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Differences in Heat Requirements of Flower and Leaf Buds Make  Hysteranthous Trees Bloom before Leaf Unfolding

叶芽花芽需热量差异导致植物先花后叶



全 文 :叶芽花芽需热量差异导致植物先花后叶∗
郭  梁1ꎬ2ꎬ Eike LUEDELING3ꎬ 戴君虎4ꎬ 许建初1ꎬ5∗∗
(1 中国科学院昆明植物研究所资源植物与生物技术重点室ꎬ 云南 昆明  650201ꎻ 2 中国科学院大学ꎬ 北京  100049ꎻ
3 世界农用林业中心ꎬ 肯尼亚 内罗毕  00100ꎻ 4 中国科学院地理科学与资源研究所ꎬ 北京  100101ꎻ
5 世界农用林业中心东亚分部ꎬ 云南 昆明  650201)
摘要: 为探究植物先花后叶的影响因素ꎬ 本研究以 1963-1988年间北京地区杏和山桃展叶和始花物候资料
及相应的日最高、 最低温度数据为基础ꎬ 利用偏最小二乘回归法确定杏和山桃叶芽及花芽的需冷期和需热
期ꎬ 进而利用动态模型和生长度小时模型分别估算叶芽和花芽的需冷和需热量ꎮ 结果表明ꎬ 依据长期物候
观测资料ꎬ 利用偏最小二乘回归法进行植物需冷和需热量的估算非常有效ꎮ 先花后叶植物叶芽和花芽需冷
量几乎相同ꎬ 需热量的差异是导致植物先花后叶的主要原因ꎮ 杏和山桃花芽的需热量分别为 2 829􀆰 7±
876􀆰 2和 1 457􀆰 2±581􀆰 2生长度小时ꎬ 而相应叶芽需热量却是花芽的两倍之多ꎮ 基于物候观测的重要性及
实用性ꎬ 中国物种水平上的地面观测应得到进一步深入发展ꎮ
关键词: 先花后叶植物ꎻ 需冷量ꎻ 需热量ꎻ 叶芽ꎻ 花芽
中图分类号: Q 948􀆰 112            文献标识码: A            文章编号: 2095-0845(2014)02-245-09
Differences in Heat Requirements of Flower and Leaf Buds Make
Hysteranthous Trees Bloom before Leaf Unfolding
GUO Liang1ꎬ2ꎬ Eike LUEDELING3ꎬ DAI Jun ̄Hu4ꎬ XU Jian ̄Chu1ꎬ5∗∗
(1 Key Laboratory of Economic Plants and Biotechnologyꎬ Kunming Institute of Botanyꎬ Chinese Academy of Sciencesꎬ Kunming
650201ꎬ Chinaꎻ 2 University of Chinese Academy of Sciencesꎬ Beijing 100049ꎬ Chinaꎻ 3 World Agroforestry Centreꎬ Nairobi
00100ꎬ Kenyaꎻ 4 Institute of Geographical Sciences and Natural Resources Researchꎬ Chinese Academy of Sciencesꎬ
Beijing 100101ꎬ Chinaꎻ 5 World Agroforestry Centreꎬ East Asia Nodeꎬ Kunming 650201ꎬ China)
Abstract: To clarify which agroclimatic requirements control the sequential occurrence of flowering and leaf unfol ̄
ding in hysteranthous plantsꎬ Partial Least Squares (PLS) regression analysis was used to identify the chilling and
forcing period of leaf and flower buds. The Dynamic Model and the Growing Degree Hour Model were applied to esti ̄
mate the chilling and heat requirement for leaf unfolding and floweringꎬ based on the phenological records of apricot
and mountain peach and daily maximum and minimum temperature data in Beijing during 1963-1988. The results
indicated that PLS regression analysis is a useful approach to calculate the chilling and heat requirements of plants
when long term phenological observations are available. Leaf and flower buds were found to have similar chilling re ̄
quirements but different heat requirementsꎬ which explained the earlier occurrence of flowering compared to leaf un ̄
folding. The heat requirements of flower buds of apricot and mountain peach were 2 829􀆰 7±876􀆰 2 and 1 457􀆰 2±
581􀆰 2 Growing Degree Hoursꎬ respectivelyꎬ while heat requirements of vegetative buds were almost twice as high. In
view of the importance and usefulness of phenological observationsꎬ species ̄level ground observations in China
should be continued and extended.
