(1990) in apple, that the non-hydraulic signal was a positive inhibitor (increased m amount from drying roots and moving to leaves to inhibit growth) rather ...
Pliylol. (1994), 127, 495 505
Non-hydraulic root-to-shoot signalling in mycorrhizal and non-mycorrhizal sorghum exposed to partial soil drying or root severing BY ROBERT C. EBEL, ANN J. W. STODOLA, XIANGRONG DUAN AND ROBERT M. AUGE* Institute of Agriculture, Unwersity of Tennessee, P.O. Box 1071, Knoxville, TN 3790'}-}07}, USA {Received 21 October 1993; accepted 28 February 1994)
SUMM.^RY
Our objectives were (1) to determine if arbuscular mycorrhizal symbiosis could modify leaf response to nonhydraulic root-to-shoot communication of soil drying in Sorghum hicolor (L.) Moench, and (2) to compare the sensitivity of leaf growth and stomatal conductance (C^) to the non-hydraulic signal. SteJlings were grown in a greenhouse with root systems split among four pots. Treatments were applied in a 2 (colonized or not colonized by Glomus iittraradices Schenck & Smith) x 2 (roots isevered or dried) x 4 (roots in 0, 1, 2 or 3 pots dried or seyered) experimental design. Plants with toots in three pots dried or severed showed reduced leaf elongation, C^ and leaf water potential (4'') compared with fully watered (control) plants and thus were prohably hydraulically affected by root treatnient. Drying ot severing roots in one pot did not affect leaf elongation. C^ or H* in either mycorrhiza! or non-mycorrhizal plants. In non-mycorrhizal plants having two pots dried, final leaf area and toiiil ieaf length were reduced by 18 and 10",,, respecti\'eh', relative to controls. Stomatal conductance of these half-dried nonmycorrhizal plants remained unchanged, suggesting that the decrease in leaf growth was not hydrauiically induced. Non-mycorrhizal plants ha\'ing root.s severed in two pots contmued to have ieaf growth sirniiar to that of the controls, suggesting that growth reductions in half-dried non-mycorrhizal pJants did not result from a reduction in root water gathering capacity. Mycorrhizal symbiosis appeared to eliminate inhibition of leaf growth that uaH not hydraulically induced, because mean leaf area and total leaf length were not reduced in half-dried mycorrhizal plants, relative to controls. However, final leaf area of mycorrhizal plants having one or two pots dried was negatively correlated with the produtt of drying root mass and the time for which roots were exposed to mild drought, suggesting that mycorrhizal plants were also susceptible to growth inhibition that was not hydraulically induced. Reductions in leaf extension rate that were not hydraulically induced, when viewed as a function of actual .soil matric potential, were similar in mycorrhiza] and non-mycorrhizal plants, suggesting that the differences in overall growth inhibition between half-dried mycorrhizal and non-mycorrhizal plants may have been related to difFerences in soil drying rate. Key words: Drought physiology. Glomus intraradices. non-hydraulic signalling. Sorghum bicolor, water relations.
