PLANT PHYSIOLOGY AND ECULU\:iYI-~ v-

PHENOLOGY OF FLOWERING IN Citrus sinensis

C.J.COVATT and S.M. STREETER Department of Botany and Plant Sciences. Cirrus Research

Cerrter and Agricultural Experiment Station, Uni';ersity ofCali/amm, Riverside. CA 92521 USA

T. C. MINTER. N. v. O'CONNELL. D. L. FLAHERTY, M. W, FREEMAN and P. B. GOODELL

Coperatille Ex tension. University ofCalifomm. CA USA

Additional index words. Citrus bloom period, an thesis, petal fall, leaf to flower ratio, relatively "leafy" and "leafless" inflorescences, degree-days.

Abstract. In preparation for developing a model for use in predicting 10% anthesis in the southwest quadrant and 75% petal fall in the northeast quadrant of commercial "Washington' navel orange trees (Citrus sinensis [L] Osbeck), we determined the time of budbreak and growth rates of developing flowers. Flower buds borne on leafless flowering shoots (Jess than 1 leaf per flowerl undergo budbreak and anthesis earlier in the bloom period than flowers borne on leafy inflorescences (greater than 1 leaf per flower). Flower buds which undergo budbreak (or anthesis) progressively later in the bloom per iod: 1) are borne on inflorescences with an increasing number of organs of which a larger proportion are leaves; 2) have a higher probability of being positioned in the axil of a leaf that develops to maturity; and 3) undergo faster calyx, corolla, and ovary growth. Faster­growing flowers and ovaries persist on the tree longer and contribute proportionally more fruit to harvest. Physiological differences betwtten leafless and leafy inflorescences are manifest by the time the inflorescence undergoes budbreak. Subsequent flower development' to anthesis or petal fall is a function of temperature only.

Introduction

In the San Jo;qurn Valley of Central California. citrus flowers produced during the spring bloom fulftll an important niche In the beekeeping industry. Citrus flowers are the only "pasture" available to pollinator bees between the end of the flowering period in the deciduous tree crops and the beginning of bloom m the next pollinated -:rop. seed alfalfa. [n addition. citrus t10wers provide a source of nectar which in some years is of commercial value to the California honey indust ry. Laws prohibiting the use of pesticides on citrus from the time the trees are at lO% anthesis in the southwest tree quadrant through the time the trees reach 75~ petal fall in the northeast tree quadranr protect bees so they can safely forage during this period. The Ume at which the citrus trees in the various districts in the citrus-growing areas of the San Joaquin Valle v meer these requirements has been diftlcult to predict. Proble~s of economic consequence have rerulted when 10% anthesis has been -:alled ~oo early in the bloom period to permit effective con trol of trulttree leafroller IArchips argyrospilus) or citrus cutworm (Xy!omyges curialis). or when 75% petal fall has been called too long after petal drop to adequately protect the developing trUlt from citrus thrips lScirtothrips citri}.

[n order to determine when these two events marking the bloom period will occur each year. considerable m<Ul hours are spent every spring making vlSU.i! observations at a number of key orchards in each district. Announcement of the two calendar dates which officially mark the beginning and end of the citrus

The ;enior author thanks Dr. C. Cogglm. Jr.. for hi, <:rill.:al n:vi.:w 01 the: manuS(;npr.

I~b

[L.J OSBECK, cv. "WASHfNGTO NAVEL ORANGE

bloom is made 72 hours in advance of each event, respective]' lo::ger period of advance notice is desirable. )

It is our objective to develop a temperature-dependent moe of citrus bloom phenology to predict 10% anthesis in the sou', west 'quadmnt of the tree and 75% petal fail in the northe" tree quadrant. Such a model will eliminate the need fo, visu data coUection and remove the subjectivity that is inherent the current prediction process. Development Of the model supported by funds from the California Statewide Integrate Pest Management Program for citrus. Thus, the model will (­available through the state IPM program to fann advisors, citr ' growers, beekeepers, and area pest control advisors alike so th. they can monitor for themselves the approach of both critic bloom dates by simple computatien of degree-days.

