THE ISOHYDRIC RESPONSE TO SHADING: PREDICTING ORCHARD WATER USE UNDER SCREENS Shabtai Cohen, Josef Tanny, and Amos Naor ABSTRACT

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1 Dhl Grednger Interntonl Symposum 9 89 THE ISOHYDRIC RESPONSE TO SHADING: PREDICTING ORCHARD WATER USE UNDER SCREENS Shbt Cohen, Josef Tnny, nd Amos Nor Insttute of Sol, Wter nd Envronmentl Scences, ARO Volcn Center, Bet Dgn, ISRAEL Goln Reserch Insttute, Ktzrn, Isrel ABSTRACT Wter use of crops shded by screens nd screenhouses hs been focus of our reserch for number of yers. All shde screens reduce rdton lod nd decrese wnd speed t the crop. If lef conductnce does not respond to shdng (.e. nsohydrc), these reductons reduce crop wter use sgnfcntly. Isohydrc plnts lmt ther lef wter potentl (LWP), preventng t from droppng below certn level. Tht level s pprently relted to the hydrulc lmts of the plnt's wter trnsport system. Isohydrc behvor s usully defned s response to reduced sol wter potentl. But well rrgted sohydrc tree crops tht we hve studed n Isrel typclly close stomt durng md-dy n hot, cler-sky condtons. For these crops, shdng cn led to ncresed lef nd cnopy conductnce nd productvty, s observed n ctrus trees. But then the decrese n wter use s mnor or even nsgnfcnt, lthough wter use effcency ncreses. Quntfyng sohydrc behvor s mportnt for relstc predctons of crop wter use nd requrements nd photosynthetc productvty under screens nd n screenhouses. Here, we show tht lef specfc hydrulc conductnce, lef re ndex, crtcl LWP nd clmte vrbles (v the Penmn-Monteth equton) cn be used to predct cnopy conductnce nd wter use n shded sohydrc crops. INTRODUCTION One of the prmry pproches to delng wth wter shortge s optmzton of rrgton n order to conserve wter. Ths nvolves contnung effort to ccurtely determne nd fterwrds predct wter use for new crops nd grculturl confgurtons s they develop. Sometmes we ntroduce the chnges, but t other tmes t seems lke we re runnng fter the frmers to optmze rrgton n the stutons tht they choose. One of the nterestng chnges n recent yers n Isrel nd other mld wnter clmte countres s the ntroducton of vrous structures tht modfy crop clmte wthout controllng t. These structures nvolve screen coverngs nd screenhouses. In the former screen s hung bove the crop nd the sdes of the structure re left open. In the ltter, the sdes re closed nd lthough the structure s technclly closed, the screens llow hgher rtes of ventlton thn those obtned n greenhouses nd the clmte n the structure s bsclly coupled to the outdoor clmte.

2 9 Crop Producton n the st Century The mn clmte vrbles chnged by the structures re rdton nd wnd speed. Both solr rdton nd wnd speed t the crop re reduced (see Fg ). Wnd speed reductons cn be descrbed by fxed lner fctor, bsed on the logrthmc profle of wnd speed observed below screens (Tnny nd Cohen, 9; Tnny et l., 9). The reducton of rdton below screens depends on the solr zenth nd zmuth ngles, but for md-dy condtons cn lso be descrbed by one fctor (Cohen nd Fuchs, 999; Tnny nd Cohen, 9). Energy blnce of the evportng surfce Solr Protected rdton cultvton + Energy to be dsspted Wnd Speed - Aerodynmc Resstnce - Crop response Lef Resstnce to wter vpor l c u r d k = H y b c d c e e r f d h y Convecton Het Storge Evpotrnsprton I so Fg. : Schemtc dgrm of the energy blnce of the crop. Postve nd negtve nfluences re noted. The mjor clmte vrbles nfluenced by shde screens re n the ovl. Crops nfluence the energy blnce through modfctons of the lef nd crop resstnce. The Isohydrc response cn be seen s feedbck from trnsprton to crop resstnce. Frst estmtes of the response of crop trnsprton to the chnged clmte below screens cn be mde usng the stndrd FAO-56 Penmn-Monteth equton, whch descrbes wter use of stndrd well-rrgted cut grss (Allen et l., 999). Wth ths nlyss (see below) reduced wnd speed (bove threshold low wndspeed) ncreses erodynmc resstnce (slghtly), whle reductons n rdton reduce trnsprton more-or-less proportontely. Thus, f crop resstnce to wter vpor trnsport s unchnged, trnsprton wll lwys be expected to declne below screens. Ths pper descrbes mesurements of trnsprton nd relted vrbles mde n severl projects n whch sp flow ws mesured below screens nd outsde of the screen structures n ctrus nd pple trees. In these cses the screen structures were open to the sdes. Anlyss of the response of crop resstnce shows tht the ltter ws ctve n stblzng trnsprton n these stutons. A recent conceptulzton of crop resstnce response to envronment, termed sohydrc', s nvoked to expln the results.

