 20.3, respectively; Fig. 4(b)). The number of roots was positively correlated with the moisture content of the media (r = 0.98, P < 0.05). The mean root dry mass per cutting and the mean length of the longest roots per treatment followed the same trend as that of number of roots, although no significant differences between treatments were observed. Mean root mass varied between 0.8 and 3.1 mg, and mean length of the longest root between 20.3 and 35.0 mm, depending on treatment. Cuttings propagated in fine sand and coarse sand displayed significantly (P < 0.05) higher percentage mortality than those in the other two treatments. The percent mortality of cuttings at the end of the propa-

30

I rl

B 0 e

ti 2 1

10

s

200 1

lso

I

Cs+SO

Propagation

,

cs

FS

,

medium

Fig. 4. Effect of propagation medium on (a) percentage rooting, (b) number of roots of Militia excelsa cuttings in a non-mist propagation system in Ghana (0, fme sand (FS); 0, coarse sand (CS); A, mixture of coarse sand and sawdust (CS + SD); 0, sawdust (SD)). Values presented are means f SE (n = 62).

300 10

15

20

25

Moisture

30 content

35

40

4.5

(76)

Fig. 5. The relationship between leaf abscission and mositure content of the propagation medium ( y = 108.8 - I .83x, r = 0.1, P = 0.004) at the fourth week after insertion of Militia excelsa cuttings in a non-mist propagation system in Ghana (Experiment 3; see text).

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gation period displayed a negative trend with pH and moisture content of the media tested. Cuttings in all the media except sawdust abscised more than 60% of their leaves. The percentage leaf abscission of cuttings rooted in pure sawdust was significantly (P < 0.05) lower than the other treatments, reaching a maximum of 36.5% by the third week, while coarse sand displayed the highest leaf loss (83.9%). A strong negative correlation was observed between percentage leaf abscission at week 4 and moisture content of the media (r = 1.0, P < 0.05; Fig. 5) and pH of the media (r = 0.93, P < 0.05). Values of foliar RWC determined at week 2 were significantly (P < 0.05) higher than those measured at time of insertion, by 2.7-13.7%, depending on treatment. Cuttings inserted in the mixture of coarse sand and sawdust displayed significantly higher values of RWC than those in fine sand and coarse sand at week 2 (Table 2).

4. Discussion

It has been widely documented that auxins promote adventitious root development of stem cuttings, through their ability to promote the initiation of lateral root primordia and to enhance transport of carbohydrates to the cutting base (Leakey et al., 1982; Hartmann et al., 1990). However, experiments with a range of tree species have indicated highly contrasting responses to IBA addition. For example, Cordia alliodora was found to require a concentration of 1.6% IBA for optimum rooting percentage, but failed to root when no auxin was applied (Me&n, 1993). In contrast, the optimum IBA concentration for rooting of Triplochiton scleroxylon was found to be 0.4% (Leakey et al., 1982). The relatively high rooting percentage recorded here in cuttings without applied IBA suggests that Militia excelsa is well supplied with endogenous auxins. Successful rooting without applied auxin has also been reported in a number of other tropical tree species, such as Shorea macrophylla (Lo, 1985>, Nauclea diderrichii (Leakey, 1990) and Vochysia hpndurensis (Leakey et al., 1990). The decline in rooting percentage with IBA concentrations greater than 0.2% suggests that such concentrations are inhibitory to root initiation,

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as has been recorded in a number of other tree species (Leakey et al., 1990). Concentrations of auxins substantially higher than those normally found in plant tissues may cause cell death (Hartmann et al., 1990). The inhibition of shoot production in Militia excelsa cuttings by higher IBA concentrations has also been recorded in other tree species, such as Cordia alliodora (Me&, 1993). This may arise as a result of IBA-induced basipetal transport of assimilates, with sink strength successively enhanced by increases in IBA concentration (Hartmann et al., 19901. This process may account for the increase in number of roots per rooted cutting with increasing IBA concentration recorded here, which is consistent with the response of other tropical tree species such as Triplochiton scleroxylon (Leakey et al., 1982), Nauclea diderrichii (Leakey, 1990), Cordia alliodora and Albizia guachepele (Mestn, 1993). The increased leaf abscission and associated cutting mortality with increasing IBA may also be attributed to this process, reflecting a depletion in foliar nutrient contents and the consequent onset of leaf senescence. In many species, the presence of leaves on cuttings exerts a strong stimulatory influence on root initiation (Hartmann et al., 1990). This reflects the role of foliage as a source of both auxins and carbohydrates. In Triplochiton scleroxylon (Leakey et al., 19821, the inability of leafless cuttings to root was associated with rapid depletion of carbohydrates in the stem, emphasizing the importance of the leaf as a carbohydrate source. The fact that 30% of leafless cuttings of Militia excelsa were found to root in this investigation suggests that both current assimilates and carbohydrates produced pre-severance and stored in the stem may influence rooting of this species. In species such as Nauclea diderrichii (Leakey, 19901 and Terminalia spinosa (Newton et al., 1992a1, variation in leaf area had little effect on the final rooting percentage. In contrast, variation in cutting leaf area had a substantial effect on the percentage rooting of Militia excelsa. This conforms with results obtained for relatively difficult-to-root species such as Triplochiton scleroqlon (Leakey et al., 1982), Cleistopholis glauca, Terminalia ivorensis (Leakey, 1985) and Khaya ivorensis (Tchoundjeu,

