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Irrig Sci (1989) 10:303-311 I r r i g a t i o n: clence Springer-Verlag 1989

Influence of water stress on kiwifruit growthM. J . Jud d 1, K. J . Mc An en ey 2 , and K. S . Wi lso n 1Ministry of Agriculture and Fisheries1 Ru ak ura Agricultural Centre, Private Bag, Ham ilton, New Zealand2 Kerikeri Hortic ultural Research Station, P.O. Box 23, Kerikeri, New ZealandReceived September 13, 1987

S ummary . An i r r i ga t i on exper iment was conduc t ed on young k iw i f ru i t v ines ove rtwo se asons to exam ine ef fects of wate r s t ress on f rui t deve lopm ent . Vines weregrown ou tdoor s i n a s andy , roo t i ng medium enc losed w i th in a po ly thene - l i nedt r ench wi th r em ovab le su r f ace cover s t o enab l e s t ri c t con t ro l o f the wa te r supp ly .Mea surem ent s o f f ru i t g rowth , l ea f wa t e r po t en t i a l, an d s t om ata l cond uc t a ncewere ma de t h r ou gh ou t t he season i n con ju nc t i on w i th pe r iods o f wa t e r s t r es simpose d a t d i ff e r en t times, and fo r va ry ing dura t i ons . F ru i t deve lopmen t was ve ryrespons ive t o wa t e r s t r e s s w i th mean f ru i t s i ze pe r v ine a t ha rves t va ry ing f rom60 to 130 cm 3 as a resul t of var iou s s t ress t rea tments . F ru i t ex pan s ion ceased wh enpred awn l ea f wa t e r po t en t i a ls f el l be low 0 .1 MP a . U po n r ewa te r ing , lea f t u rg o twas rega ined wi thin 24 h even af ter severe, pro lon ge d s t ress . An y tur go r lossas soc i a t ed w i th f ru i t so f t en ing was qu i ck ly made up , and t he rea f t e r f ru i t g rowthcon t inued a t t he same r a t e concur r en t l y exh ib i t ed on con t inuous ly we l l -wa te r edv ines . S ugges t i ng t ha t s t omata l conduc t ance d id no t fo l l ow the r ap id r ecovery o fl ea f wa t e r po t en t i a l s and f ru it expans ion m ay be mo re c lose ly l i nked t o wa t e rsupp ly t han t o t he concur r e n t r a t e o f pho tosyn thes i s . Desp i t e t he la rge r ange i nme an f rui t s ize , the shape o f the f rui t size di s t r ibu t ion a t harves t w as not af fec tedby wa te r s tr e ss and i t is conc lu ded t h a t h a rves t y i e lds can be adequa t e ly mod e l l edby as suming a normal d i s t r i bu t i on w i th a f i xed s t andard dev i a t i on .

E c o n o m i c r e t u r n s f r o m k i w i fr u it (Actinidia deliciosa (A. Chev. ) C . F . Laing & A. R.F e r g u s o n v a t deliciosa) depend s t rong ly on i nd iv idua l f ru i t s i zes , which a r e i n t u rnd i r ec t l y a f f ec t ed by wa te r ava i l ab i l i t y ( Judd and McAneney 1987) . F or t h i s r easonk iwi f ru i t a r e usua l l y ir r iga t ed . I n som e a r eas t h i s p r ac ti ce has l ed t o dema nds fo r wa t e rt ha t exceed l oca l r e sources and t hus t he r equ i r ement fo r communi ty -based i r r i ga t i onschemes . I n o rde r t o p rov ide some es t ima te o f t he econo mic v i ab il it y o f p rov id ings u c h ir r ig a ti o n , J u d d a n d M c A n e n e y ( 1 9 87 ) p r o p o s e d a f r u it g r o w t h m o d e l b a s e d o nwate r ba l ance ca l cu l a t i ons and t he fo l l owing p ropo s i t i ons r e l a t i ng wa te r s t re s s andf ru i t g rowth :1 . Mo is tu re s t ress hal t s f rui t growth .2 . Once water -s t ress i s re l ieved, f rui t expans ion resumes a t the same ra te as would beconcur r en t l y exh ib i t ed on con t inuous ly we l l -wa te r ed v ines .

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304 M.J. Judd et al.3. Harvest fruit weights are Normally distributed about their mean value.4. Fruit size distributions have a constant standard deviation independent of theirmean weight.The first two of these propositions were suggested by limited fruit volume measure-ments on water-stressed vines (Van Oostrom 1985; Prendergast et al. 1987) while thelatter two are consistent with results from a number of trials on irrigated vines (Juddand McAneney 1987). These last two propositions are equivalent to assuming thatwater-stress does not change the shape of the fruit size dis tribut ion - only the mean size.

