Effect of hand pinching and plant growth regulators on ...

New Zealand Journal of Crop and Horticultural Science

ISSN: 0114-0671 (Print) 1175-8783 (Online) Journal homepage: http://www.tandfonline.com/loi/tnzc20

Effect of hand pinching and plant growth regulators on seed production of field grown hybrid dahlia S. Phetpradap , J. G. Hampton & M. J. Hill To cite this article: S. Phetpradap , J. G. Hampton & M. J. Hill (1994) Effect of hand pinching and plant growth regulators on seed production of field grown hybrid dahlia, New Zealand Journal of Crop and Horticultural Science, 22:3, 313-320, DOI: 10.1080/01140671.1994.9513840 To link to this article: http://dx.doi.org/10.1080/01140671.1994.9513840

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New Zealand Journal of Crop and Horticultural Science, 1994, Vol. 22: 313-320 0114-0671/94/2203-0313 $2.50/0 © The Royal Society of New Zealand 1994

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Effect of hand pinching and plant growth regulators on seed production of field grown hybrid dahlia

S. PHETPRADAP* J. G. HAMPTON1 M. J. HILL Seed Technology Centre Department of Plant Science Massey University Palmerston North, New Zealand *Present address: Maejo Institute of Agricultural Technology, Chiangmai, Thailand. Abstract The effects of hand pinching above node four on the main stem and the application of the plant growth regulators paclobutrazol and chlormequat chloride on seed production of field grown hybrid dahlia 'Unwins Mixed' were investigated over two seasons at the same site. Pinching reduced the spread of flowering and resulted in seed heads at approximately the same height above ground level because of the promotion of lateral branch length, but did not increase seed yield. Seed yield perplant was significantly increased following the application of chlormequat chloride at 1.5 kg a.i./ha at stem elongation in the first season, and by paclobutrazol applied at 1.0 kg a.i./ ha at the appearance of the first visible flower bud in both seasons, the former through the production of more seed heads/plant, and the latter through a significant increase in seeds/seed head in one season and seed heads/plant in the following season. Other seed yield components did not differ among treatments. Plant growth retarding effects were transitory and floral development and flower numbers were not altered by the plant growth regulators. Although results were inconsistent,

1 To whom correspondence is to be addressed. H93066 Received 27 September 1993; accepted 16 March 1994

chemical manipulation of this hybrid dahlia shows promise for seed production and warrants further investigation. Keywords dahlia; Dahlia pinnata x Dahlia coccinea; seed production; plant growth regulators; paclobutrazol ; chlormequat chloride ; hand pinching INTRODUCTION Dahlias, popular as cut flowers, bedding, and potted plants (Salunkhe et al. 1987) because of their diversity of form, colour, and size (Runger & Cockshull 1985) are most often and easily grown from seed. However as a species, dahlia produces many problems for seed production (Still 1988). Indeterminacy leads to lack of uniformity in growth habit, variability in time to flowering, and hence varying proportions of flower buds, open flowers, and immature, ripening, ripe, and shattered seed heads can be present on the same plant at the same time (Phetpradap 1992). This results in a wide range of seed maturity within and among plants, and therefore hand harvesting is common in dahlia, because the crop cannot be effectively mechanically harvested. Methods for manipulating the plant to increase uniformity of growth, induce greater uniformity of flowering, and decrease the range of seed maturities would be beneficial for seed production. Hand pinching (the removal of the apical bud of the main shoot) is a common floricultural practice (Ecke & Matkin 1976) designed to encourage axillary shoot development and greater floral synchrony (Larson 1980). However, pinching is usually aimed at increasing flower number for display purposes (Love 1975) and little information is available as to any effects on seed yield. Chemical manipulation of plant growth is also receiving increasing attention in floriculture (Davis & Andersen 1989), particularly the use of plant growth regulators (PGR) to suppress vegetative growth,

