Effects of duration of bulb chilling on dry matter
distribution 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 gesnerianaL. `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 100mmol mÿ2sÿ1. 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-weektc, enlargement of daughter
bulbs had already started and their DWp was high. The days from planting to anthesis (tpa)
decreased hyperbolically with increasingtc. The dry weight at anthesis (DWa) of shoots increased
with increasingtcup to 12 weeks and then decreased.DWaof ¯oral organs decreased and that of
daughter bulbs increased with increasingtc. After a 30-weektc, theDWavalues 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, andRpawhich were shown as functions oftc.#2000 Elsevier Science
B.V. All rights reserved.
Keywords: Bulb chilling; Daughter bulbs; Dry matter distribution; Growth analysis; Regression;
Tulipa gesnerianaL.
*
Corresponding author. Tel.:81-722-54-9424; fax:81-722-54-9423.
E-mail address: [email protected] (K. Inamoto)
1. Introduction
Bulbs of tulip (Tulipa gesneriana L.) require chilling for shoot elongation following the completion of the ¯oral organ formation (Rees and Charles-Edwards, 1975; Le Nard and De Hertogh, 1993). The duration of bulb chilling greatly 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 matter allocated to the shoots including ¯oral organs. This experiment was conducted to examine: (1) how the duration of bulb chilling at 28C in¯uences the dry matter distribution at planting and anthesis, (2) whether the growth analysis method is useful to estimate dry matter accumulation in plant parts, and (3) whether individual parameter in growth analysis can be expressed as a function of the duration of bulb chilling.
2. Materials and methods
2.1. Plant materials
Bulbs of `Gander' (11±12 cm in circumference) produced in sandy ®eld in Toyama Prefecture in Japan were purchased. The bulbs were divided into each treatment lot uniformly in weight and stored under dry conditions. To promote the development of the shoots and the ¯oral organs, the bulbs were stored at 308C for 1 week starting on 15 June 1993, and then transferred to 208C. This storage method was similar to that previously described by Imanishi et al. (1993) and Le
Nard (1980). Following the storage at 208C for 8 weeks, when trilobed
gynoecium 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 lot
were inserted into holes (85) on a foamed polystyrene plate (620 mm
L278 mm W15 mm T). A plastic container (620 mm L278 mm W153 mm
H) was ®lled with nutrient solution containing 1000 mg lÿ1 CaNO3, and the
2.3. Fresh and dry weight
In all experimental plots, 10 bulbs or plants were sampled at the time of planting and at weekly intervals after planting, and 20 plants were sampled at anthesis when the tip of the perianths dehisced. The length of stem and perianth and the fresh weights of whole plants and their component parts, i.e., mother bulb scales and basal plates, ¯oral organs, leaves, stems, roots and inner daughter bulbs, 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 plotted against the days after planting and the relations were shown by ®tting them on exponential curves:
when the part was growing,
DW b eat (1)
and when the part was senescing,
DW cÿb eat (2)
whereDWis the dry weight,tthe days after planting,a,bandcare constants and
e the base of natural logarithms.
2.4. Relative growth rate
When the regression to Eq. (1) was signi®cant, relative growth rates during the period from planting to anthesis (Rpa) of the whole plant and each part were
calculated as follows:
Rpa lnDWaÿlnDWp=tpa (3)
whereRpais the relative growth rate from planting to anthesis,tpa the days from
planting to anthesis,DWa the dry weight at anthesis andDWp the dry weight at
planting.
2.5. Regression of growth parameters
The relations between the growth parameters (Table 1), i.e.,tpa,DWp,DWaand
Rpa, and the duration of bulb chilling at 28C (tc) were formulated by regression analysis. We applied linear, quadratic, hyperbolic and logistic curves to them, and chose the best ®tting model which had the smallestp value.
3. Results
3.1. Flowering and quality of cut ¯owers
weight of shoots including a stem, leaves, and ¯oral organs at anthesis, increased with the increase in duration of bulb chilling at 28C (tc) up to 12 weeks, but decreased when tc was longer than 18 weeks (Table 2). The perianth length at
anthesis decreased with the increase in duration of bulb chilling (Table 2). The correlation coef®cient between the fresh weight and the dry weight of shoots at anthesis was 0.972 (p<0.01) and that between the perianth length and the dry weight 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 including scales and basal plate slightly decreased with increasingtc(Fig. 1A and B).DWp
of shoots and ¯oral organs increased linearly with increasingtc(Fig. 1C and D). TheDWpvalue of inner daughter bulbs (we call them `daughter bulbs' simply in
this report) increased slightly with increasing tc up to 18 weeks and became extremely large whentcwas 30 weeks (Fig. 1E).DWpof each plant part was well illustrated as a function oftc(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
3.3. Days from planting to anthesis
The days from planting to anthesis (tpa) decreased hyperbolically with increasingtc(Fig. 2, Table 3).
3.4. Dry weight at anthesis
The dry weight at anthesis (DWa) of shoots initially increased with increasingtc
and then decreased showing the maximum value of 2.9 g at a 12-week chilling (Fig. 3).DWa of ¯oral organs consistently decreased with increasingtc. DWa of mother bulbs decreased with increasing tc although the values showed some
¯uctuation.DWa of daughter bulbs increased with increasing tc and showed an extremely high value of 2.5 g at a 30-week chilling.DWaof each plant part was well illustrated as a function oftc(Table 3). No clear relationship betweenDWaof
whole plants or roots andtcwas 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.