For detailed discussion of the model and its practical implications, the reader isreferred to Judd and McAneney (1987). The following description is included only toillustrate the role of the above propositions within the framework of the model.Briefly the model uses water balance calculations to first determine periods of limitedwater availability within a kiwifruit growing season. Assumptions (1) and (2) are thenemployed to calculate reductions in fruit growth on unirrigated vines and so predictthe mean fruit size for a particular season. By repeating this procedure using historicalrainfall records, the model is used to create a frequency distribution of mean fruit sizesappropr iate to the term of the record which, mathematically combined with the fruitsize distribution of well-watered vines (employing Assumpt ions (3) and (4)), is thenused to calculate a longterm, fruit size distribution characteristic of unirrigated pro-duction. Finally, the financial advantage of irrigat ion is calculated by difference, afterapplying a pricing function to both the unirrigated and irrigated, fruit size distribu-tions.

The present study records the response o f kiwifruit vines to controlled periods ofwater stress with the aim of examining the adequacy of the four propositions used inthe model o f Judd and McAneney (1987). Indicator s o f physiological stress (leaf waterpotentials and stomatal conductances) were measured in conjunction with both therate of fruit growth and fruit size distributions at harvest, for both stressed andwell-irrigated vines.

ExperimentalTrial managementThe trial was established in 1982 at the Rukuhia Horticultural Research Orchard,Hamilton (3751'S; 17520'E) in a manner which enabled control of both the onsetand du rat ion of water stress. The roo ts of kiwifrnit vines were restricted to a poly-thene lined trench from which rainfall could be excluded by ground covers sealedaround vine trunks. This arrangement provided only a small reservoir of available soilwater for each vine which thus became totally dependent on irrigation within a fewdays of applying covers. The trench (90 m long, 1 m average depth, 1 m width) wasorientated north/ south , lined with heavy-duty polythene and had a porous drainagepipe laid along the bottom to prevent possible waterlogging. The lower 0.5 m wasfilled with a layer of coarse pumice and then back-filled with equal parts of pumicesand and the original topsoil (Horitiu sandy loam - Typic Vitrandept). Eighteengrafted vines ('Hayward' cultivar on seedling rootstocks) were planted at 5 m inter-vals and trained on a pergola system in bays 5 m wide. Until January 1986, irrigation

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Water stress on kiwifruit growth 305was provided by microtube trickle emitters (5 per plant), and thereafter by pressure-compensating emitters.

Vines were managed according to standard practices with care being taken toensure adequate nutrition in view of the low fertility medium in which they weregrown. Flowers were band-pollinated to avoid interactions between fruit size and seednumber (Hopping and Hacking 1983). At harvest all vines were strip picked and thefruit graded using a modified commercial grader which provided individual fruitweights with 1 g accuracy.

Water stress was first imposed three years after grafting during the 1984/85 grow-ing season, and again during the subsequent season by withholding irrigation fromselected vines with the covers in place. Once wilting was observed, vines were irrigatedfor the remainder of the stress period at approximately one half of the control rateto ensure their survival.

Daily water requirements were calculated following Judd et al. (1986) usingprojected canopy areas and evaporation estimates (Priestley and Taylor 1972). Thelatter were calculated from on-site radiation and air temperature measurements.Measurements of the projected canopy area of each vine were made for irrigationmanagement in late January of 1985, and in early December, late January and earlyApril the following season. The horizontal pergola trellis allowed these canopy areasto be estimated to within 10% either directly or from measurements of middayshadow areas. Since vines were either well-watered or heavily stressed, the experimentwas not heavily dependent on the accuracy of this measurement nor on the resultingestimated water requirement.Plant measurementsLeaf water potentials (LWP) were measured at irregular intervals during the firstseason and at least twice weekly throughout the 1985/86 season from exposed posi-tions near the top of the canopy. Since previous LWP measurements on kiwifruit(Judd et al. 1986; Prendergast et al. 1987) had shown only a small variation over theplant, measurements were typically made on only one or two fully expanded leavesper vine. The procedure used followed that of Baughn and Tanner (1976), i.e. theselected leaf was wrapped in a damp cloth, enclosed in a plastic bag, excised, and themeasurement made immediately.Stomatal conductances were measured with a Licor 1600 (Licor Inc., Lincoln,Nebraska) using the broadleaf (2 cm 2) aperture. Measurements presented in thispaper are averages of at least 5 measurements on the underside of exposed leaves nearthe top of the canopy: kiwifruit leaves are hypostomatous.