Key words: Hysteranthous plantꎻ Chilling requirementꎻ Heat requirementꎻ Leaf budꎻ Flower bud
植 物 分 类 与 资 源 学 报  2014ꎬ 36 (2): 245~253
Plant Diversity and Resources                                    DOI: 10.7677 / ynzwyj201413081

∗∗
Funding: The National Natural Science Foundation of China (NSFC) (31270524)
Author for correspondenceꎻ E ̄mail: jxu@mail􀆰 kib􀆰 ac􀆰 cn
Received date: 2013-04-08ꎬ Accepted date: 2013-07-10
作者简介: 郭  梁 (1984-) 男ꎬ 博士ꎬ 从事气候变化对作物物候及产量影响研究ꎮ E ̄mail: guoliang@mail􀆰 kib􀆰 ac􀆰 cn
  In natureꎬ plants have different strategies to de ̄
termine the chronological sequence of leafing and
flowering. Most woody plants unfold leaves firstꎬ be ̄
fore initiating bloom. Howeverꎬ some trees display
hysteranthous behaviorꎬ meaning that bloom early in
springꎬ before leaves have developed. Earlier flower ̄
ing than leaf unfolding hold important ecological
significance on pollination since abundant flowers
sprouting together is prone to attract more insects
and no leaf covering facilitates wind pollination
meanwhile. Most hysteranthous plants belong to the
Rosaceaeꎬ Calycanthaceaeꎬ Magnoliaceae and Olea ̄
ceae families (Kang and Jinꎬ 2009). Even though
many hysteranthous trees are widely used as orna ̄
mentals or for fruit productionꎬ few studies have in ̄
vestigated the causes of this behavior.
Most woody plants from temperate and cool sub ̄
tropical climates fall dormant in winter to resist unfa ̄
vorable conditions and to protect the sensitive leaf
and flower buds from frost (Luedeling et al.ꎬ 2013ꎻ
Jones et al.ꎬ 2013). It is commonly assumed that
dormancy is composed of an endodormancy phaseꎬ
followed by an ecodormancy period ( Lang et al.ꎬ
1987). The fulfillments of the chilling and heat re ̄
quirements of plant buds help to break endodorman ̄
cy and ecodormancy (Campoy et al.ꎬ 2011ꎻ Luedel ̄
ingꎬ 2012). After breaking dormancyꎬ buds can
sprout and develop into leaves and flowers. Numer ̄
ous studies have been conducted to estimate the
chilling requirement of flower budsꎬ since insuffi ̄
cient chilling can cause uneven and even failed
bloomꎬ potentially threatening the production of
some horticultural crops (Lang et al.ꎬ 1987). Com ̄
parative studies of the chilling and heat requirements
of leaf and flower buds are rare. Gao et al. (2001)
compared the chilling requirements of leaf and flower
buds for some hysteranthous fruit treesꎬ and found
that flower buds need equal or slightly more chilling
units than leaf budsꎬ indicating that different heat
requirements of leaf and flower buds might dominate
the sequential occurrence of flowering and leafing.
The objectives of the present study were to com ̄
pare the chilling and heat requirements of leaf and
flower buds of two hysteranthous plantsꎬ and to iden ̄
tify the agroclimatic triggers that are responsible for
earlier occurrence of flowers compared to leaves. In
this analysisꎬ we used a chilling and a forcing model
to calculate daily chilling and heat accumulations
during 1963-1988 in Beijing based on daily maxi ̄
mum and minimum temperatures. A novel method
(Partial Least Squares regression) was applied to i ̄
dentify the chilling and forcing periods by relating
the leaf unfolding and flowering dates of two hyster ̄
anthous trees to daily chilling and heat accumula ̄
tions. Based on regression resultsꎬ chilling and heat
requirements of both leaf and flower buds of apricot
and mountain peach were estimated and compared.
1  Materials and methods
1􀆰 1  Phenological and climatic data
There is a long history of phenological observa ̄
tions in China. The Chinese Phenological Observa ̄
tion Network (CPON)ꎬ which was established in the
early 1960sꎬ conducted standardizedꎬ systematic and
comprehensive phenological observations of plants
and animals across Chinaꎬ but unfortunatelyꎬ was
interrupted after 1988 ( Lu et al.ꎬ 2006). Since
thenꎬ no detailed and nationwide observations have
been conducted. Consequentlyꎬ CPON records be ̄
fore 1988 are highly valuable resources for phenology
research in China.
