Effects of Duration of Bulb Chilling on Dry Matter

Effects of duration of bulb chilling on dry matterdistribution in hydroponically forced tulips

Katsuhiko Inamoto*, Takanori Hase, Motoaki Doi,Hideo Imanishi

College of Agriculture, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan

Accepted 10 December 1999

Abstract

Bulbs of tulip (Tulipa gesneriana L. `Gander'), in which the formation of ¯oral organs had been

completed, were chilled at 28C for 3±30 weeks and then forced hydroponically at 208C under a 12 h

photoperiod with the light at 100 mmol mÿ2 sÿ1. The fresh weight and the perianth length of the cut

¯ower highly correlated with the dry weights of shoots and ¯oral organs at anthesis, respectively.

The dry weights at planting (DWp) of shoots, ¯oral organs and daughter bulbs increased with

increasing the duration of bulb chilling at 28C (tc). After a 30-week tc, enlargement of daughter

bulbs had already started and their DWp was high. The days from planting to anthesis (tpa)

decreased hyperbolically with increasing tc. The dry weight at anthesis (DWa) of shoots increased

with increasing tc up to 12 weeks and then decreased. DWa of ¯oral organs decreased and that of

daughter bulbs increased with increasing tc. After a 30-week tc, the DWa values of shoots and ¯oral

organs were markedly small and that of daughter bulbs was very large. The dry weights of shoots,

¯oral organs and daughter bulbs increased exponentially after planting, and the relative growth rate

during the period from planting to anthesis (Rpa) in each plant part was calculated. The changes of

DWa of each plant part after various tc could be explained by their changes in the growth

parameters, i.e., DWp, tpa, and Rpa which were shown as functions of tc. # 2000 Elsevier Science

B.V. All rights reserved.

Keywords: Bulb chilling; Daughter bulbs; Dry matter distribution; Growth analysis; Regression;

Tulipa gesneriana L.

Scientia Horticulturae 85 (2000) 295±306

* Corresponding author. Tel.: �81-722-54-9424; fax: �81-722-54-9423.

E-mail address: [email protected] (K. Inamoto)

0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 4 2 3 8 ( 0 0 ) 0 0 1 2 8 - X

1. Introduction

Bulbs of tulip (Tulipa gesneriana L.) require chilling for shoot elongationfollowing the completion of the ¯oral organ formation (Rees and Charles-Edwards, 1975; Le Nard and De Hertogh, 1993). The duration of bulb chillinggreatly in¯uences forcing duration and cut ¯ower quality (Moe and Wickstùrm,1973; Charles-Edwards and Rees, 1975; Aoba, 1976; Hobson and Davies, 1978).We assumed that the quality of cut tulips is determined by the dry matterallocated to the shoots including ¯oral organs. This experiment was conducted toexamine: (1) how the duration of bulb chilling at 28C in¯uences the dry matterdistribution at planting and anthesis, (2) whether the growth analysis method isuseful to estimate dry matter accumulation in plant parts, and (3) whetherindividual parameter in growth analysis can be expressed as a function of theduration of bulb chilling.

2. Materials and methods

2.1. Plant materials

Bulbs of `Gander' (11±12 cm in circumference) produced in sandy ®eld inToyama Prefecture in Japan were purchased. The bulbs were divided into eachtreatment lot uniformly in weight and stored under dry conditions. To promote thedevelopment of the shoots and the ¯oral organs, the bulbs were stored at 308C for1 week starting on 15 June 1993, and then transferred to 208C. This storagemethod was similar to that previously described by Imanishi et al. (1993) and LeNard (1980). Following the storage at 208C for 8 weeks, when trilobedgynoecium had been formed in the ¯ower buds, chilling at 28C for 0, 3, 6, 9, 12,15, 18 and 30 weeks started. The rooms for bulb storage were well ventilated.Following the chilling, the bulbs were forced hydroponically.