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reduce plant height, alter plant shape, size, and form, and promote flower initiation and uniformity. Previously published PGR research on growth and development of dahlia has concentrated on enhancing rooting of cuttings (e.g., Biran et al. 1973) or tuberisation (e.g., Moser & Hess 1968) but not seed production. However in non-floricultural crops, PGRs such as paclobutrazol and chlormequat chloride have been reported to increase seed yield in indeterminate species such as Vicia faba L. (Attiya et al. 1983) and Lotus uliginosm Schkr. (Tabora & Hampton 1992; Tabora & Hill 1992). In this paper we report the effects of hand pinching and PGR application on the synchronony of flowering and seed yield of field grown dahlia 'Unwins Mixed'. This cultivar, a hybrid (Dahlia pinnata L. x D. coccinea Cav.; Rowell 1981) is an extensively-grown bedding type (Hammett 1980), 60 cm tall and with semi-double mixed colour flowers (Phetpradap 1992). MATERIALS AND METHODS The trials were conducted at Massey University, Palmerston North, New Zealand (40°S 175°E) on a soil type classified as an Ohakea silt loam (gleyed yellow grey earth) in 1988/89 and 1989/90. In both seasons seed bed preparation involved ploughing in early October and two passes with power harrows.

Trifluralin (1.2 litres/ha) was soil incorporated on 30 October 1988 and 25 October 1989 (Phetpradap 1992). Seeds of dahlia 'Unwins Mixed' were thiram dusted (0.8 g a.i./lOO g seed) and hand sown 2 cm deep at 30 x 30 cm spacings on 21 December 1988 and 1 November 1989 (Table 1). Approximately four seeds were sown per position and seedlings thinned to a single plant 14 days after sowing. Any missing plants were replaced by transplanted seedlings of the same age (Phetpradap 1992). In both seasons overhead irrigation was applied for 30 min every 3 days during the first 2 weeks after sowing, and then every 5-7 days as required until peak flowering, using a system which delivered 10 litres water/m2 per h. Weeds were hand removed as required. Methiocarb (20 kg a.i./ha) was broadcast 1 week after sowing. Benomyl (0.25 kg a.i./ha) or chlorothalinol ( 150 ml a.i./ha) and methomyl (0.25 kg a.i./ha) or fluvalinate (0.1 kg a.i./ha) were applied alternately every 2 weeks in each season to prevent the increase of diseases and pests. Ammophos fertiliser (Table 1) was hand broadcast whereas two foliar fertiliser applications (Table 1) were also made via a fertiliser dispenser attached to a soft spray wand (Phetpradap 1992). For 1988/89 there was a total of 10 treatments including a control i.e., ( 1 ) hand pinching—removal of the apical tip between nodes 4 and 5 (cotyledonary

Table 1 Management and treatment information. (PGR = plant growth regulator.) 1988/89 Sowing date Plant spacing Plot size Plants/plot Replicates Fertiliser N. P. K. S. N. P. K.

Treatments and application date

(1)

(2) (1) (2) (3)

Peak flowering (control) Harvest

21 Dec 1988 3 0 x 3 0 cm 1.5x2 m 30 3 12.10.10.8 100 kg/ha 30 Jan 1989 12.8.16 (foliar) 35 kg/ha 10 Jan, 25 Jan 1989 hand pinching above node 4—21 Feb 1989 PGR first visible flower bud—8 Feb 1989 PGR stem elongation 21 Feb 1989 28 Apr 1989 19 May 1989

1989/90 1 Nov 1989 30 x 30 cm 3 x 3m 100 3 12.10.10.8 100 kg/ha 1 Dec 1989 12.8.16 (foliar) 35 kg/ha 21 Nov, 29 Nov 1989 (1) PGR vegetative stage 1 Dec 1989 (2) PGR first visible flower bud—16 Dec 1989 (3) PGR stem elongation 3 Jan 1990 10 Feb 1990 22 Mar 1990