Fruit were tagged in early December as soon as the success of pollination becameobvious. Volumes of tagged fruit (10 per vine in 1984/85 and 15 per vine in 1985/86),randomly positioned within the canopy, were measured weekly by water displacementin a measuring cylinder. To reduce percentage errors in measurement, different diame-ter cylinders were used according to fruit size. As an element of judgement is involvedin this measurement, only one operator was used throughout each season whichresulted in a consistency of approximately 2% and an overall accuracy in fruit sizeestimation of 7%. This resolution meant that twice-weekly estimates of fruit growthwere impractical for most of the season as the growth increment would have beenmasked by measurement errors.

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306 M.J. Judd et al.Resu l t s

Phys i o l og ica l meas ur emen t sPre-dawn LWP's were used as the primary indicator of water stress. Irrigated vinestypically exhibited values of between -0.03 and -0 .08 MPa, which corresponded withmid-day values of between -0.4 and -0.8 MPa. On withholding irrigation, pre-dawnLWP's fell below -0.1 MPa within four days in mid-summer as a consequence of thesmall water reservoir available to the vines. Once pre-dawn values dropped below thisvalue, mid-day readings fell dramatically below the range measured on irrigated vines- the specific value depending on the particular days evaporative demand (Fig. I).Leaf wilting was associated with LWP's of -0.9 MPa and lower.

Stomatal conductances of irrigated vines (both day and night-time) were similarto those measured in other studies (Judd et al. 1986; van Oostrom 1985). Valuesranged from greater than 1.0 cm s ~ for well-lit leaves on unstressed vines, down to0.1 cm s- 1 for stressed vines. Night-time values for both stressed and unstressed vineswere also 0.1-0.2 cm s- 1 as previously observed (Judd et al. 1986). Stomatal responseto water stress generally followed falls in LWP but was more difficult to define owingto greater variability over the plant. This variability increased markedly as vines cameunder stress making stomata l conductance the least sensitive indicator of waterstress.

The time for kiwifruit to recover from water stress was measured by supplyingwater to vines that had been water-stressed for a month. Their visual recovery wasrapid and matched by LWP's which attained values similar to those on well-wateredcontrol vines within 24 h (Fig. 2) and thereafter remained identical. This rapid recov-ery of turgor was reflected in a similar recovery of fruit growth rates (see later) andis undoubtedly facilitated by the kiwifruit vines' extremely high stem and roo t hydrau-lic conductivities (McAneney and Judd 1983). In contrast to LWP's, the completerecovery of stomatal conductance took several weeks.

- 0 " 8| i

W i l t i n g o c c u r s '

xx X

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x x I - 1 " 2x x Ix III - t . 6IIS t r e s s

t h r e s h o l d 2 . 0Fig. 1. Mid-day versus pre-dawn leafwater potentials for a range of vinesand days

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W ate r s t re ss on k iw i f ru i t g row th 307

Fig . 2 . Change i n LW P o f a v ineafter be ing water ed to excess a t 6 .00a m ( 4 M a r c h 1 9 8 6 ) f o l lo w i n g o nemon ths w a te r s t re s s . A l so show na r e m e a s u r e m e n t s m a d e o n a c o n -t ro l v ine . W ea the r cond i t i ons du r -i ng t h i s expe r imen t s w ere ove rcas tw i th f requen t show ers

o 01 3 .I E

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Fig . 3 . A com par i so n o f t he f ru i te x p a n s i o n o f w e l l - w a t e r e d c o n t r o lv ines ve rsus t ime a f t e r f l ow er ing( e a r ly N o v e m b e r ) f o r t h e t w o y e a r so f t he expe r im en t ( - - 1984 /85 ,. . . . 1985 /86 ). A l so show n i s t hec o r r e s p o n d i n g c u r v e f r o m O o s t r o m(1985) ( . . . . . . )

120-v

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' d o ' 8 ' o ' 1doDays a f ter f lower ing