3 Dhl Grednger Interntonl Symposum 9 9 THEORY A smplfed model whch ccounts for the clmtc nfluences on potentl trnsprton but lmts trnsprton to the hydrulc lmt of trees ws ppled to condtons outdoors nd under shdng tretment. The model ssumes the followng: Trnsprton of non hydrulclly lmted tree, E pm, proceeds s tht of crop wth mxmum cnopy conductnce, nd s smlr to tht for stndrd grss crop descrbed by the FAO56 mplementton of the Penmn-Monteth (PM, Allen et l., 999 for detls) equton,.e., E pm R n c Dg p ( Trnsprton s lmted to mxmum vlue descrbed by the hydrulc equton, where: E mx = k ψ *l crtcl Where k l s lef specfc hydrulc conductnce (kg m s P ), nd ψ crtcl g g ) c () potentl dfference between sol nd leves (for well wtered condtons) bove whch cnopy conductnce s djusted n order to mntn ths grdent. s threshold wter Both vlues re clculted nd E s tken s E pm when t s less thn E mx. Otherwse E=E mx. Cnopy conductnce (g c ) nd resstnce ( s: r c gc cr g ) cn be computed from nverson of the P-M equton R c p Dg n * E ( ) () METHODS The ctrus experment ws crred out n lte July nd August n 994 nd 995 n n orchrd t the Volcn Center, Bet Dgn, Isrel (359'N, 3449'E, 5 m bove men se level). The trees were Ctrus lmon cv. Vllfrnc on Volcmrnn rootstock plnted n Oct 99. The orchrd ws drp rrgted nd the sol s sndy lom. Shde nets were hung horzontlly bove two trees n the hedgerow nd two nets of dfferent denstes were used, both of hghly reflectve lumnzed polypropylene (Polysck Ltd., Nr Ytzhk - Suf, Isrel). The shde nets were the wdth of one hedgerow gvng shde mostly durng md-dy, so tht before 9 Am nd fter 4 PM much of the tree ws not shded. Stem sp flow ws mesured contnuously wth het pulse technque clbrted for ctrus (Cohen, 994) n sx trees; two under ech of the nets nd two s control trees. Clmte condtons were montored, ncludng net rdton wth nd wthout the net. Full detls re gven n Cohen et l (997). The pple experment ws crred out t En Zvn n the Goln Heghts of Northern Isrel n the summer of 8. A cler whte % shde net (Polysck Ltd.) ws hung horzontlly bove