In our analysisꎬ two hysteranthous plants (apri ̄
cot and mountain peach) in Beijing Summer Palace
(40°01′Nꎬ 116°20′Eꎬ 50 m a􀆰 s􀆰 l.) were chosen
for analysis since these two trees had the longest
phenological records there. First leaf unfolding and
flowering dates of apricot (Prunus armeniaca L.) and
mountain peach (Prunus davidina Franch.) during
1963-1988 were acquired from the CPON. Details of
the phenological observation method have been de ̄
scribed by Lu et al. (2006).
Daily minimum and maximum temperatures in
Beijing during 1963 - 1988 were obtained from the
Beijing Meteorological Station which is only 2􀆰 5 km
642                                  植 物 分 类 与 资 源 学 报                            第 36卷
from the Summer Palaceꎬ so that temperatures recor ̄
ded there should closely mirror conditions at the ob ̄
servation site. Since most common chilling and forc ̄
ing models require hourly temperature dataꎬ idealized
daily temperature curves with an hourly resolution
were constructed based on daily minimum and maxi ̄
mum temperatures as suggested by Linvill ( 1989ꎬ
1990). Other inputs for the calculation also included
sunrise and sunset timeꎬ as well as day lengthꎬ and
computed according to the methods used by Spencer
(1971) and Almorox et al. (2005).
1􀆰 2  Chilling / forcing periods and chill / heat re ̄
quirements for leafing and flowering
Based on the hourly temperature dataꎬ the Dy ̄
namic Model was used to calculate daily chilling ac ̄
cumulations during 1963-1988. This model was cho ̄
senꎬ because it has repeatedly been shown to be the
most robust and accurate among commonly used
chilling models (Ruiz et al.ꎬ 2007ꎻ Campoy et al.ꎬ
2011ꎻ Zhang and Taylorꎬ 2011ꎻ Luedeling and
Gassnerꎬ 2012). The Growing Degree Hour Model
(GDH) was used to calculate daily heat accumula ̄
tions. The mathematical functions of the chilling and
forcing models were given in Luedeling et al.
(2009aꎬ b). Daily chilling and heat values were
subjected to a 15 ̄day running mean to ensure the e ̄
mergence of recognizable response patterns in subse ̄
quent statistical analyses (Luedeling et al.ꎬ 2013ꎻ
Luedeling and Gassnerꎬ 2012).
Partial Least Squares (PLS) regression was used
to identify the chilling and forcing periods for leafing
and flowering by relating leaf ̄unfolding and flower ̄
ing dates of trees to daily chilling and heat accumu ̄
lations during 1963 - 1988ꎬ respectively. The PLS
regression has proven to be a useful and reliable
method when independent variables are highly auto ̄
correlated and when the number of independent vari ̄
ables exceeds the number of dependent variables
(Yu et al.ꎬ 2010ꎬ 2012ꎻ Luedeling et al.ꎬ 2013ꎻ
Luedeling and Gassnerꎬ 2012). The two major out ̄
puts of PLS analysis are the variable importance in
the projection (VIP) and standardized model coeffi ̄
cients. The VIP values reflect the importance of all
independent variables for explaining variation in the
dependent variables. A threshold value of 0􀆰 8 is typi ̄
cally used for determining importance. The standard ̄
ized model coefficients indicate the strength and di ̄
rection of the effects (Luedeling et al.ꎬ 2013).
In the output of PLS regression analysisꎬ VIP
values greater than 0􀆰 8 and negative model coeffi ̄
cients indicate that positive deviations of the respec ̄
tive independent variable are correlated with early
occurrence of bloom or leaf unfolding. In other
wordsꎬ the method identifies periods during which
high accumulation rates of chill or heat are related to
early occurrence of phenological stages. According to
common understanding of the progression of trees
through the dormancy periodꎬ this should occur during
the chilling and forcing periodsꎬ respectively. Ac ̄
cordinglyꎬ a period when VIP scores for daily chill
accumulation rates are predominantly high and model
coefficients negative can be considered as the chilling
phase. During the forcing phaseꎬ the same pattern
should emerge for daily heat accumulation rates.
We identified chilling and forcing periods for
generative and vegetative buds of apricot and moun ̄
tain peach and calculated chilling and heat require ̄
ments as the total chilling and heat units accumula ̄
ted during these phases.
All analyses were conducted in the R 2􀆰 15􀆰 2
programming language. All procedures used in this
study were contained in the R package ‘ chillR’
(Luedeling et al.ꎬ 2013)ꎬ available at http: / / cran.
r ̄project􀆰 org / web / packages / chillR / .