Fruit growthF i g u r e 3 s h o w s th e m e a n g r o w t h c u r ve s o b t a i n e d f r o m w e e k l y m e a s u r e m e n t s o fw e l l - i r r i g a t e d c o n t r o l v i n e s fo r e a c h y e a r o f th e t r i a l. F r u i t e x p a n s i o n o n w e l l - w a t e r e dv i n e s i s n o n - l i n e a r w i t h t i m e ( H o p p i n g 1 97 6; J u d d a n d M c A n e n e y 1 98 7) , e x h i b i t i n gr a p i d g r o w t h f r o m D e c e m b e r t h r o u g h t o th e e n d o f J a n u a r y , f o l lo w e d b y a d e c r e a s in gr a t e o f e x p a n s i o n u n t i l h a r v e s t in M a y . A s c a n b e s ee n ( F i g . 3) , f r u i t g r o w t h w a s v e r ys i m i l a r d u r i n g b o t h y e a r s o f t h e t r i a l a n d i n i ti a l g r o w t h r a t e s a l s o a g r e e c lo s e l y w i t ht h e d a t a o f v a n O o s t r u m ( 19 85 ). S m a l l d i f f er e n c e s b e t w e e n t h e v a r i o u s d a t a s et s,w h i c h a p p e a r t o w a r d s t h e e n d o f e a c h s e a s o n in F i g . 3 , a r e p r e s u m a b l y r e l a t e d t of a c t o r s s u c h a s a g e o f v i ne , le a f - t o - f r u i t r a t i o s a n d f r u i t l o a d p e r v i n e .

A v e r a g e f r u i t v o l u m e s a t h a r v e s t v a r i e d f r o m 6 0 t o 1 30 c m 3, a n d r e f l ec t e d t h ed u r a t i o n a n d t i m i n g o f w a t e r s t r e ss tr e a t m e n t s . F r o m a c o m m e r c i a l p e r s p e c ti v e t h isis a fa r g r e a t e r r a n g e t h a n w o u l d b e e x p e c t e d o n a n y c o m m e r c i a l o r c h a r d a n d i n d e e dt h e r a n g e o f m e a n f r u i t si ze s c o v e r a l m o s t t h e e n t i r e e x p o r t r a n g e a l l o w a b l e f o ri n d i v i d u a l f r u i t .

W i t h i n t h e w e e k l y r e s o l u t i o n o f t h e m e a s u r e m e n t s , f r u i t g r o w t h o n s t r es s e d v i n e sa p p e a r e d t o s t o p a n d r e s t a r t a b r u p t l y i n r e l a t io n t o w a t e r a v a i l a b i l i t y ( F i g . 4 ). T h e r ew a s n o g r a d u a l r e d u c t i o n i n g r o w t h r a t e a s w a t e r s t re s s w a s a p p l i e d . I n s o m e s c a s e ss tr e s s w a s o f s u f f ic i en t d u r a t i o n t o e v e n t u a l l y r e s u lt i n f ru i t s h r i n k a g e . O n r e - i r r i g a t i o n

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308 M .J . Judd et al .

~ 9 03

v

-= 60o

= 3 0ID

0 0 40 80 120Days after flowering 160

Fig. 4. Gro wth curve of vinestressed at two periods of fruit de-ve lopmen t ( e - . - . - -o ) comparedwi th the models predic tion ( - - - - )and the growth on a control vine( ), Shaded porti ons refer toperiods when pre-dawn leaf waterpotentia ls on the stressed vine wereless that -0 .1 M Pa

A

~, 2 4o

o"oc

86 0e ,

A q ~ A& e& &&A&e o

o

8'o ' l o'Mean fruit weight (g)

Fig. 5. Stan dar d deviations versus the mean of fruitsizes from t he 1984/85 (o) an d 1985/86 (zx) seaso ns.Solid symbols refer to water stressed vines