In preparation. for developing a temperature-depende~ phenology model of flowering in the 'Washington' navel oran~

variety (Citrus sinensis [L.] Osbeck), we studied tne influen~ of environmental and tree factors on the phenology a, physiology of the flowering process. Potential topophysic effects on flowering. fruit set, and yield were also examine ~ While suspected. such effects have not been demonstrated c'

quantified for the 'Washington' navel orange.

Materials and Methods

'Washington' navel orange trees (Citrus sinensis [LI Osbe-:~ in commercial production were se lected to represent areas the Central Valley with potentially different temperatu­patterns both prior to and dUring the citrus flowering perine Sites. rncluded: Lindcove, Lind~.YJVisajia,andFresno.Riversic was rncluded to provide a site typically warmer in the spring thJ those in the Central Valley. Approximately 25 innorescen-:e were tagged. ideally before any of the flower buds rea-:h<.' I mm in length. in both the southwest and northeast quadr:m: of each of 20 trees. Typical inflorescences contained from I . 5 !lowers with 0 to 5 developing leaves. The position of e:.l. flower and each leaf within the inl10rescence was recorde­Each int10rescence was checked 2 times J week. and t;-, following data were recorded: 1) for each tlower in an :­tlorescence, bud length to the nearest 0.1 mm. date ofanthes (f1ower opening), date of petal fall. and date ot' nower abscissior 2) date of abscission for each leaf in the int1orescence: :lr

3) date of in t10rescence abscission. After petal fall. the dia etc of each remaining ovary (fruit) was measured llnce a week un; mid-July (end of the June-drop period in Cali!.) and the once a month until harvest. At each site. Jmbient air ternper:ltur :.tir temperature within the canopy. and sou temperature beic the canopy were recorded. [n addit ion, irngatlon. precipita tic!

relative humidity. and solar radiarion were monitored. Da were collected for both 1983 and Il)X~.

Degree-days were calculated using a double SlOe wave abo' the threshold temperature 9.-PC pgoF j l.7 Lising this threshoi the start date which minimized the coefficient of variation 1', the mean number of degree-days was determined

Results

[n this paper. the term intlores.:ence applies to JII tlower!1 shoots :lfising t'rom lXillarv buds as defined bv Reece - r range of intlorescence types' observed in thiS study was ..:onsiste with those described bv Sauer 10 . At the time of visual (mad s.:opic) budbreak. inn~rescences had from 1 to many tlow buds. Each tlower was borne in the axil of a leaf prunordiul By the time of macros.:opic budb reak. it was apparent that sur leaf prim ord ia had aborted. while others had continued develop in [u ~mall leaves'. Thus. when int10rescences bec:u

hoc. lne. Soc. Cilriculrure. IlJi'f-J. Vul.

--------------------------------

Table 2 Relationship between leaf to flower ratio of the inflorescence and the time of !lower budbreak and anrhesis

I, .

, t

I•!

, t

Mean date of for position

I flowers

Mean date of an thesis for pllsition

I tlowers

Site Year <I >1 < 1 > I

Riverside Lindsay Lindcove Lindcove Visalia Fresno Fresno

1984 1983 1983 1984 1984 1983 1984

reb. ~b.r. ~1ar. ~Iar.

Mar. \lay Mar.

27 29 25

8 14

20

Mar. Apr. Mar. Mar. Mar.

Mar.

3 2

30 9

16

25"

Mar. Apr. Apr. Apr. Apr. .-\pr. Apr.