4 9 Crop Producton n the st Century pproxmtely 4 dunms (.4 h) of n pple (Mlus domestc) orchrd. Four rrgton tretments were ppled outsde nd below the screen, ncludng % (regonl recommendton) nd 3% rrgton n splt plot desgn wth 7 replctes. Detls re gven n Tnny et l. (8). Stem sp flow ws mesured n trees of the Golden delcous cultvr wth the therml dsspton technque (Grner, 983) usng mm long probes. Sp flow for the mm nnulus of xylem ws computed bsed on Grner's generl equton. RESULTS H ) L..8 W m 8 6 6% shde Unshded Globl w (.6 o n t 4, S p F l / -.4. Before 6% Shde After R d Tme of Dy Tme of dy b. 8 n c co n d u c t m p e r s e f m L e c e o % shde 4% shde Unshded Tme of dy Fg : () Sp flow n one ctrus lemon tree on three dys; before, durng nd fter shdng wth 6% shde screen (b) Globl, nd net rdton mesured bove unshded nd shded trees, nd (c) lef conductnce t two shde levels. After Cohen et l. (997). c. Dly course of sp flow for trees mesured wth nd wthout screen shdng re presented n Fgs. nd 3. For ctrus shded wth 6% screens, whch reduce both net rdton (Fg. b) nd wnd speed by pproxmtely 5% (Tnny nd Cohen, 3), no chnge n the dly course of sp flow ws observed. Anlyss of md-dy sp flow for the other trees shded nd for the whole perod of shdng showed tht the shde reduced verge md-dy sp flow by bout % (Cohen et l., 997). Results for pple trees (Fg. 3) show smlr phenomenon,.e. durng md-dy the shde screen hd no nfluence on sp flow, even n the tretment wth full rrgton. In the pple experment the sp flow n the mornng under shde lgged behnd tht outdoors. The lg cn be explned by the shdng of the screen n ths experment nd tht screen shdng s more ntense when solr elevton s low due to the geometry of the screen (see Tnny nd Cohen 8). In the

5 Dhl Grednger Interntonl Symposum % rrgton tretment shdng dd not chnge the dly course of sp flow sgnfcntly. Sp flow peked n the mornng nd fterwrds declned mrkedly durng the dy. Ths cn be explned by depleton of wter n the root zone durng the dy becuse of the defcent rrgton. Apples, En Zvn, Sept s, L / h u.75 _ u w, cm n n S p f l. o 5.5 Control Shde Control 3 Shde Tme of dy Fg 3: Hourly sp flow n pple trees verged for 4 dys. Vertcl lnes stndrd errors of men wth (n=9). % Shdng screen. % nd 3% rrgton. Lef conductnce durng the course of the dy ws mesured wth porometer n the ctrus experment nd these results (Fg. c) show tht lef conductnce decresed t md-dy. Ths clssc md-dy stomtl closure stblzes sp flow so tht n the full rrgton tretments t remns more-or-less constnt (wthn % of mxmum) for mny hours (Fg. nd 3). Mddy lef conductnce ncresed under shde, nd ths ccounts for the fct tht sp flow ws the sme s n the unshded tretment. f c r b o n d e l t C 3-5 Fg. 4: Delt C3 of lef nd frut Apple smples from four shdng Grpefrut experments. Smples were tken Grpefrut frut -6 from lef nd frut tssue tht Tngor developed n the upper porton of the trees durng the hot summer -7 months. Tngor s ctrus vrety. Grpefrut dt from Cohen et l. (5). -8 L e Crbon sotope rtos (δc Approxmte shde level ) mesured n severl of our shdng experments. Results for grpefrut trees (from Cohen et l., 5) nd pple trees re gven n were Fg. 4. In generl, δc 3 s negtvely correlted wth the CO concentrton n the leves when the photosynthess tht fxed crbohydrtes n the plnt mterl ws done. In ll the orchrd shdng 3 3 experments where we hve nlyzed δc we hve found sgnfcnt decrese n δc wth

6 94 Crop Producton n the st Century ncresed shdng. In generl, we tke ths response s n ndcton of ncresed lef conductnce t md-dy ledng to hgher nternl CO concentrtons durng photosynthess. The model clcultons of trnsprton for pple orchrds were mde usng pproxmte vlues of hydrulc conductnce, K (.e-4 kg m s l MP ) nd crtcl (. MP) tht we mesured n our prevous work on pple trees t En Zvn (Cohen nd Nor, nd Cohen et l., 7), nd LAI=. For clmte condtons we took vlues mesured t our wether stton n summer 8 nd used the rto of net rdton bove the orchrd to globl rdton wth (.4) nd wthout (.63) shdng observed n our experments. Results (Fg. 5) show the sme fetures for the dly course of trnsprton tht we observed n the shdng experments for well rrgted trees. / m, W n o t d R Globl rdton Net Rd Net Rd, shde.4e-4.e-4.e-4 8.E-5 6.E-5 4.E-5.E-5.E Tme of dy T r n sp r t o n, Kg/ m / s Outdoors Shde Tme of dy 5 ce, m / s.6.4. ce, s/ m 6 4 n. n C n o p y co n d u ct Outdoors Shde Tme of dy C n o p y r e s st Vpor pressure defct, P Fg. 5: Results of the smulton of trnsprton bsed on the Penmn-Monteth model nd hydrulc lmts. Rdton vlues bsed on the rtos of net to globl rdton observed n the orchrd shdng experments. DISCUSSION The results of the well rrgted shdng tretments n orchrds show tht trnsprton ncreses n the mornng but levels off lte n the mornng nd s reltvely constnt for mny hours n the remnder of the dy. No dfference n trnsprton, t lest for the md-dy porton of the dy, ws observed between the shded nd non-shded trees. Thus, snce the lrge chnges n wnd