a n y s u c h s h r i n k a g e w a s q u i c k l y r e g a i n e d , a n d t h e r e a f t e r f r u i t g r o w t h r e s u m e d a t ar a t e s i m i l a r t o t h a t o b s e r v e d o n v i n e s t h a t h a d b e e n c o n t i n u o u s l y w e l l - w a t e r e d( F i g . 4 ). W i t h t h e e x c e p t i o n o f t h is r a p i d r e c o v e r y f o l l o w i n g s h r i n k a g e , s t re s s e d f r u i tn e v e r e x p a n d e d f a s t e r t h a n t h o s e o n i r r i g a t e d v i ne s .Fruit size distributionsE x a m i n a t i o n o f t h e l im i t e d a m o u n t o f h ar v e s t d a t a t h e n a v a i la b l e , l e d J u d d a n dM c A n e n e y ( 19 87 ) t o a s s u m e t h a t k i w i f r u i t si ze d i s t r i b u t i o n s c o u l d b e a d e q u a t e l yd e s c r ib e d b y a G a u s s i a n d i s t r i b u t i o n w it h a s t a n d a r d d e v i a t i o n o f a p p r o x i m a t e l y 17 g .W h i l e t h e i r m o d e l w a s i n t e n d e d t o b e a p p l i e d o n a r e g i o n a l s c al e , f r u i t si ze d i s t r ib u -t i o n s f r o m i n d i v i d u a l v in e s h a v e b e e n e x a m i n e d i n t h e c u r r e n t t r ia l . P r e l i m i n a r ya n a l y s e s s h o w e d o c c a s i o n a l p o p u l a t i o n s o f s m a l l f r u i t s e p a r a t e d f r o m t h e p r i n c i p a lf r u i t d i s t r i b u t i o n . T h e s e s m a l l f r u i t w o u l d n o t n o r m a l l y b e h a r v e s t e d o n a c o m m e r c i a lo r c h a r d a n d a r is e f r o m i n a d e q u a t e p o l l i n a t i o n o r p o o r f l o w e r d ev e l o p m e n t . S u b se -q u e n t a n a l y s e s e x c l u d e d t h e se f ew f r u i t t o g e t h e r w i t h a s m a l l p r o p o r t i o n o f o v e r s iz e da n d m i s - s h a p e n f r u i t b y r e q u i r i n g t h a t :1 . f ru i t we igh t s exceed 40 g and /o r2 . F r u i t w e i g h t s l ie w i t h i n 3 s t a n d a r d d e v i a t i o n s o f t h e v i n e ' s m e a n w e i g h t .T h e r e s u l t i n g s t a n d a r d d e v i a t i o n s a n d m e a n w e i g h t s ar e s h o w n i n F ig . 5 a n d i n d i c a t en o o b v i o u s d e p e n d e n c e o f s t a n d a r d d e v i a t i o n o n m e a n w e i g ht , e i th e r f o r w a t e rs t r e s sed o r con t ro l v ines .

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W ate r s t re ss on k iw i f ru i t g row th 309

Fig . 6 . Fr u i t s ize d i s t r i bu t i o n f rom a w a te r ~"stres sed ( m ean = 85 g, std. dev. 13 g, 185 frui tp e r v in e) a n d a c o n t r o l ( m e a n = l l 0 g , s td .dev. = 12 g, 205 frui t pe r vine) vine. S olid l iness h o w n o r m a l c u r ve s w i th m a t c h i n g m e a n s a n ds t a n d a r d d e v i a t i o n s

4 0

O

O

| | | | , |8 0 1 2 0 1 6 0F r u i t w e i g h t ( g )

D e s p i t e t h e a p p e a r a n c e o f th e f r u i t si ze d i s t r i b u t i o n s ( F i g . 6) t h e s m a l l n u m b e r o ff r u i t p e r v i n e m a d e s t ri c t n o r m a l i t y u n l i k e l y a n d f o r t hi s r e a s o n , s k e w n e s s a n dk u r t o s i s w e r e c a l c u l a t e d f o r al l f r u i t d i s t r i b u t i o n s ( b o t h s e a s o n s d a t a ) a n d c o m p a r e dw i t h t h e s t a n d a r d v a l u e s f o r a N o r m a l d i s t r i b u t i o n ( 0 a n d 3 r e s p e c ti v e ly ) . T h e s k e w -n e s s te s t s h o w e d t h a t 8 3 % o f th e f r u i t s iz e d i s t r i b u t i o n s w e r e s t a t i s t i c a l l y i n d i s t i n -g u i s h a b l e f r o m t h e N o r m a l ( 5 % l e ve l o f si g n if i ca n c e ) , w h e r e a s o n l y 4 7 % o f d i s tr i b u -t i o n s s a t is f i e d t h is c r i t e ri a f o r k u r t o s i s . I n n e i t h e r c a s e h o w e v e r , w e r e d e p a r t u r e s f r o mn o r m a l i t y a s s o c i a te d w i t h a n y p a r t i c u l a r t r en d .