2  Results
2􀆰 1  Chilling and forcing periods for leafing and
flowering of apricot
During 1963 - 1988ꎬ the average first leafing
and flowering dates of apricot in Beijing Summer
Palace were the 18th and 7th of Aprilꎬ respectivelyꎬ
with flowers appearing on average 11 day before
leaves unfolded. Daily chilling and heat accumula ̄
tion rates between the previous May and April were
7422期      GUO Liang et al.: Differences in Heat Requirements of Flower and Leaf Buds Make Hysteranthous 􀆺       
used as independent variables in the PLS regressionꎬ
while dependent variables were apricot leaf unfolding
and flowering datesꎬ respectively. Based on the VIP
and standardized model coefficients of the PLS re ̄
gressionꎬ chilling and forcing periods for leafing and
flowering of apricot were identified (Fig􀆰 1-2).
Fig􀆰 1  Results of the PLS regression analysis for flowering of apricot in Beijingꎬ China during 1963-1988. Blue bars in the top row mean
that VIP is above 0􀆰 8ꎬ the threshold for variable importanceꎻ while the grey bars mean the VIP is below 0􀆰 8. In the second rowꎬ red col ̄
ors mean the model coefficients are negative and important (VIP>0􀆰 8)ꎬ while the green colors indicate positive and important relation ̄
ships between flowering and daily chilling and heat accumulations. In the third and bottom rowsꎬ the greyꎬ red and green bars indicate the
standard deviation of daily chilling and heat accumulation and mean temperature. The left part of the figure is the PLS analysis result for
the chilling periodꎬ while the right part is for the forcing period. CP means Chill Portions and GDH means Growing Degree Hours
Fig􀆰 2  Results of the PLS regression analysis for leaf unfolding of apricot in Beijingꎬ China during 1963-1988.
See captions of Fig􀆰 1 for a full explanation
842                                  植 物 分 类 与 资 源 学 报                            第 36卷
    In Fig􀆰 1ꎬ the left panals showed the results of
chilling analysis for flowering. Between 17th Septem ̄
ber and 27th Februaryꎬ model coefficents were mostly
negative and VIP values exceeded 0􀆰 8 ( the thresh ̄
old for variable importance). Howeverꎬ this period
was interrupted by some phases with positive coeffi ̄
cients indicating delaying effects of high chill accu ̄
mulation on flowering. Since these unexpected peri ̄
ods were short and always unimportant (VIP < 0􀆰 8)ꎬ
we interpreted the entire period (17th September to
27th February) as the chilling period of apricot flow ̄
er buds. The right part of Fig􀆰 1 showed that the for ̄
cing period started in early Januaryꎬ but heat accu ̄
mulation before February was low. Considering the
average flowering date (7th April) of apricotꎬ we re ̄
garded the period between 15th February and 7th A ̄
pril as the forcing period of apricot flower buds.
Similarlyꎬ the chilling and forcing periods of a ̄
pricot leaf buds were from 17th September to 25th
February and from 18th February to 18th Aprilꎬ re ̄
spectively (Fig􀆰 2). The chilling periods of leaf and
flower buds were almost the sameꎬ while the forcing
period of leaf buds of apricot seemed longer than
flower budsꎬ which could partially explain the later
occurrence of leaf unfolding.
2􀆰 2  Chilling and forcing periods for leafing and
flowering of mountain peach
During 1963 - 1988ꎬ the average leafing and
flowering date of mountain peach in Beijing Summer
Palace was the 8th April and 28th Marchꎬ respective ̄
ly. For the flowering analysisꎬ daily chilling and heat
accumulation rates between the previous April and
March were used as independent variables in the
PLS regressionꎬ while dependent variables were
mountain peach flowering dates. For the leafing ana ̄
lysisꎬ daily chilling and heat accumulation rates be ̄
tween the previous May and April were used. Based
on the VIP and standardized model coefficients of
the PLS regressionꎬ chilling and forcing periods for
leafing and flowering of mountain peach were identi ̄
fied (Fig􀆰 3-4).
According to the regression resultsꎬ the chilling
and forcing periods of flower buds of mountain peach
were from 20th September to 28th February and from
15th February to 28th Marchꎬ respectively. The chill ̄
ing and forcing periods of leaf buds were from 17th
September to 28th February and from 16th February to
8th Aprilꎬ respectively. The chilling periods for lea ̄
fing and flowering of mountain peach were the same.