Phetpradap et al.—Seed production of dahlia


leaves = node 0) when the plants had 6-7 pairs of fully expanded true leaves; (2) PGR application at the first visible bud stage (when the plants had 4-5 nodes)—paclobutrazol at 0.5 and 1.0 kg a.i./ha, and chlormequat chloride at 1.5 and 3.0 kg a.i./ha; and (3) PGR application at stem elongation (when the plants had 6-7 nodes and branches were beginning to extend—PGRs and rates as for (2)). For 1989/90 the pinching treatment was deleted and the two PGR (paclobutrazol at 1.0 kg a.i./ha and chlormequat chloride at 1.5 and 3.0 kg a.i./ha) were applied at the first visible bud and stem elongation growth stages, whereas paclobutrazol was also applied during the vegetative stage (when the plants had 2-3 pairs of true leaves). There were therefore eight treatments including the control. The PGRs were applied via a 5 litre pressurised sprayer in 1 litre water/plot. Application dates are presented in Table 1. These rates and growth stages were chosen on the basis of previous work with China aster (Callistephus chinensis Nees.—L. Phetpradap 1992) and other field crops (e.g., Attiya et al. 1983; Hampton & Young 1988) for the first season, and on the previous season's results for the second season. In both years each treatment was replicated three times in a randomised complete block design. Days to first flowering was taken as the number of days from sowing until 50% of the plants in each plot had the first flower open. Four randomly chosen plants in each plot were identified by means of a cane placed beside each of them, and flowers/plant counted every 4 days. Peak flowering was taken as the date at which the maximum number of flowers were present on these four plants. In the second season, three plants were sampled at random from each plot at peak flowering (10 February 1990) and plant height, main stem length, number of main stem nodes, lateral branch number and length, and plant component dry weights were recorded. Plots were judged ready for seed harvest when 80% or more of the seed heads had turned brown (Phetpradap 1992). In both seasons, five plants were removed at random from each plot and the morphological characters previously recorded at peak flowering (1989/90) re-evaluated. In addition seed head number/plant was also counted. Seed heads were hand removed from each plant and the number of seeds/seed head counted from 20 randomly chosen seed heads/plant. All remaining plants per plot (excluding border rows) were hand cut at ground level and seed heads hand removed from the plants. All seed heads were then dried at 28-30°C in an air flow of 34 m3/sec

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(Phetpradap 1992). One week after drying, seeds were hand removed from the seed heads, weighed, and seed moisture content determined (ISTA 1985). The seed samples were then air-blast cleaned to remove inert matter using a seed blower (Burrows Model No. 1836-4) at an air flow setting of 85 m3/ min, and finally air-screen cleaned (Clipper Seed Cleaner) using a 2 mm oblong screen. This processing resulted in a pure seed content of over 99% for all treatments (Phetpradap 1992). Seed yields were corrected to 0% seed moisture content. Percentage seed cleaning losses were calculated by subtracting cleaned seed yield/plant from precleaned seed yield/plant and dividing by the latter. Thousand seed weight and germination were determined using internationally standardised methodology (ISTA 1985).

RESULTS 1988/89 Growth retardation (reduced plant height) was first observed 2-3 weeks after PGR application (Phetpradap 1992). However by seed harvest, only the main stem of pinched plants (5.1 cm) and plants treated with paclobutrazol 1.0 kg a.i./ha applied at the first visible bud stage (26.0 cm) differed (P < 0.05) significantly from the control (32.0 cm). PGRs did not significantly reduce total plant height and lateral branch lengths did not differ. Pinching increased (P < 0.05) lateral branch length (by 15, 10, and 33 cm for basal, mid, and top lateral branches respectively) so that the net effect was no difference in total plant height (range 56-74 cm) at final harvest. Days to first flowering were increased (P < 0.05) from 67 (control) to 81 (pinching) and 71 by paclobutrazol 1.0 kg a.i./ha applied at stem elongation and chlormequat chloride 3.0 kg a.i./ha applied at first visible bud. However, as days to peak flowering (mean =100 days) did not differ among treatments, pinching reduced (P < 0.05) the spread of flowering by 14 days. No differences in flowers/plant (mean = 34.7) were recorded. Seed yield per plant was increased (P < 0.05) by two PGR treatments (Table 2), paclobutrazol 1.0 kg a.i./ha applied at first visible bud (+ 72%) and chlormequat chloride 1.5 kg a.i./ha applied at stem elongation (+ 66%). No significant differences in seed heads/plant or thousand seed weight were recorded (Table 2) but paclobutrazol 1.0 kg a.i./ha applied at first visible bud increased (P < 0.05) the

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number of seeds/seed head (+ 29%). Seed cleaning losses ranged from 1 to 45% (Table 2). Germination (range 79-84%) did not differ significantly among treatments.


1989/90 Chlormequat chloride treatments had no significant effects on plant height, main stem length, or lateral branch length. However, paclobutrazol reduced (P < 0.05) plant height and main stem length at peak flowering (data not presented), and although plant height was still significantly shorter than that of the control at final harvest for all three application

times (Table 3), main stem length was shorter only for the vegetative growth application (Table 3). At final harvest, paclobutrazol-treated plants had lateral branches at the basal and mid plant positions which were shorter (P < 0.05) than those of the control (Table 3). No differences were recorded among treatments for the number of nodes on the main stem, number of lateral branches, or total plant dry weight, although paclobutrazol applied at the vegetative stage did reduce (P < 0.05) main stem dry weight (data not presented). Control plants took 75 days to reach first flowering and 100 days to reach peak flowering.