The concept of non-hydraulic root-to-shoot communication of soil moisture availability', with leaf sensitivity dependent upon leaf water potential (*f). „ _
,
,
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" l o whom correspondence should be addressed. Abbreviations: C, fully watered mnrrol; T,, stomatal conduciance; D l , D2, D3, plants with I. 2, or 3 pots dried. respectLvdy-; LER, leaf extension rat.; J - - "^ptnc- potential; M, m'V'Corrhizai; N, non-tnycorrhizal; b l , bZ, hi, plants vvuh roots severed in !, 2, or 3 pots. respectL\'ely; "P, water potential. ,34
has received considerable attention in the recent literature (e.g, Bano et al.. 1993; Blum & Johnson, 1993 ; Gowing, Jones ii Davies, 1993 ; Gowing et al.., 1993 ; Khalil & Grace, 1993 ; Tardieu, 1993 ; Tardieu & Davies, 1993). It is still unclear how leaf responses vary in sensitivity to the signals and how \'arious environmental factors affect the process. For inStance, s o m e t i m e s stomata react to n o n - h y d r a u l i c signals before leaf g r o w t h is affected ( Z h a n g & ^^^. | 990), o t h e r times in t h e s a m e species leaf g r o w t h IS reduced in t h e absence of any change in Q A N P 127
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R. C. Ehel and others
(Saab & Sharp, 1989). Since the initial perception by the plant of soil water deficit is thought to occur in the root (Davies & Zhang. 3 991), any edaphic factor that affects root physiology might in turn be expected to affect non-hydraulic root-to-shoot communication. Arbuscular mycorrhizal symbiosis affects hosi plant physiology in several ways, with the most studied influence being an enhancement of phosphorus nutrition. Mycorrhizae can modify whole plant response to drought (Bethlenfalvay, 1992), including stomatal response to possible nonhydraulic signals (Auge 8c Duan, 199]). Our purpose here was to determine whether colonization of roots by arbuscular mycorrhizal fungi would modify either leaf growth or stomatal response ro non-h\'draulic signals of soil drying in sorghum, and to deterniine whether leaf growth and C^ varied in sensitivity to the signalling process. Roof severing treatments were included to test the possibility that leaves of partly dried plants may be responding to reduced water gathering capacity of their root systems rather than to non-hydraulic signals. AND METHODS
Plant materials and culture Seeds of Sorghum bicolor (L.) Moench cv. G522A (Delta and Pine Land Co., Scott. M S . USA) were planted on 2 March 1992, with five seeds per pot in a 2:1 (v:v) mixture of autoclaved quartz sand and calcined montmorillonite clay (Turface; Industrial Materials Corp., Deerfield, IL, USA). Fertilization for both mycorrhizal (M) and non-mycorrhizai (N) plants included a weekly feeding of 2'0 mM P from KHjPOj, and daily feeding of 11 mM N as Peter's (Grace-Sierra, Mitpitas, CA, USA) 15-0-15 and soluble trace elements as Peter's S.T.E.^L at D'Ol mM Mn. The plants were grown in a glasshouse coated for tooling purposes with whitewash that reduced ambient irradiance by about 50",,. Maximum daily temperatures over the course of the experiment ranged from 25 to 35 "C, and night temperature was maintained near 20 °C. On 30 March. 4 wk after seeding, 96 seedlings were transplanted with roots divided among four 1-0] square pots, taped together into a square. All four pots of each plant contained autoclaved sand and Turface (2:1, v:v} mixed with either mycorrhizal (48 plants) or non-mycorrhizal (48 plants) pot culture (1:1, sand/Turface mix:pot culture). A 2-5-3-O cm layer of sterile sand was placed above and below the pot culture mix to minimize the risk of contamination of N plants. Mycorrhizal pot cultures were Giomiis intrafadices Schenck & Smith isolate UT143 grown for 2 months on cowpea \Vigna unguiculata (L.) Walp. cv. White .Acre], five plants per pot, in a sand and Turface (2:1, v:v) culture medium. Pot culture cowpea roots had 70"(, colonization rates (70 of 100, 1 cm root pieces
contained mycorrhizal structures). Xon-mycorrhizal pot cultures were grown under the same conditions as inyeorrhizal pot cultures except without mycorrhiza] fungi m the rhizosphere. After transplanting, plants were irrigated two or three times a week to maintain high soil moisture, with a modified nutrient solution developed for sorghum (Clark, 1982). The nutrient solution was modified for both M and N plants b>' using i)-Z m\i P and 0-065 mM Fe as sodium ferric diethylenetriamine pentacetate ('Sprint 330', Ciba-Geigy. Greensboro, NC, USA). M plants had 73 "crimpn?«/ Botany 36: 1087-1098. London iBiulogy) 341. 57 66. Ludtow MM. 1989. Strategies in response to water stress, in: Tardieu F, Davies WJ. 1993. Integration of hydraulic and Kreeb UK, Richter H, Hinkley TM, eds. Structural and chemical signalling in the control of siomatal conductance and functional response to enTironTnental stresses ' Kater shortage. Thewater status of droughted plants. Plant, Cell and Environment Hague, The Netherlands: SPB Academic Press, 264-2^1. 16: 341 349. Ludiow MM, Sommer KJ, Flower DJ, Ferraris R, So HB. Turner NC, Begg JE. 1981. Plant-water relations and adaptation 1989. Influence (if r