Howeverꎬ the forcing period of leaf buds was longer
compared with the flower buds.
For apricot and mountain peach in Beijingꎬ the
chilling periods and the initial dates of the forcing
periods for flowering and leafing were almost identi ̄
cal. Varied end dates of the forcing periods for lea ̄
fing and flowering were the dominant factors that ini ̄
tiated bloom earlier than leave unfolding.
2􀆰 3  Chilling and heat requirements for leafing
and flowering of apricot and mountain peach
The chilling and heat requirement values of leaf
and flower buds gave clearer explanations for the
earlier occurrence of flowering than leaf unfolding
(Table 1). The chilling requirements of leaf and
flower buds of apricot and mountain peach were al ̄
most the same. The greater differences of heat re ̄
quirements between leaf and flower buds determined
the chronological sequence of flowering and leaf un ̄
folding.
Table 1  Estimation of the chilling and heat requirements for leafing and flowering of apricot and mountain peach in Beijing
Summer Palace during 1963-1988. CP means Chill Portionsꎬ and GDH means Growing Degree Hours
Species (Buds)
Chilling period
Start End Requirement (CP)
Forcing period
Start End Requirement (GDH)
Apricot (Flower) 17th Sep 27th Feb 75􀆰 1±5􀆰 9 15th Feb 7th Apr 2829􀆰 7±876􀆰 2
Apricot (Leaf) 17th Sep 25th Feb 73􀆰 5±6􀆰 3 18th Feb 18th Apr 5209􀆰 7±1268􀆰 6
Mountain peach (Flower) 20th Sep 28th Feb 75􀆰 7±5􀆰 9 15th Feb 28th Mar 1457􀆰 2±581􀆰 2
Mountain peach (Leaf) 17th Sep 28th Feb 75􀆰 7±5􀆰 9 16th Feb 8th Apr 2992􀆰 6±925􀆰 0
9422期      GUO Liang et al.: Differences in Heat Requirements of Flower and Leaf Buds Make Hysteranthous 􀆺       
Fig. 3  Results of the PLS regression analysis for flowering of mountain peach in Beijingꎬ China during 1963-1988.
See captions of Fig􀆰 1 for a full explanation
Fig􀆰 4  Results of the PLS regression analysis for leaf unfolding of mountain peach in Beijingꎬ China during 1963-1988.
See captions of Fig􀆰 1 for a full explanation
3  Discussion
3􀆰 1   Identification of the chilling and heat re ̄
quirementsꎬ and usefulness of our PLS analysis
Different methods have been used to identify
the chilling and heat requirements of plantsꎬ espe ̄
cially in fruit trees. Valentini et al. (2004) collected
apricot and peach twigs with dormant buds weekly
from November to February and put them into forcing
conditions to release dormancy. Sample buds were
weighed before and after the forcing period to deter ̄
052                                  植 物 分 类 与 资 源 学 报                            第 36卷
mine the time of endodormancy breaking. Chilling
and heat requirements of the sample buds were then
calculated during the respective periods. In Chinaꎬ
this approach was used widely to identify the chilling
requirements of different fruits including grapeꎬ
sweet cherryꎬ peachꎬ plumꎬ apricotꎬ jujubeꎬ pome ̄
granateꎬ and fig ( Gao et al.ꎬ 2001ꎻ Cui et al.ꎬ
2009). Other experimental methods are also in use.
To evaluate the chilling requirements of some Asian
pear cultivars in Iranꎬ shoot cuttings with flower
buds were kept at 4±1 ℃ for different time periodsꎬ
and subsequently forced in a greenhouse with high
temperatures. If 50% of the flower buds sprouted af ̄
ter the forcingꎬ the previous chilling period was as ̄
sumed to be enough to break the dormancyꎬ and
used to calculate chilling requirements (Arzani and
Mousaviꎬ 2008). Both methods may not be useful
for determining chilling and heat requirementsꎬ be ̄
cause they include artificial temperature treatments
that do not represent conditions in orchards. Such ar ̄
tificial treatmentsꎬ especially during the chilling
phaseꎬ have been shown to lead to misleading results
(Luedeling et al.ꎬ 2009b).
Long ̄term phenological records offer an oppor ̄
tunity for statistically determining the climatic re ̄
quirements of treesꎬ based on statistical analysis of
correlations between the timing of phenological events
and daily temperatures. This method has proven useful
for estimating chilling and heat requirements of
cherry flower buds in Klein ̄Altendorfꎬ Germany
(Luedeling et al.ꎬ 2013).