Table 2 Effect of hand pinching and plant growth regulator application on dahlia seed yield and its components, 1988/89. (NS = not significant.)

kg a.i./ha

Treatment

Chlormequat chloride

Seed yield (g/plant)

% seed lost

37.6 40.3

36.0 34.6

6.74 6.50

4.99 6.28

44.2 31.8

0.5Ba 1.0B 0.5Sb 1.0S

45.6 31.3 37.7 30.7

38.3 46.4 37.1 35.4

6.50 6.78 6.85 6.57

6.02 8.62 6.58 7.67

45.0 11.0 18.7 1.4

1.5Ba 3.0B 1.5Sb 3.OS

40.0 39.3 53.3 39.0

39.6 35.3 32.2 39.9

5.54 5.70 8.29 5.72

42.9 32.2 28.0 44.2

NS 26.8

8.54 13.9

6.18 6.14 6.76 6.61 NS 8.2

LSD P < 0.05

%cv a

Thousand seed weight (g)

-

Control Pinching Paclobutrazol

Seeds/seed Seed heads/plant head

2.96 28.2

Applied at the first visible bud stage. Applied at stem elongation.

b

Table 3 Effect of paclobutrazol ( 1.0 kg a.i./ha) on dahlia plant height, main stem length, and lateral branch length at final harvest, 1989/90. (Data for chlormequat chloride not presented as they did not differ significantly from the control.) (NS = not significant.) Application time Control During vegetative growth First visible bud Stem elongation LSD P < 0.05

%cv a

Plant height (cm)

Main stem length (cm)

78.8 64.7 70.0 66.8 6.0 4.8

Lateral branch length (cm) Basea

Mida

Topa

33.7

74.8

58.7

52.7

23.0 29.0 30.0 6.9 12.9

59.1 64.7 62.5 8.8 7.5

67.2 69.3 56.5 7.0 6.2

47.1 49.2 50.1 NS 6.7

Lateral branches from the base, middle, and top of the plant respectively.

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PGR-treated plants reached the same stages 1 or 2 days earlier than the control, but these differences were not significant. At peak flowering total flower number did not differ among treatments, the range being from 36 to 44 per plant. Paclobutrazol applied at the first visible bud stage and chlormequat chloride 1.5 kg a.i ./ha applied at stem elongation significantly increased the number of seed heads/plant at final harvest (Table 4), but seeds/seed head and thousand seed weight did not differ significantly. Seed yield/

plant was significantly increased (+ 42%) by paclobutrazol applied at the first visible bud stage (Table 4). Seed cleaning losses ranged from 15 to 42% and unlike the previous year, were not necessarily highest for the lowest seed yields. Germination did not differ among treatments, ranging from 85 to 89%. Paclobutrazol applied at the first visible bud stage increased (P < 0.05) the number of harvested ripe seed heads at each of the three different lateral

Table 4 Effect of plant growth regulators on dahlia seed yield and its components, 1989/90. (NS = not significant.)

Treatments Control Paclobutrazol


42.0 45.0 62.1 45.7 51.7 50.3 55.5 45.8

35.7 36.7 42.3 38.3 41.3 40.3 44.3 35.0

Thousand seed weight (g) 6.01 6.17 6.14 6.09 6.27 6.04 5.72 6.03

11.4 13.6

NS 14.7

NS 3.9

Seed heads/ kg a.i./ha plant 1.0Va 1.0Bb 1.0Sc 1.5B 3.0B 1.5S 3.0S

LSD P < 0.05

%cv

Seeds/ seed head

Seed yield (g/plant) 7.62 7.28 10.85 7.35 9.02 8.36 8.13 7.79

15.4 28.2 33.3 30.4 32.3 31.7 41.9 17.6

2.1 14.7

a

Applied during the vegetative growth stage. Applied at the first visible bud stage. c Applied at stem elongation.

b

Table 5 Effect of plant growth regulators on number of harvested ripe dahlia seed heads from different lateral branch positions, 1989/90. (Seeds which had turned brown and/or were beginning to open.)

Treatment Control Paclobutrazol


LSD P