19 26 24

9 14 30 16

Mar. 23*z Apr. 30" Apr. 30* Apr. 8 Apr. 16 May 6* Apr. 19*

Z Paired dates followed by an asterisk l"j are. significantly different (p < 0.05 by Student's r test!.

y Data not available; the population of !lowers ragged for obscrvatoin before the bud rea~hed 3 mm in k~th was wo small to provide statistically valid results.

visible on the tree. they had from a to many developing leaves. It is in this context that we use the relative terms "leatless" and "leafy" to describe inl1orescen-:es with leaf to flower ratios of less than I leai per tlower or greater than I leaf per tlower. respectively. since both leaves and tlowers abscice throughout the bloom period. ' _._

The position within tile intlorescence occupied by a tlower had considerable intluence on when budbreak and anthesis occurred (Table 1). For this study. budbreak. is defined as the date on which the tlower bud reached 3"t 0.1 mm in length. For all intlorescence types. apical flowers Iposition I) were precocious; position :2 tlowers. just below the apex. were typically delayed in their development relative to position 1 and most other positions. This situation is consistent with apical dominance orten observed in tlowering shOalS. Development of !lowers in position other than I and::; was highly vanable Position .3 tlowers were often as delayed in lheir development as position ::; tlowers. Flowers more basal in the intlorescence appeared to be intluenced to a lesser degree by apical dominance and were often intermediate in their development to both positions I and 2. The results provided evidence of other sites of dominance. [n intlorescences with 7 or more !lowers. the flower in position 6 typically reduced the rate of develupment of the nower immediately below it and. to a lesser extenr. the other tlowers basal to it. Thus. nowers in position 7 often had budbreak dates 3.5 late :lS those in position 2. The variabiJity associated with the development of !lowers in positions .3 th rough 7 is evident even when the data are averaged for these pnsllinns (Table I). These results were consistent for all site ror both years of the study.

The development ot tlowers in each position within in· tlorescences having different leaf to tlower ratios was examined to determ ine the influence exerted by the in!lorescences ty pe on flower developmenl. Generally. t10wers in all positions uf leafy inflorescences were delayed in their development by approximately ~ days over their counterparts in leatless

Proc: Int. Sue. CirriLuliure. I ')84. Vol. J

I :IDle 1 lnllUCII\:t'" V1 ll'lV"\". t' ........ ~ ....... .. ....... .-.- ... - .... -.-_ .._- _.. -.- ..

~I~an date of budbreak Mean dat~ of anthesis

Position Position

Site Year 2 3·7 :. 3-7

Riverside Lindsay Lindcove Lindco·.~

Visalia Fresno Fresno

1934 1983 1983 1984 1984 1983 1984

Feb. Apr. Apr. Mar. ~1ar. May Mar.

27 a I a :. a

IS a 9 a

21 J

Mar. Apr. Apr. Mar. Mar.

Mar.

6 6 9

27 23

30

'"b b b b

b

Mar. Apr. Apr. Mar. Mar.

.-\pr.

5 b 3 a

15 b 28 " :!2 b

:. c

Mar. Apr. Apr. Apr. .-\pr. May Apr.

20 a 27 a 26 a

9 a 14 a

:. a i 7 a

Mar. Mav May Apr. Apr. May Apr.

26 9 7

19 25 12,­~:J

'" c b b

c c

b

.-\pr. ~Iay May Apr. AfJr. Ma\' Apr.

24 7 6

19 23 10 24

b b b b b h

c

Z For budbreak or anthesis. dates followed by dillerent letters are significantly different at the 5% level by Duncan's multiple range analysis. y Data not available; the population of flowers tagged for observation before the bud reached 3 mm in lenght was too mlall to provide statisticall\

valid results. .

inflorescences. This delay was not significant at the 5% level (Student's t test) at budbreak but was for anthesis (Table 2). Only data for the apical (position l) Howers are reported; the influence of inflorescence type was sinlilar for all positions.