D i s c u s s i o n s a n d c o n c l u s i o n s

T h e e n c l o s e d t r e n c h p r o v e d a n e f f ec t iv e m e t h o d f o r i m p o s i n g w a t e r s t r es s a t d e f i n e dt im e s t h r o u g h o u t t h e s e a s o n . C a r e f u l a t t e n t i o n w a s n e c e s s a r y to m a i n t a i n v i n e s i n a' s t a b l e ' c o n d i t i o n a n d i n s o m e i n s t a n c e s t h e d e g r e e o f i m p o s e d s tr e ss r e s u l te d i n s e v e red e s i c ca t i o n a n d l e a f f al l. M i d - s u m m e r p r e - d a w n L W P ' s n o r m a l l y fe ll b e l o w - 0 . 1 M P a( se e la t e r d i s c u s si o n ) w i t h in 4 d a y s o f c e a s i n g to i r r i g a te . L e a f c o n d u c t a n c e p r o v e d t ob e th e m o s t l a b o r i o u s m e a s u r e m e n t t o p e r f o r m , a n d t h e le a s t s e n si ti v e i n d i c a t o r o fw a t e r s t re ss . V a lu e s o f a r o u n d 1 .0 c m s - 1 a l t h o u g h h i g h w h e n c o m p a r e d w i t h m a n yp l a n t s p e c i e s ( K 6 r n e r e t a l. 1 9 7 9) , a r e c h a r a c t e r i s t i c o f i r r i g a t e d k i w i f r u i t . I n m o s tp l a n t s s e v er e w a t e r s t re s s is a s s o c i a t e d w i t h c o m p l e t e s t o m a t a l c l o s u r e ( H s i a o 1 97 3) ,w h e r e a s c o n d u c t a n c e s o n t h e se v i n es n e v e r f el l b e l o w 0 .1 c m s 1 d e s p i t e a p p l y i n gs tr e ss o f s u f f ic i e n ct s e v e ri t y t o c a u s e m a n y l e a v es t o d e s s i c a t e a n d d r o p . R e d u c t i o n si n c o n d u c t a n c e , w h i l e le s s s h a r p l y d e f i n e d , f o l l o w e d r e d u c t i o n s i n L W P ' s a s w a t e rs t r e s s w a s a p p l i e d .

P r e - d a w n L W P ' s e x h i b i t e d a s t e a d y d e c r e a s e t o 0 .1 M P a f o l l o w e d b y a m o r er a p i d d e c l in e . T h i s t h r e s h o l d w a s a s s o c i a t e d w i t h a r a p i d d e c l in e i n m i d d a y L W P ' s a n da l so w i t h l e a f a n d s h o o t w i l t i n g , d u r i n g p e r i o d s o f p e a k e v a p o r a t i v e d e m a n d . V a nO o s t r u m ( 19 85 ) o b s e r v e d a s i m i l ar p r e - d a w n L W P t h r e s h o l d in a c o m m e r c i a l o r c h a r do n a s o il t y p e w h i c h a l l o w s u n r e s t r ic t e d r o o t i n g a n d w h e r e w a t e r e x t r a c t i o n o c c u r r e dt o a t l e a s t 2 .8 m . I t is p o s s i b l e t h a t t h e s i m i l a r r e s p o n s e , o b t a i n e d w i t h t w o m u c hw i d e l y d i f f e r in g s o i l / r o o t c o m b i n a t i o n s , m a y b e r e l a t e d t o t h e f a c t t h a t m a n y s o il sh a v e s i m i l a r h y d r a u l i c c o n d u c t i v i ti e s a t th e s e h i g h w a t e r p o t e n t i a l s ( G a r d n e r / 9 6 0 ) .

F r o m t h es e m e a s u r e m e n t s a p r e - d a w n L W P o f - 0 . 1 M P a s e e m e d a n a p p r o p r i a t et h r e s h o l d t o u s e f o r d e f i n i n g w a t e r s t re s s p e r io d s . N e v e r t h e le s s , i s o l a t e d d a y s o nw h i c h L W P ' s f e ll b e l o w t h is t h r e s h o l d a p p e a r e d t o h a v e n o i n f lu e n c e o n m e a s u r e df r u i t g r o w t h . W h e t h e r t h is re f le c t s t h e r e s o l u t i o n o f t h e g r o w t h m e a s u r e m e n t s , o r

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310 M .J . Judd e t a l.w h e t h e r a s i n gl e d a y o f ' s t r e s s ' is in s u f f i c i e n t t o i n s t i g a t e a g r o w t h r e s p o n s e , i su n k n o w n . T h u s , f o r m o d e l l i n g p u r p o s e s , w a t e r s t re s s w a s d e f in e d b y p r e - d a w n L W P ' so f l es s t h a n - 0 . 1 M P a f o r m o r e t h a n t h r e e c o n s e c u t iv e d a y s . U s i n g t h i s d e fi n it i o n ,m e a s u r e d f r u it e x p a n s i o n a g r e e d c l o s e l y w i t h t h e p r e d i c t i o n s o f th e m o d e l ( F ig . 4 ).T h e a b r u p t g r o w / n o g r o w b e h a v i o u r , i n h e r e n t i n t h e m o d e l , w a s r e f l e c t e d i n t h em e a s u r e d g r o w t h a s s o c i a t e d w i t h s t r e s s p e r i o d s . F r u i t g r o w t h f o l l o w e d t h e o b s e r v e dr e c o v e r y o f L W P r a t h e r t h a n s t o m a t a l c o n d u c t a n c e a n d w e s p e cu l at e t h a t i t is m o r ec l os e ly l i n ke d w i t h th e w a t e r s u p p l y t h a n w i t h t h e c o n t e m p o r a r y r a t e o f p h o t o -s y n t h e s i s . J o n e s a n d H i g g s ( 1 9 8 5 ) r e a c h e d a s i m i l a r c o n c l u s i o n f o r a p p l e s a f t e r t h eo b s e r v a t i o n t h a t f r u it g r o w t h c o n t i n u e d u n a b a t e d f o r a f u r t h e r t w o w e e k s a f t e rc o m p l e t e d e f o l i a t i o n o f t re e s.