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, ¯oral organs 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 plants and mother bulb scales could be ®tted to the exponential curves represented by Eq. (2), and the increasing dry weights of shoots, ¯oral organs and daughter bulbs could 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
Mother bulb Hyperbolic y4.235.01/tc 0.741 p<0.05
Shoot Quadratic y2.000.130tcÿ4.93
10ÿ3
tc2
0.991 p<0.001
Floral organs Logistic y0.594/(10.137e0:130tc) 0.917 p<0.001
Daughter bulbsb Linear yÿ0.3120.0861tc 0.925 p<0.001
Roots ±d ±d ±d ±d
Rpa
Shoot Quadratic y0.02810.0111tc
ÿ2.68810ÿ4
Daughter bulbsb Quadratic yÿ0.04700.0307tc
ÿ7.7710ÿ4tc2
Roots were not observed at planting.
d
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. (n20). 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. (n20). The equations
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 during the period from planting to anthesis (Rpa) were calculated (Fig. 5). These growth parameters were well illustrated by a quadratic equation with the maximum value againsttc(Table 3). The maximum value ofRpawas observed at 18 weeks of tc in shoot and daughter bulbs and at 12 weeks of tc in ¯oral organs. The maximum value of Rpa was greater in daughter bulbs than in shoots and ¯oral
organs.
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 byDWbeat for shoots, ¯oral organs and daughter bulbs orDWcÿbeatfor the whole plant and mother bulb, whereDWis the dry weight,t
4. Discussion
Considering previous reports about contribution of photosynthesis to growth and 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 bulb scales in this experiment.
Judging from the shoot fresh weight and its perianth length, the best cut ¯ower was obtained from the bulbs chilled at 28C for 12 weeks. Longer chilling lowered cut ¯ower quality. Although many workers have reported on the relationship between 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 them discussed the negative effects of excessive bulb chilling on the quality of forced cut 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
weight and the perianth length with dry weight of ¯oral organs. This means that dry 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 matter
accumulation in newly developed plant parts at anthesis (Fig. 3) and the relationship between them was represented by linear or non-linear curves at a highly signi®cant level (Table 3). As shown in Fig. 3, the dry matter translocated
from 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 small amount of dry matter was supplied to shoots and ¯oral organs from the mother bulbs. 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 bulbs in allocation of dry matter. Tsutsui (1974) observed that cultivars with high ability to produce daughter bulbs are liable to abort ¯owers. Rees (1971) reported that heat treatment to kill the ¯ower buds in mother bulbs enhanced growth of daughter bulbs after planting.
Shoots, ¯oral organs and daughter bulbs grew slowly even under low temperature 28C. The dry weight at planting (DWp) of whole plants decreased with increasing tc (Fig. 1A, Table 3). This may be due to the respiration loss
during chilling. DWp of shoots and ¯oral organs increased linearly with
increasingtc. DWp of daughter bulbs increased exponentially with increasing tc
showing a very high value at 30 weeks oftc(Fig. 1C±E, Table 3). Development of
daughter bulbs is induced by low temperature and they can enlarge even under dry conditions (Aoba and Shibuya, 1976; Le Nard and Cohat, 1968, cited in Le Nard and De Hertogh, 1993). The results obtained in this experiment show that, in cases of bulbs maintained in dry condition, although the chilling requirement for the development of daughter bulbs is higher than that for development of shoots, once the chilling requirement has been satis®ed the daughter bulbs collect the materials stored in mother bulbs rapidly and in large amount.
The dry weights of shoots and ¯oral organs increased exponentially after planting. The dry weight of roots increased rapidly after planting but ceased to increase within a short period (Fig. 4D±F). We calculated the relative growth rates during the period from planting to anthesis (Rpa) for each plant part except for roots using Eq. (3).Rpais 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 amount of dry matter allocated to each tulip plant part at anthesis (DWa) is a function of to
the initial size at planting (DWp), relative growth rate (Rpa) and days to anthesis (tpa).
DWaDWpeRpatpa (4)
The duration of bulb chilling at 28C (tc) apparently affected Rpa (Fig. 5) of
equation with the maximum value was highly signi®cant (Table 3). Rpa of
daughter bulbs was consistently higher than that of shoots suggesting that the growth activity of daughter bulbs is higher than that of shoots.
tpa decreased with increasing tc and their relationship represented by a hyperbolic curve was highly signi®cant (Table 3). Eq. (4) implies that decreasing
tpa results in a decreasing DWa.
DWaof shoots increased with increasingtcup to 12 weeks and then decreased thereafter (Fig. 3). According to Eq. (4), this phenomenon can be explained as follows: (1) whentcis shorter than 12 weeks, the effects of the increasing DWp
and Rpa are superior to the effect of decreasing tpa, resulting in the increasing DWa, (2) when tcis between 12 and 18 weeks, the effect of the decreasing tpa
compensates the effects of the increasing DWp and Rpa, resulting in a slight decreasing of DWa, and (3) whentcis longer than 18 weeks, the decreasingRpa
andtparesults in the decreasing of DWa.
The effects oftcon DWaof ¯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 ¯oral organs continued to decrease with increasingtc. This might be due to a lowerRpa
of ¯oral organs as compared with that of shoots and daughter bulbs. DWp of
daughter bulbs was markedly high whentcwas 30 weeks, and this may result in their largeDWavalue in spite of relatively small Rpa andtpa values.
We can correlate the ®nal dry weights of plant parts to several growth parameters. Most of these growth parameters can be represented as a function of the duration of bulb chilling (tc, Table 3). This indicates the possibility of the establishment of growth models by estimating the effects of environmental conditions on the individual growth parameters and by reconstructing them.
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