7 Dhl Grednger Interntonl Symposum 9 95 speed nd rdton below the screen should hve led to reducton n trnsprton, t s cler tht cnopy conductnce ws modultng trnsprton so tht t proceeded t more-or-less the sme rte n both cses. Durng the pst decde much ttenton hs been gven to the hydrulc lmts of trees, nd n prtculr we re now wre of the fct tht wter potentl grdents exceedng certn vlues wll led to dmge to the trees hydrulc system. Although t s not cler how mechnstclly plnts regulte trnsprton n order to lmt wter potentl grdents to sfe levels, t s cler tht ths hppens. The terms 'sohydrc' nd 'nsohydrc' hve been ntroduced to descrbe responses of dfferent plnt speces to drought condtons. Jones nd Trdeu (998) were mong the frst to mke ths dstncton. They wrote tht the frst group (of plnts), so-clled sohydrc' plnts, mntn dytme lef wter sttus reltvely constnt regrdless of sol wter sttus s result of ctve stomtl control; exmples nclude pple or mze (Trdeu nd Dves, 99). The second group, nsohydrc' plnts such s sunflower or sorghum, do not mntn lef wter potentl constnt nd t tends to correlte wth stomtl conductnce (Trdeu et l., 996)ǁ. Schultz (3) wrote sohydrc, tht s, when wter s scrce, the stomt ct to prevent lef wter potentl from droppng below crtcl threshold levelǁ (H.R. Schultz, 3). It s cler tht the response to shdng n the orchrd trees tht we observed cn be termed sohydrcǁ. In ddton, the perhps oversmplfed ntl model of trnsprton presented here s bsed on the sohydrc prncple tht dytme lef wter sttus (or wter potentl) s mntned reltvely constnt s long s the potentl trnsprton exceeds the hydrulc threshold. Md-dy stomtl closure of stomt hs been observed n ctrus snce the begnnng of reserch on ctrus wter reltons, nd hs often perplexed reserchers. Tody t s cler ths s prt of the sohydrc response. In ddton, cyclng of lef conductnce t the lef nd cnopy level hve been observed. The cyclng cn be ttrbuted to mperfecton of the feedbck control loop tht nvolves cnopy conductnce modulton of trnsprton through some currently unknown wter potentl sensor n the plnt. The prmeters used to determne E mx re vtl for predctng tree trnsprton both wth nd wthout shde. As noted bove, much ttenton hs been gven to the crtcl lef wter potentl nd ts reltonshp to structurl prmeters of the xylem (e.g. Tyree nd Sperry, 988). Hydrulc conductnce of plnts hs lso been quntfed for mny speces, nd we hve good de wht vlues to expect (Tyree nd Ewers, 99). But lef specfc hydrulc conductnce depends not only on the xylem, but lso on the ntercton between lef re, root re nd sol hydrulc conductnce (L et l., 5). Thus, t s possble tht f the trees under the screens were pruned k l