U n s t r e s s e d i r r ig a t e d f r u i t w e r e f o u n d t o g r o w a t a r a t e w h i c h w a s q u i t e r e p e a t a b leo v e r th e t w o m e a s u r e m e n t s e a s o n s a n d w h i c h a l s o a g r e e d cl o s el y w i t h o t h e r p u b l i s h e dd a t a f r o m m u c h o l d e r v in e s in a d if f e re n t l o ca l it y . T o a fi rs t a p p r o x i m a t i o n w e c a nt h u s t r e a t t h e i r r i g a t e d g r o w t h c u r v e a s a ' p o t e n t i a l ' g r o w t h c u r v e f o r k i w i f r u i t -i n d e p e n d e n t o f lo c a t i o n , s e a s o n a n d v i n e a ge . T h e s t a bi l it y o f t hi s c u r v e is i m p o r t a n ts in c e t h e m o d e l a s s u m e s t h a t, i n th e a b s e n c e o f w a t e r s tr es s, g r o w t h c a n b e c h a r a c t e r -i s e d i n t h i s s i m p l e w a y .

T h e s e o b s e r v a t i o n a l a n d p h y s i o l o g i c a l d a t a t h u s s u p p o r t t h e f ir s t t w o p r o p o s i t i o n so f th e m o d e l w h i c h p r e d i c t e d t h e e f f ec ts o f s tr e ss p e r i o d s o n f r u i t g r o w t h , a n d h e n c em e a n f r u i t s iz e a t h a r v e s t , v e r y c l o s el y .

D e s p i t e t h e f a c t t h a t m e a n f r u it s iz es w e r e m a n i p u l a t e d o v e r a v e r y w i d e r a n g et h e r e w a s n o e v i d en c e f o r a n i n t e r a c t i o n b e t w e e n m e a n f r u it s iz e a n d s t a n d a r d d e v i a -t i o n ( F ig . 5). S i ze d is t r i b u ti o n s w e r e s h o w n t o b e a p p r o x i m a t e l y n o r m a l d e s p i te t h el o w n u m b e r s o f f r u it p e r v in e a n d f o r p r a c t i c a l a p p l i c a t i o n s i n w h i c h m u c h l a r g ers a m p l e s w o u l d b e t a k e n , t h e a s s u m p t i o n o f n o r m a l d i s t r i b u t io n s e e m s w e ll j u st if ie d .T h u s t h e d a t a s u p p o r t s t h e l a t t e r t w o p r o p o s i t i o n s u s e d i n t h e m o d e l s u g g e s t i n g t h a tw a t e r s t re s s, w h i l e r e d u c i n g m e a n f r u i t si ze d o e s n o t s t r o n g l y i n f l u e n c e t h e s h a p e o ft h e f r u it s iz e d i s t ri b u t i o n . T h e p r o p o s i t i o n a l l o w s c o m p u t a t i o n a l a d v a n t a g e s w h e ni m p l e m e n t i n g t h e m o d e l .

I n s u m m a r y , k i w i f r u i t f r u it e x p a n s i o n a p p e a r s t o b e r e m a r k a b l y i n t o l e r a n t o fw a t e r s t re s s. F r u i t g r o w t h c e a s e s, a n d w i l t in g o f l e a v es a n d s h o o t s o c c u r a t r el a t i v e lym o d e s t L W P ' s . S t o m a t a l c o n t r o l o f t r a n s p i r a t i o n is p o o r , w i t h l ea v e s u n d e r s e v er es t re s s c o n t i n u i n g t o l o s e w a t e r u n t i l t h e y d e s i c c a te a n d d r o p . A s i m i l a r l a c k o fs t o m a t a l c o n t r o l i s a l s o o b s e r v e d a t n i g h t w h e n s t o m a t a l c o n d u c t a n c e s r e m a i n h i g ha n d c o m p a r a b l e t o v a l ue s a c h ie v e d o n m a n y p l a n t s u n d e r f ul l s u n li g h t. N o f r u i td r o p p e d e v e n u n d e r s e v e r e a n d p r o l o n g e d w a t e r s t r e s s . O n i r r i g a t i n g s t r e s s e d v i n e s ,v i su a l re c o v e r y a n d L W P r i se w a s r a p i d , w i t h f r u i t g r o w t h r e s u m i n g , a f t er r e g a i n i n gt u r g o r , a t a r a te s i m i l ar t o t h e c o n c u r r e n t r a t e o n c o n t i n u o u s l y w e l l i rr i g at e d v in e s.Acknowledgements.The a uthors are gra tefu l to Allan W ilson for h is ass is tance in the f ie ld andto Isobel Gravett for her statistical analyses.