1989), where optimum leaf areas for rooting were found to be 50 cm2, 50 cm2, 100 cm* and IO-30

DA.

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cm2, respectively. The tendency to display an optimum leaf area reflects the balance between photosynthesis and transpiration, since the retention of large leaves on cuttings has been found to result in increased water loss and a consequent reduction in photosynthetic activity (Leakey and Coutts, 1989; Newton et al., 1992a). The negative relationship between foliar RWC and leaf area recorded in this investigation is consistent with this hypothesis. The type of rooting medium used can have a major influence on the rooting capacity of cuttings (Hartmann et al., 1990). Since aeration and water holding capacity of the media are often negatively correlated, a balance between these must be achieved to ensure optimal rooting, which will most often depend upon whether the species is xeromorphic or hydromorphic (Loach, 1986). In the experiment reported here, the addition of sawdust to coarse sand (which was used as the rooting medium for the other experiments) improved its moisture holding capacity from 14.1 to 24.2% by volume, representing an increase of 71.6%. Furthermore, pure sawdust was found to have the highest moisture holding capacity and also the highest air content of the media tested, but had a lower air/water ratio than coarse sand and fine sand. These results indicate the importance of the organic fraction of a rooting medium for water retention (Loach, 1986; Leakey et al., 1990). In this investigation, cuttings inserted in media with relatively low water contents (coarse sand and fine sand) displayed lower values of foliar RWC, which were associated with lower rooting percentages and root mass. The positive correlation recorded between moisture content of the rooting media and mean number of roots per rooted cutting further emphasizes the importance of moisture availability for rooting. Experiments with other tree species have shown that uptake of water by cuttings is positively related to the volumetric water content of the medium (Newton et al., 1992b), and that this may enhance rooting by reducing water deficits (Loach, 1986). The ability of cuttings to absorb water from the media is illustrated by the increases in RWC recorded in all three experiments during the first 2 weeks after insertion, and particularly in sawdust in Experiment 3. Water uptake is particularly important for overcoming the initial physiological shock when cuttings are taken, which can lead to high moisture deficits,

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leaf abscision and cutting death in drought-sensitive species (Newton and Jones, 1993b). The results of this investigation indicate that concentrations of not more than 0.2% IBA should be used for mass propagation of Militia excelsa cuttings. A leaf area of around 40 cm* is recommended, as more cuttings may be accommodated on the propagation bed than if larger leaf sizes are adopted. Media with a high moisture holding capacity (24% or greater water by volume) should be used, as cuttings inserted in media with lower moisture contents showed signs of moisture deficits. These results also confirm that the non-mist propagation system used in these experiments is appropriate for propagation of this species, although it should be recognized that the optimum post-severance treatments for propagation will be highly dependent on propagation microclimate (Newton and Jones, 1993a). In general, values of propagator microclimate recorded here were in a similar range to those recorded for a non-mist propagation system in the UK (Newton and Jones, 1993a). All mean RWC values recorded were over 80%, which suggests that severe water deficits, which have been recorded in other tropical tree species (Newton and Jones, 1993b), did not occur during these experiments. However, the fact that higher relative humidity was recorded in Experiment 1 than in Experiments 2 and 3, may account for the relatively high values of RWC observed in the former experiment.

Acknowledgements

We thank the Director and Management of Forestry Research Institute of Ghana and the World Bank/Forest Resource Management Project who funded this project. We are also grateful to Dr. J.R. Cobbinah and Augustina Gyimahl of the Forestry Research Institute of Ghana for their respective assistance in supplying us with seedlings of Militia excelsa and provision of technical staff.

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