To examine whether leaf to nower ratio intluenced !lower development before budbreak, from _budbreak to anthesis. or postanthesis. we analyzed the rate of development of individual flowers as a function of temperature. Calculation of the number of degree-days above the threshold temperature of 12.7°(' (55° F} reqUired by individual flowers to develop from budbreak to aothesis or petal fall for 3 sites studied in [983 confirmed that the rate of flower development during this p~riod was solely. dependent on the daily temperatures experienc~d; the nUlIl ber of degree-days was the same at the 5% level (StJdent"s ( test) regardless of the type of inflorescence on which the !lower was borne or its position within the intlorescence (Table 3). Thus. the effect of ""eafiness" on flower development occurred prior to budbreak of the apical flower. e.g .. intlorescenn~

budbreak. The delay in development associated with "leafness' of an

innorescence is consistent with the fact that shoot developmenr in citrus is a continuum; the shoot apex continues to initiate new leaf primordia until it differentiates into a tlower. Sometime later. nowers :lre initiated in the axil of each le:lr" below the apex of what is now an infloresce!lce. These events occur prior tu visual (macroscopic) budbreak'. Since intlorescence le;Jves du not intluence nower development from budh reak to petal fall. the date of budbreak of the apical nower of ar. intlvrescence is a guml indicator of the relative date that an apex becam<: '\;ommitted ,. to tlowering. Thus. the later the date for hud­break. the more leaves per nower.

Since tlowering shoots have the pvtenrial to haw a leaf :lnci a tlower at every node. which would give a le:lf ttl tlower ralll1 vf lust under \' I the apical t10wer is -nut burn~ in lh~ 3.."\il ", a leaf)'. the ubserv:ltion that early developing int1,>r~~ct'rh:<:' tearly date of buubreak or anthesis) were preduminanlh leatle~,

while later ones were leafy suggests that :In earl~ (Ornlliitment r,\ tluwering. by rhe ;Jpex C;Juses leaf primordia tl' aht 1 r[. while a later commitment to tluwering inhibits develt.>pl!1<:lll ,'I. Itll'

X\i1larv tlowers. ('ornparison of the mean number of leaves anJ mean nUlllber

of 110wers fur learless and leatv intlorescence<; cuntirms that there is both :l loss in tlowers and :In increase in leaves to achieve the higher leaf tv tlnwer ratio observed for iater·developin~

intlorescen..:es (Table ~l. Cunsistent With these changes. tluwers hurne on larer·ueveloping int10rescences typIcally had :I hilZher probabilit\ of heing in the 3..xil 'Jf a leaf that developed rll maturirv !Table ~).

Afte~ pel:!1 fall. Ie:!'· ro tlower ratio had considerable influence on the growth rate uf the young ovary. Flpwers borne on leat,­intlurescences exhibited faster ovat:' gruwth pllstanthesi, (Table ~l. This latter ohservatiun is Df IntereSl since (nllt ,1' the 'Washingtun" navel vrange which have higher g.ruwth rate, during the 2nd week after petal fall have a higher probabilir::­of £.ersistin!! on the tree to harvest~.

The results ur' the field studies provide evidence that in an::­

_ _

&f I

Tabk 3 Mean number.pf dC(rree-days requireo b~ md,,·,duaJ !lowen Il' !!~' !!T<>nJ budbreak 10 i1nthcsis (A, or petal lall T'PII as a function ul no"er

I• ~

position and inflorescences typt> i I

X dCl!ree·days+ std. deY. (number of !lowers)

Leaf to flower ration Event

F1l1·...'U position within the inflorescence, t I

Lindsay - 1983 r

LIt- 0 <

Lit- I <

A PI

A PI

~2~o2i'65, 37 = J 1 (651

23(;::Qt471 40:151431

240, 29 (26) 41 ! 17 (261

244 : 34 (30) 42 ! 17 (30/

243= 291411 41 : 16141)

240:351711 45 = I ~ 1711

r I

LIt­ "" 1 A rl

246 = 341421 44 : J ~ '42,

247:34'161 45:1~1161

lifO"'" .\ PI

271>:21,J(l4, 311, 1\, I 1If"

270 : 2:' .1 I) :< I : I I , 111

27: : 2~ '01 32: 121\:>,

L;I· 1 < ,-\ PI J:; : 1:; t 2h I

::5 =~.'ljOI 34 : 15 II 01

277: ~5 13), 37:22(31;