In our analysisꎬ using daily chilling and heat
accumulations instead of pure daily temperatures in
the PLS regression allowed more accurate estimation
of chilling and heat requirementsꎬ because these val ̄
ues should be more representative of the speed of
physiological processes in plants than unprocessed
temperatures. When long ̄term phenological observa ̄
tions are availableꎬ this approach provides a rapid
way to estimate the chilling and heat requirements of
plants without extensive experimentationꎬ and could
be a more accurate method since it just bases on the
natural response of phenology events to chilling and
heat accumulations.
3􀆰 2  Chilling and heat requirements of leaf and
flower buds
Most studies of the climatic requirements of
temperate trees have focused on the chilling require ̄
ments of flower buds. Howeverꎬ heat requirements of
plants are equally important for initiating flowering.
The fulfillments of chilling and heat requirements of
leaf buds are also vital for leaf unfoldingꎬ photosyn ̄
thesis and vegetation growth of plants. Yet most past
studies have focused exclusively on chilling require ̄
ments.
Both leaf and flower buds chilling requirements
of 65 fruit tree cultivars were estimated experimen ̄
tally in Chinaꎬ with results indicating that chilling
requirements of flower buds were generally slightly
higher than leaf buds. The chilling requirements of
flower buds of different apricot cultivars varied from
790 to 920 Chill Units ( calculated with the Utah
Modelꎬ another chilling model)ꎬ while they were
790- 910 Chill Units for leaf buds ( Gao et al.ꎬ
2001). Similar chilling requirements of both leaf and
flower buds were also observed in our study. Howe ̄
verꎬ the fruit trees in our analysis displayed huge
differences in the heat requirements for flowering and
leaf unfolding. Earlier flowering followed by leaf un ̄
folding could be attributed to lower heat require ̄
ments compared with leaf buds.
3􀆰 3  Importance and urgency of the phenologi ̄
cal observations in China
Phenological observations have provided impor ̄
tant guidance for agricultural practicesꎬ advising
farmers on the timing of spring sowingꎬ irrigationꎬ
fertilization and crop protection. They are also impor ̄
tant for evaluate the risk of frost damage and for fore ̄
casting plant development and harvest dates. Recent
climate warming has aroused public attention across
the world. As the most responsive and easily obser ̄
vable indicator in natureꎬ phenology records have
been used to ascertain impacts of climate changes.
Variation of plant phenology induced by current
1522期      GUO Liang et al.: Differences in Heat Requirements of Flower and Leaf Buds Make Hysteranthous 􀆺       
global warming can have significant impacts on plant
production (Lobell and Asnerꎬ 2003)ꎬ plant com ̄
petition (Rathcke and Laceyꎬ 1985) and interac ̄
tions (Primack et al.ꎬ 2009)ꎬ shifts in agricultural
zoning ( Badeck et al.ꎬ 2004)ꎬ pest and disease
control (Luedeling et al.ꎬ 2011) and pollen dispers ̄
al forecasts (Traidl ̄Hoffmann et al.ꎬ 2003).
Compared with systems in place in Europeꎬ A ̄
merica and Japanꎬ where long ̄term phenological ob ̄
servation records are being collected from widely dis ̄
tributed observation sitesꎬ systematic phenological
observations in China started laterꎬ and unfortunate ̄
ly stopped in 1988 after observations had been con ̄
ducted for about 25 years. Abundant biodiversityꎬ a
vast land areaꎬ and various climatic types in China
provide unparalleled opportunities to conduct pheno ̄
logical observations. So it is quite important and ur ̄
gent to restart the standardizedꎬ systematicꎬ compre ̄
hensiveꎬ and nationwide phenological observations
across China.
4  Conclusions
PLS regression between phenological dates and
daily chilling / heat accumulation rates proved useful
for identifying the chilling and heat requirements of
leaf and flower buds when long ̄term phenological
observations are available. For hysteranthous plantsꎬ
lower heat requirements of flower buds determined
the earlier occurrence of flowers compared to leavesꎬ
while chilling requirements for leaf unfolding and
flowering were similar. Consistent phenological ob ̄
servations should be conducted in China since phe ̄
nological variation of plants can provide direct evi ̄
dence of climate changeꎬ which is expected to affect
plant growthꎬ development and survival in the near
future.
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3522期      GUO Liang et al.: Differences in Heat Requirements of Flower and Leaf Buds Make Hysteranthous 􀆺