2.2. Forcing methods

Forty bulbs with tunica and external offsets removed per each treatment lotwere inserted into holes (8�5) on a foamed polystyrene plate (620 mmL�278 mm W�15 mm T). A plastic container (620 mm L�278 mm W�153 mmH) was ®lled with nutrient solution containing 1000 mg lÿ1 CaNO3, and thefoamed polystyrene plates with bulbs were ¯oated on the solution. The forcingcondition was 208C and 12 h photoperiod with the light at 100 mmol mÿ2 sÿ1

PPFD at bulb level supplied from the combination of three-band ¯uorescent tubes(FL20SS EEXN/18-H; Toshiba Company, Tokyo, Japan) and incandescent lamps(LW110V100W; Matsushita Electric Company, Osaka, Japan).

296 K. Inamoto et al. / Scientia Horticulturae 85 (2000) 295±306

2.3. Fresh and dry weight

In all experimental plots, 10 bulbs or plants were sampled at the time ofplanting and at weekly intervals after planting, and 20 plants were sampled atanthesis when the tip of the perianths dehisced. The length of stem and perianthand the fresh weights of whole plants and their component parts, i.e., mother bulbscales and basal plates, ¯oral organs, leaves, stems, roots and inner daughterbulbs, were measured. Each part was dried in an oven at 808C for 72 h and weighed.

The dry weights of whole plants and their parts measured weekly were plottedagainst the days after planting and the relations were shown by ®tting them onexponential curves:

when the part was growing,

DW � b eat (1)

and when the part was senescing,

DW � cÿ b eat (2)

where DW is the dry weight, t the days after planting, a, b and c are constants ande the base of natural logarithms.

2.4. Relative growth rate

When the regression to Eq. (1) was signi®cant, relative growth rates during theperiod from planting to anthesis (Rpa) of the whole plant and each part werecalculated as follows:

Rpa � �ln DWa ÿ ln DWp�=tpa (3)

where Rpa is the relative growth rate from planting to anthesis, tpa the days fromplanting to anthesis, DWa the dry weight at anthesis and DWp the dry weight atplanting.

2.5. Regression of growth parameters

The relations between the growth parameters (Table 1), i.e., tpa, DWp, DWa andRpa, and the duration of bulb chilling at 28C (tc) were formulated by regressionanalysis. We applied linear, quadratic, hyperbolic and logistic curves to them, andchose the best ®tting model which had the smallest p value.

3. Results

3.1. Flowering and quality of cut ¯owers

The bulbs which had not been chilled completely failed to ¯ower, and only halfof those chilled for 3 weeks ¯owered (Table 2). The length of stems and the fresh

K. Inamoto et al. / Scientia Horticulturae 85 (2000) 295±306 297

weight of shoots including a stem, leaves, and ¯oral organs at anthesis, increasedwith the increase in duration of bulb chilling at 28C (tc) up to 12 weeks, butdecreased when tc was longer than 18 weeks (Table 2). The perianth length atanthesis decreased with the increase in duration of bulb chilling (Table 2). Thecorrelation coef®cient between the fresh weight and the dry weight of shoots atanthesis was 0.972 (p<0.01) and that between the perianth length and the dryweight of the ¯oral organs was 0.996 (p<0.01).

3.2. Dry weight at planting

The dry weights at planting (DWp) of whole plants and mother bulbs includingscales and basal plate slightly decreased with increasing tc (Fig. 1A and B). DWp

of shoots and ¯oral organs increased linearly with increasing tc (Fig. 1C and D).The DWp value of inner daughter bulbs (we call them `daughter bulbs' simply inthis report) increased slightly with increasing tc up to 18 weeks and becameextremely large when tc was 30 weeks (Fig. 1E). DWp of each plant part was wellillustrated as a function of tc (Table 3).