L!F ~ 1 A PI

2~.:l

~ '7 : : .. \ ~ ~ t

: .: 1 1:;~ I

L,'f 0 < -\ PI

~59:3.JllIIJ

33:1~l](11

:'~ : 4£, II::', 43=20,51

LFl< 270:44,111' 44 = 1'111<,

2~3 , .. J t J-II -Ill : ) 'J t:- I

L/J­ ~ 1 A PI­

2F:4::',lj, 49: :0 .l'il

Table 4 Characteristics ~ssocialed'~'iih Iealkss' less than 1 leaf per !lower I and leaf~ I greatfr than) leaf per 110werl inl1orescence~

Site ' ear

\1ean numberleave~

Mean number of Ouwen

< 1 >\ .:: I

nuwcrs In

leaf :l' its (", I.~

," I <I

Ovary. I'ru ill !U()wth rdteZ

I mm per da\"

Riverside

Lindsay

Lindcove

Lindcove

Visalia

Fresno

Fresno

1984

1983

1983

1984

1984

1983

1984

I 4' , '4

.2..< 3

3 5 3 5

4

3 5

4 T 5 T 4 1 4 T 4 '4 6 T 5 4

.-~ .­-;;;

.­., .­

*' * .­

.' -;

_.

"

34

17

68

67

39

55

sc,

415

21

5S

47

8'.'

67

0.U9

v.13

0.28

\E

0.11*

Z

y :

Growth rate of the ovary (fruit) was determined by measuring fruit uanS\cr>e diam,'ter 1 limes a week bc~inning at pedal fall. GW\o\lh rate during the 2nd week after petal fall is repar ,ed. . Paired values ;ollowed by an asterisk (") are significantly differcnt Ip < 0.05 by Sluden(~ ( ::.-st). Statistical analysis was not done. Data have not been entered into the computer for analysis at this lime.

I•

population of flowers of the 'Washington' navel orange variety. the first 10% of the flower population to reach anthesis will be dominated by apical Oowers borne on leafless inflorescences (Table 5).

Using 9.4°C as the threshold temperature, the best dates to ._~ • .:_.~ .. L .... r ..J_ .............. ...a .... ~.,. •• ' ... _~ .J_ ...

January :;Q. which gave a mean value of 505.5 degree-days to 1O'7c anlhesis in the southwest tree quadrant. and February 1 which gave a mean value of 744,4 degree-days to 757c petal fall in the northeast tree quadrant. Comparison of the observed and predicted dates for these two bloom events is presented

"T' t ' ''''' ,.. - -_. ­ -, • ~ •••

1

not vet been cakulated 1111 twu sites for the 1Qr;4 seas:m. The Tabk 5 Charactcristi<:~ of the fl,'wer, "\lmrn'Jn~ Ifl. JI111,..", In till' southweS1 tree qu~dranlmodel will be field-testt>d in the spring of lq~:,: subsequent

modification rna) be requued. At JO';; ~nthesls In the southweST quadran'Discussion

Characleristic, oj the !lower' 'P.1e topophysical effects on flower development ohserved that have open,'d

during bloom California have

of the 'Washington' navel orange variety in been reponed previously for the 'Valencia·:1· 4 Site Year

'if in pusition I

"; bornc un JeaJles' mfloresccnces (LlJ < I)

and 'Hamlim '3 sweet 'Marsh' grapefruit (C

orange varieties (C sinensis) and paraJisr/. Differentiation between

lor the Riverside 1984 93 93

effects that inflorescence type and flower position within the inflorescence have on flower development in the "Washington'

Lindsay 1983 100 ~I

navel orange along with su bsequent determination of the degree to which flower development in this variety was affected make~

Lind,ovc Lmd"ove

J983 19&4

JOO JU(l

it possible to predict bloom phenology for this crop. example. in our particular case. the flowers \:omprising

For I O(~

I: resnu I· resno

J983 1984

100 Ion

97 88

anthesis on the southwest quadrant of the tree for most sites

Table 6 l:sc of de!!ree-days tll predic,t the \It'~innIn~ and cnd of the l1()werin~ period 'in the '\\ ashin!!10n' navel oran!!c "arietl

111"; a nthe"s ":­ . petal fall

'."llthwest trec 4uadrant ~<lrlhea'i tree quadrant

Site " ear Ohserved Prcdkt"d bror

1,:J1endar da tc I (days) Observed Predi,ted Erwr

(calendar datt~1 (dap)

Riverside Riverside Riverside Riverside

19!:il Jn~ 19~3

1984

.\:~

-\Pl. . pr \far

12 I! I ~

Arr -\p1. \lar.