8 96 Crop Producton n the st Century would ncrese (Ptk et l., 998) nd the response to shdng mght be more coupled to the clmte chnge n shde. The current study shows tht the 'sohydrc' prncple s pplcble to shdng studes nd tht f we re to properly ntcpte wter use of crops n the shde t s mportnt to understnd nd quntfy ther hydrulc prmeters nd scertn to wht extent they re sohydrc. REFERENCES Allen RG, Perer LS, Res D, Smth M (998) Crop evpotrnsprton: gudelnes for computng crop wter requrements. FAO Irrgton nd Drnge Pper 56, FAO, Rome Cohen, S. nd M. Fuchs. (999). Mesurng nd predctng rdometrc propertes of reflectve shde nets nd therml screens. Journl of Agrculturl Engneerng Reserch 73: Cohen, S., S. Moreshet, L. LeGullou, J.-C. Smon, nd M. Cohen. (997) Response of ctrus trees to modfed rdton regme n sem-rd condtons. J. Exp. Bot. 48(36): Cohen, S., Rveh,E., L,Y., Grv, A., nd E.E. Goldschmdt (5). Physologcl responses of leves, tree growth nd frut yeld of grpefrut trees under reflectve shde screens. Scent Hortculture 7():5-35. Cohen, S. nd A. Nor (). The effect of three rootstocks on wter use, cnopy conductnce nd hydrulc prmeters of pple trees; nd predctng cnopy from hydrulc conductnce. Plnt Cell nd Envronment 5():7-8. Cohen, S., A. Nor, J. Bennnk, A. Grv nd M. Tyree. (7). Hydrulc resstnce components of mture pple trees on rootstocks of dfferent vgours. Journl of Expermentl Botny 58(5-6): Jones HG, Trdeu F Modelng wter reltons of hortculturl crops: revew. Scent Hortculture 74(): -46. L, Y., H. Xu, nd S. Cohen (5). Long term hydrulc cclmton to sol texture nd rdton lod n cotton. Plnt, Cell nd Envronment 8: Ptk, D.E., R. Oren, nd N. Phllps. Responses of sp flux nd stomtl conductnce of Pnus ted L. Trees to stepwse reductons n lef re.. J. Exp. Bot. 49 (3):87-878, 998. Rveh, E., S. Cohen, T. Rz, D. Ykr, A. Grv nd E.E. Goldschmdt (3). Incresed growth of young ctrus trees under reduced rdton lod n sem-rd clmte. Journl. J. Exp. Bot., 54 (38): Schultz, H.R., 3. Dfferences n hydrulc rchtecture ccount for nersohydrc nd nsohydrc behvour of two feld-grown Vts vnfer L. cultvrs durng drought. Plnt, Cell nd Envronment (3) 6,

9 Dhl Grednger Interntonl Symposum 9 97 Tnny, J., S. Cohen (3). The effect of smll shde net on the propertes of wnd nd selected boundry-lyer prmeters bove nd nsde ctrus orchrd. Bosystems Engneerng 84(): Tnny, J., A. Nor, S. Cohen, E. Rveh nd A. Grv 8. Optmzton of pple rrgton under shdng screens. Annul rpt 7, Chef Scentst of the Mnstry of Agrculture, project number (n Hebrew). Tnny, J., nd Cohen, S. (9). Mcroclmte nd crop wter use under screen constructons. In: Agrculture n Ard nd Semrd Zones: Sol, Wter, nd Envronment Aspects. Reserch Sgnpost. edted by Dr. Men Ben-Hur (n press). Tnny, J., Moller, M., nd S. Cohen (9). Aerodynmc propertes of boundry lyers long screens. Bosystems Engneerng (n press). Trdeu F, Dves WJ. 99. Stomtl response to ABA s functon of current plnt wter sttus. Plnt Physology 98, Trdeu F, Lfrge T, Smonneu Th Stomtl control by fed or endogenous xylem ABA n sunflower: nterpretton of observed correltons between lef wter potentl nd stomtl conductnce n nsohydrc speces. Plnt, Cell nd Envronment, 9, Tyree, M.T. nd F. W. Ewers. The hydrulc rchtecture of trees nd other woody plnts. New Phytol 9:345-36, 99. Tyree, M.T. nd J. S. Sperry Do Woody Plnts Operte Ner the Pont of Ctstrophc Xylem Dysfuncton Cused by Dynmc Wter Stress? Plnt Physol. 88(3):