Table 1

List of abbreviations of parameters

Abbreviation Description Unit

tc Duration of bulb chilling at 28C week

tpa Time from planting to anthesis day

DWp Dry weight at planting g

Dwa Dry weight at anthesis g

Rpa Relative growth rate from planting to anthesis dayÿ1

Table 2

Effects of duration of bulb chilling at 28C on ¯owering and cut ¯ower quality of `Gander' tulips

Duration of chilling

at 28C (week)

Flowering

(%)

Stem

length (cm)

Perianth

length (cm)

Shoot fresh

weight (g)

0 0 ± ± ±

3 55 28.7 5.5 23.0

6 100 37.7 5.0 23.5

9 100 41.4 5.0 27.8

12 100 46.4 4.8 30.0

15 100 47.4 4.4 28.6

18 100 47.5 4.0 28.8

30 100 32.5 3.5 15.9

Analysis of variance p<0.001 p<0.001 p<0.001


3.3. Days from planting to anthesis

The days from planting to anthesis (tpa) decreased hyperbolically withincreasing tc (Fig. 2, Table 3).

3.4. Dry weight at anthesis

The dry weight at anthesis (DWa) of shoots initially increased with increasing tcand then decreased showing the maximum value of 2.9 g at a 12-week chilling(Fig. 3). DWa of ¯oral organs consistently decreased with increasing tc. DWa ofmother bulbs decreased with increasing tc although the values showed some¯uctuation. DWa of daughter bulbs increased with increasing tc and showed anextremely high value of 2.5 g at a 30-week chilling. DWa of each plant part waswell illustrated as a function of tc (Table 3). No clear relationship between DWa ofwhole plants or roots and tc was observed.

Fig. 1. Effects of duration of bulb chilling of `Gander' tulips at 28C (tc) on dry weights of whole

plants and their component parts at planting (DWp). The bulbs had no emerging roots at planting.

Vertical bars indicate S.E. (n�10). The equations are shown in Table 3.


3.5. Change in dry weight after planting

The dry weights of whole plants and mother bulbs decreased after planting(Fig. 4A and B) accompanied by increase of the dry weights of shoots, ¯oralorgans and daughter bulbs (Fig. 4C±E). The increase or decrease rate was greater,the longer the bulbs had been chilled. The decreasing dry weights of whole plantsand mother bulb scales could be ®tted to the exponential curves represented byEq. (2), and the increasing dry weights of shoots, ¯oral organs and daughter bulbscould be ®tted to the curves represented by Eq. (1). These regressions were

Table 3

Equations showing the relationships between weeks of bulb chilling at 28C (tc) and growth

parameters of forced `Gander' tulips

Variable Equation

Type Expressiona Coefficient of

determination (R2)

Significance

tpa Hyperbolic y�6.84�227/tc 0.995 p<0.001

DWp

Whole plant Linear y�10.9ÿ3.69�10ÿ2tc 0.742 p<0.05

Mother bulb Logistic y�12.0/(1�0.101 e0:0358tc ) 0.869 p<0.01

Shoot Linear y�ÿ0.0144�0.0107tc 0.991 p<0.001

Floral organs Linear y�4.26�10ÿ4�1.31�10ÿ3tc 0.995 p<0.001

Daughter bulbsb Logistic y�0.279/(1�208 eÿ0:289tc ) 0.952 p<0.001

Root ±c ±c ±c ±c

DWa

Whole plant ±d ±d ±d ±d

Mother bulb Hyperbolic y�4.23�5.01/tc 0.741 p<0.05

Shoot Quadratic y�2.00�0.130tcÿ4.93

�10ÿ3tc2

0.991 p<0.001

Floral organs Logistic y�0.594/(1�0.137 e0:130tc ) 0.917 p<0.001

Daughter bulbsb Linear y�ÿ0.312�0.0861tc 0.925 p<0.001

Roots ±d ±d ±d ±d

Rpa

Shoot Quadratic y�0.0281�0.0111tcÿ2.688�10ÿ4t2

c

0.974 p<0.001

Floral organs Quadratic y�0.0392�9.32�10ÿ3tcÿ2.78�10ÿ4t2

c

0.931 p<0.01

Daughter bulbsb Quadratic y�ÿ0.0470�0.0307tcÿ7.77�10ÿ4t2

c

0.988 p<0.001

a 0�tc�30.b Inner daughter bulbs only.c Roots were not observed at planting.d p�0.05 in either linear, quadratic, logistic or hyperbolic regression.