1~

13

... " - 6 :;

Apr. -\pr. Apr. ·-\P1.

20 :In 28 4

-\rr -\rr \bl Apr.

16

:; ..

.. + 4

II

Lindsa~ 1910 Apr Apr 2(1 ,- Ma~ 13 \lal 1 J

Lindcove Lindcovc

1983 J9h4

-\I'r \1J1.

~( I

31 -\pr. Apr

12 J

- 8 ... I

\lay Apr

9 15

\lay -\pr.

5

" ..

Visalia 1984 .-\pr. 9 "'pr. 14 ... 5 \il)

Fresno 19h3 Fresno 1984

Mean: standard deviation -Coefficient of I'ariatkln

Apr \pr.

201 12

AI'/ Apr.

21 15

- 3 ... 3

4 ~ :; 0.8

Ma~' SF

13 Ma~ 14 + I

1.2

1. Data not al'aiJable tor the early part ot' the hloum periud m 1981. }' Data have not been enten,d into the l'ompu ter 10r analy sb at thb time.

in California will be predominantly api<.:al flowers in relatively leafless inflorescences. Any environmental parameters or cultural practice that significantly decreases the population of leafless intlorescences will concomitantly delay bluom. Furthermore. it is apparent that the length of the bloom period is a function of I) the proportion of flowers borne on leafy inflorescences in the total flower population and 2) Jaily temperatures just prior to or during this period. For the ~ years of this study, the length of the bloom period from 107< anthesis in the southwest Jree quadrant to 757c petal fall in the northeast tree quadrant averaged 20 ± 2.5 days.

Because the period of !lowering in this variety is relatively long, accuracy is needed in predicting the end of bloom. By the time 757c petal fall has been reached in the northeast tree quadrant. nowers on the rest of the tree have already passed this point. Since young fruit are susceptible to damage by dtrus thrips from petal fall until they reach 4 cm transverse diameter. the time at which a grower can start thrips population control by the application of pesticides is inif-lortan t.

In addition to providing the data necessary to construct the temperature-<iependent phenology model of flowering i:1 the 'Washington' navel orange which wilJ be field-tested for the bloom of 1985, the present study also provided information important to understanding the physiology of the flowering process in citrus.

Demonstration that flower development from nower 1 ,,"_11 - ... .., ....a.A th_ c-';ITTlPo nlln,hpr of

and degrees-day analysis support the interpretation thaT leal~ intlorescences arise from a late commitment 10 nowerinl:! h~ the shOOI apex which is accompanied by reduced axillary nowc'} development. As a consequence. flowers in leaf~ innores~'t'llce~ undergo anthesis and petal fall later than their "lllll1lerpartS irr leafless inflorescences even though their rate of growth frl\111 budbreak to anthesis may be faster due to the warmer dail~

temperatures that occur later in the spring. Since the r<'sulh of this research demonstrated that nower development JuriT1!-, the periud fwm budbreak to anthesis or petal rail wa~ sC'lely _ dependent on temperature. they rule out the possibilitie, that inflorescence leaves enhance or inhibit !lower development i'

during this period. Thus, contrary to suggestiolls in the Inel­attire. intlorescence leaves do not serve as a significant sourc:e 4

nor a competitive sink 9 for photosynthate homlllnes or mineral elements during this 6- 10 8-weeks period in the Washington' navel orange variety. However. the results suggest that devehlpin!Z tlowers and leaves might be significant competitive sinks prior to budbreak. !lowers being the stronger sir.ks in early-deve)nping inflorescences and leaves the stronger sinks in later-oeve!t'pin!, inflorescences. .