Re f e r e n c e sBaughn JW, Tanner CB (1976) Lea f wa te r po ten t ia l : C omp ar i son o f p re ssu re cham ber andin-s i tu hygr om eter on f ive herbaceous species . Cro p Sci 16:181Ga rdne r W R (1960) D ynam ic aspects o f water avai lab il ity to p lants . S oi l Sci 89:63

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W a t e r s t re s s o n k i w i f r u i t g r o w t h 3 11H o p p i n g M E ( 19 76 ) E f f e c t o f e x o g e n o u s a u x i n s , g i b b e r e ll i n s a n d c y t o k i n i n s o n f r u i t d e v e l o p -

m e n t o f c h in e s e g o o s e b e r r y (Actinidia chinensis P l a n c h . ) N Z J B o t 1 4 : 6 9H o p p i n g M E , H a c k i n g N J A (1 98 3) A c o m p a r i s o n o f p o ll e n a p p l i c a ti o n m e t h o d s f o r t h e a rt i-f i c ia l p o l l i n a t i o n o f k i w i f r n it . A c t a H o r t 1 39 :4 1

H s i a o T C ( 19 73 ) P l a n t r e s p o n s e s t o w a t e r s t r e ss . A n n R e v P l a n t P h y s i o l 2 4 : 5 1 9J o n e s H G , H i g g s K H (1 98 5) W a t e r m o v e m e n t i n to a n d o u t o f a p p le f r ui ts . A c t a H o r t 1 7 1: 35 3J u d d M J , M c A n e n e y K J ( 1 98 7) E c o n o m i c a n a ly s i s o f k i w i f r u i t i r r i g a t i o n in a h u m i d c l i m a t e . In :H i l l e l D ( e d ) A d v a n c e s i n ir r i g a t io n , v o l 4 . A c a d e m i c P r e ss , N e w Y o r k , p p 30 7 3 30

J u d d M J , M c A n e n e y K J , T r o u g h t M C T ( 19 86 ) W a t e r u s e b y s h e l te r e d k i w i f r u it u n d e r a d v e c t i v ec o n d i t i o n s . N Z J A g r i c R e s 2 9 :3K 6 r n e r C h , S c h e e l J A , B a u e r H ( 19 79 ) M a x i m u m l e a f d i f f u s iv e c o n d u c t a n c e i n v a s c u l a r p l a n t s.

P h o t o s y n t h e t i c a 1 3 : 4 5M c A n e n e y K J , J u d d M J (1 98 3) O b s e r v a t i o n s o n k iw i f r u it Actinidia Chinensis ( P l a n c h ) r o o te x p l o r a t i o n , r o o t p r e s s u r e , h y d r a u l i c c o n d u c t i v i t y a n d w a t e r u p t a k e . N Z J A g r i c R e s 2 6 : 7P r e n d e r g r a s t P , M c A n e n e y K J , A s t i l l M S , W i l s o n A D , B a r b e r R F ( 1 9 8 7 ) K i w i f r u i t w a t e re x t r a c t i o n a n d f r u i t e x p a n s i o n . N Z J A g r i c R e s 1 5 : 3 4 5P r i e s t le y C H , T a y l o r R J ( 19 72 ) O n t h e a s s e s sm e n t o f s u r fa c e h e a t f l u x a n d e v a p o r a t i o n u s i n gl a r g e - s c a le p a r a m e t e r s . M o n W e a t h e r R e v 1 0 0:8 1v a n O o s t r o m A J ( 19 85 ) K i w i f r u i t w a t e r u s e c h a r a c t e r i s ti c s a n d p h y s i o l o g i c a l re s p o n s e s t o t h e

p r e s e n c e a n d a b s e n c e o f i r r i g a t i o n in a n O h i n e p a n e a L o a m y s a n d . M S c T h e s i s, U n i v e r s i t yo f W a i k a to