Fig. 2. Effects of duration of bulb chilling of `Gander' tulips at 28C (tc) on days from planting to

anthesis (tpa). Vertical bars indicate S.E. (n�20). The equation is shown in Table 3.

Fig. 3. Effects of duration of bulb chilling of `Gander' tulips at 28C (tc) on dry weights at anthesis

(DWa) of whole plants and their component parts. Vertical bars indicate S.E. (n�20). The equations

are shown in Table 3.


statistically signi®cant (p<0.05). The changes in dry weight of roots could not be®tted to the exponential curve applied here.

3.6. Relative growth rate

The relative growth rates of shoots, ¯oral organs and daughter bulbs duringthe period from planting to anthesis (Rpa) were calculated (Fig. 5). These growthparameters were well illustrated by a quadratic equation with the maximumvalue against tc (Table 3). The maximum value of Rpa was observed at 18 weeksof tc in shoot and daughter bulbs and at 12 weeks of tc in ¯oral organs. Themaximum value of Rpa was greater in daughter bulbs than in shoots and ¯oralorgans.

Fig. 4. Changes in dry weights of whole plants of `Gander' tulips and their component parts after

planting. Data are ®tted to exponential curve shown by DW�beat for shoots, ¯oral organs and

daughter bulbs or DW�cÿbeat for the whole plant and mother bulb, where DW is the dry weight, t

the days after planting, a, b and c are constants, and e the base of natural logarithms. The durations

at 28C were 3 (*), 6 (*), 12 (&), 18 (&) and 30 weeks (D).


4. Discussion

Considering previous reports about contribution of photosynthesis to growthand development of tulip plants (Rees, 1966; Ho and Rees, 1975; Benschop,1980), we premised that most of the dry matter in developing parts, i.e., shoots,roots and daughter bulbs, originated from the storage materials in mother bulbscales in this experiment.

Judging from the shoot fresh weight and its perianth length, the best cut ¯owerwas obtained from the bulbs chilled at 28C for 12 weeks. Longer chilling loweredcut ¯ower quality. Although many workers have reported on the relationshipbetween cut ¯ower quality and bulb chilling in tulips (Moe and Wickstùrm, 1973;Charles-Edwards and Rees, 1975; Rees and Charles-Edwards, 1975; Aoba, 1976;Hobson and Davies, 1978; Le Nard and De Hertogh, 1993), few of themdiscussed the negative effects of excessive bulb chilling on the quality of forcedcut tulips. At anthesis, the shoot fresh weight was highly correlated with its dry

Fig. 5. Effects of duration of bulb chilling of `Gander' tulips at 28C (tc) on the relative growth rates

during the period from planting to anthesis (Rpa) in plant component parts. The equations are

illustrated in Table 3.


weight and the perianth length with dry weight of ¯oral organs. This means thatdry matter accumulation in shoots and ¯oral organs is essential to improve the cut¯ower quality. The duration of bulb chilling at 28C (tc) in¯uenced dry matteraccumulation in newly developed plant parts at anthesis (Fig. 3) and therelationship between them was represented by linear or non-linear curves at ahighly signi®cant level (Table 3). As shown in Fig. 3, the dry matter translocatedfrom mother bulbs to shoots increased with increasing tc up to 12 weeks.However, in the plants raised from the bulbs chilled for 30 weeks, only a smallamount of dry matter was supplied to shoots and ¯oral organs from the motherbulbs. On the other hand, in the plants raised from bulbs chilled for 30 weeks,large amount of dry matter was accumulated in the daughter bulbs at anthesis.Some researchers pointed out that developing shoots compete with daughter bulbsin allocation of dry matter. Tsutsui (1974) observed that cultivars with highability to produce daughter bulbs are liable to abort ¯owers. Rees (1971) reportedthat heat treatment to kill the ¯ower buds in mother bulbs enhanced growth ofdaughter bulbs after planting.