Taken together, our data suggest that the implllveo (ruit Sl';

and yield observed, for later-opening flowers. i.e .. thosl' i.n position 2 of the inflorescence and thuse bl'rne 011 lealy inflorescences, of the Washingtun' Ilavel llT:Jnge J

• ~ and other citrus varieties t

,3.4 is probably due to the more favorahle rpmt"lpr"tllf"~ pxnpripn,'p/i hv thl'<o;e !lowers uurilH' (heir

__

"Duences ova')' '(ffUit) growth rate. Faster.growing fruit are ,orne on leafy inflorescences. Ovaries with higher growth rates s. _Jrin6 their early development (:2 weeks after petal faU) tend :' reinain faster growing. to persist on the tree longer. and to 6, •1ntribute more fruit to harvest 2 • 5 ,b.

7.Literature cited

Haaas, A. R. C. 1949. Orange fruiting In rdatlOn to the blossom 8. opening period. Plant Phvsio I. 24: 481-493. Harri~. M., C. Lovatt. P. Tomlinson. S. Streeter. 'and W. Dugger. 9. 1984. Criteria for th~ early identifi<:ation of ab~elsmg and perslstmg fruit in navel orange. Proc. Int. Soc. Citriculture I In press!. . J ahn, O. T. 1973. Infiorescence t\'pc, and !ru Itffi!': pattern~ In 10. 'Hamlin' and 'Valencia' orange~ and 'Marsh grapefruit. A mer. J. Bot. 60: 663,<)70. . 11. Lenz. F. 1966. Flower and fruit development In 'ValencUi Late orange as affected by type of Inflore,cenee and nutritional status.

Hurt. Res. 6: 65·78. '. Lord. 1:. and K. Lckard. Shoot development In Cirrus slllen~i5 I L I (\.... ashington navel orange) 1. Floral and m!1ore!><.·en<:e ontogeny. iSubmmitted to A mer. J. ·Bot.) Ramina. A. and A. Masia. 19HO. Level~ of extractabk absisic aCid In the mesocarp of persisting and abscising pl'adl lruit. J . .4 mer. Soc. Horr. Sci. 105; 465-468. Reece, P. C. 1945. Fruit set in the ,"tel prang~ In relation to flowering habit. Prac. A mer. Soc. Hart. Sci. 4t>. Ii I-fit>. . Reuther, W. 1970. Temperatur~ effect> on ,'ltru, lrull gro..... th and development. ]8th Inri. Hart. Conp. ~71-24 j Rhoades. W. and R. Wedding. 1953. Thr photosynthetll' and n.-spirator)' rates of citrus leaves of four different it!!",. Cirrus I.eo!'es 33: 10·11. Sauer. R. M. 1951. Growth or orange shoots. .4u~·rr. J. A.I'!'. Res. 2: 105·117. Zuc<:oni. F .. S. P. Monselise. and R. GOfen. 197tl. Gro",th­abscission relationships in developIng orange frun SClenrlO Han 9: 137-146.

INTER-RELATIONSHIP BETWEEN FLOWERING AND FRUITING IN SWEET ORANGE, . CULTIVAR NAVELINA

S. BECERRA Insriruto Nacional de Investigaciones Agrz'colas

Aparrodo Posra188. Tecomdn. Colima. Jf'£XICO

J. L. GLARDIOLA Unil'ersidad Polirecnica de I 'alencia

Camino de Vera s.'nP, Valencia. ESPAIVA

:. Jditional index words. ~avelina orange. sprouting, nowering. ·uit·setting.

-. bstract. Fruit exert an inhibitory effect on bud sprouting '1e follOWing spring. Nevertheless, at fruiting densities up to o fruits/m 2 cup external area, both parameters are not inter­elated. The relation between'mese two parameters varies with "le year and the orchard productive potential, and at constant -uiting, bud sprouting has been observed to be more inhibited 1 low-bearing orchards. High bud sprouting densities tend a increase the number of leafless one-flower and multi-flower ;)routs, while the number of vegetative sprouts decreases, This ..J ggests that a reversion of floral buds to vegetatibe buds occurs ... ith low sprouting densities. Flowering densities lower than 20 f1owersl1 00 nodes restr ict the number of fru its at harvest. ~t higher flowering densities, fruit number is independent of ,ower number in productive orchards, or is reduced in low·

.. saring orchards. Total fruit·set per tree was the principal factor

.etermining tree productive potential.

Introduction

Flowering is a critical phase in the determination of crop _e1d; without it, fruit formation is impossible. However. ~ Jctification is correlated to flowering only when the latter is .ther too sciuce21 or excessively heavy J. As in other species fming a large amount of flowers, fruit set in citrus species

:curs only in a very low percentage of the initially fo~~ed :wers, thus fruit set is normally the central factor determmmg lal yield IS. On the other hand, fruit load has an inhibitory effect on

'wering20 , in. such a way that a large number of fruits

,;pecially in the more sensitive cultivars) may significantly :uce flowering and th~s initiate an alternate bearing cycle l7 .

)\i.. ~ring and fruiting are therefore inter-related and an dilibrium between them is necessary to obtain high yields a continuous manner. The present work involves a comparative study of flowering,

:it set and crop yield in trees from productive and improductive .eet orange cv. Navelina groves. Observations included :-ferences between groves as well as between trees within the 'ne grove. The study was conducted over a oeriod of .. " .."r~

as in Ouenced by lruit load; b) relatiunsh ips be {ween ill)Wer1Jl~

and fruiting.

Materials and Methods

The investigation was conducted in three adult sweet \.H;JIl!!e (Orms sinensis [L.] Osbeck) groves of the ~a\'elma (UllJ\'a~. located in the regions of Picassent (PIC ASSl::I'T}. Quart de Ie, Valls (QL:ARTJ and Vila-Real (VILA.REAL). 10 the PW\'lIl<.:e of Valencia. Spain. . ..

The PIC ASSEl\'T grove was a low-bearing one. with a 1rUltlng densit\' under 12 fruits/m 2 of canopy. and with average yield, under' 42 kg/trees. The QUART and Vl~A.RfAL groves. on the other hand. produced wigh yields, WIth a fruItmg densIty between 20-30 fruits/m 2 of canopy. and average yIelds hIg.her than 65 kg/tree. . .

Shoot growth (vegetative and floral) characterIStICS .....·ere studied during spring of 1981 and 1982. FIfteen trees were evaluated during 1981 at the PICASSENT and QL:~RT groves. but this number was increased to 30 and 26, respectIvely. dunng 1982. Studies at the VILA·REAL grove were conduded on the same 16 trees during both years. .

Prior to shoot emergence, four branches were selected t rom around each tree; each had a diameter between 2.0 and 2.:' cm. and contained from 1500 to 3000 nodes. Imediately before anthesis. once shoot growth had been completed. the following infonnation was 'recorded: a) total number of nodes per branch; b) number of shoot-forming nodes; c) number and types of shoots formed; d) number of organs (flowers and leaves) per node.

Shoots were classified according t0 6 in the following types: a) multiflowered 1eafles's shoots '(M. 1':s. S); b) one·nowered leaf· less shoots (0. Ls. S.); c) multiflowered leafy shoots (M. Ly. S.): d) one·flowered leafy shoots (0. Ly. S.). and e) vegetative shoots (Y. S.). .

In order to compare yields among trees. data were adjusted to tree size, by determining fruiting density as the number of harvested fruits per unitary area of ex terna1 canopy J~umber of fruits/m 2 of canopy). Yields from the PIC ASSLNT nnd QUART groves were evaluated during 1980, 1981 and 1982. while at VILA-REAL they were evaluated only during 19~1

and 1982.

. Results and Discussion

Relationship between yield and shoot gro".... rh . . The percentage of shoot-forming no~e.s dunng I% I was

inversely related to the previous year fruItIng densIty (Flg. II. However, such relationship was not clear durm~ 198; (Fig. 2). ~n~ '::If tho f"'\t T A DT ...I ' ''' A nr ... ,