Shoots, ¯oral organs and daughter bulbs grew slowly even under lowtemperature 28C. The dry weight at planting (DWp) of whole plants decreasedwith increasing tc (Fig. 1A, Table 3). This may be due to the respiration lossduring chilling. DWp of shoots and ¯oral organs increased linearly withincreasing tc. DWp of daughter bulbs increased exponentially with increasing tcshowing a very high value at 30 weeks of tc (Fig. 1C±E, Table 3). Development ofdaughter bulbs is induced by low temperature and they can enlarge even underdry conditions (Aoba and Shibuya, 1976; Le Nard and Cohat, 1968, cited in LeNard and De Hertogh, 1993). The results obtained in this experiment show that, incases of bulbs maintained in dry condition, although the chilling requirement forthe development of daughter bulbs is higher than that for development of shoots,once the chilling requirement has been satis®ed the daughter bulbs collect thematerials stored in mother bulbs rapidly and in large amount.

The dry weights of shoots and ¯oral organs increased exponentially afterplanting. The dry weight of roots increased rapidly after planting but ceased toincrease within a short period (Fig. 4D±F). We calculated the relative growth ratesduring the period from planting to anthesis (Rpa) for each plant part except forroots using Eq. (3). Rpa is the index of the ef®ciency of dry matter accumulation,and is considered to be an index of growth activity of each plant part. The amountof dry matter allocated to each tulip plant part at anthesis (DWa) is a function of tothe initial size at planting (DWp), relative growth rate (Rpa) and days to anthesis(tpa).

DWa � DWp eRpatpa (4)

The duration of bulb chilling at 28C (tc) apparently affected Rpa (Fig. 5) ofshoots, ¯oral organs and daughter bulbs and the regression to the quadratic


equation with the maximum value was highly signi®cant (Table 3). Rpa ofdaughter bulbs was consistently higher than that of shoots suggesting that thegrowth activity of daughter bulbs is higher than that of shoots.

tpa decreased with increasing tc and their relationship represented by ahyperbolic curve was highly signi®cant (Table 3). Eq. (4) implies that decreasingtpa results in a decreasing DWa.

DWa of shoots increased with increasing tc up to 12 weeks and then decreasedthereafter (Fig. 3). According to Eq. (4), this phenomenon can be explained asfollows: (1) when tc is shorter than 12 weeks, the effects of the increasing DWp

and Rpa are superior to the effect of decreasing tpa, resulting in the increasingDWa, (2) when tc is between 12 and 18 weeks, the effect of the decreasing tpa

compensates the effects of the increasing DWp and Rpa, resulting in a slightdecreasing of DWa, and (3) when tc is longer than 18 weeks, the decreasing Rpa

and tpa results in the decreasing of DWa.The effects of tc on DWa of ¯oral organs and daughter bulbs can be explained

by DWp, Rpa and tpa in the same way as that on DWa of shoots. DWa of ¯oralorgans continued to decrease with increasing tc. This might be due to a lower Rpa

of ¯oral organs as compared with that of shoots and daughter bulbs. DWp ofdaughter bulbs was markedly high when tc was 30 weeks, and this may result intheir large DWa value in spite of relatively small Rpa and tpa values.

We can correlate the ®nal dry weights of plant parts to several growthparameters. Most of these growth parameters can be represented as a function ofthe duration of bulb chilling (tc, Table 3). This indicates the possibility of theestablishment of growth models by estimating the effects of environmentalconditions on the individual growth parameters and by reconstructing them.

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