Plant and Soil 41, 425-427 (1974) Ms. 2342

SHORT COMMUNICATION

O r g a n i c acid s p e c t r u m of root e x u d a t e s of w h e a t

in re la t ion to i ts root rot d i sease

Summary The range of hydroxy-di, and tricarboxylic acids present in the root exudates

of wheat in 'health' and under the stress of its root rot infection have been correlated at four stages of plant growth, both qualitatively and quanti ta- tively. Although, under the stress of infection, the exudation of most of the organic acids identified were suppressed, increase in the exudation of glycolic acid and succinic acid were observed to be pronounced. The selective suppres- sion of lactic acid exudation was of interest to note.

Introduction

The establishment of a soil-borne plant pathogen on its potential host is mainly a function of tile nutr i t ional environment existing in the soil. And one of the vital factors that influences this soil nutri t ional environment is the root exudation of the host plants. Although responses of soil-borne fungal patho- gens to root exudates of their hosts have been reported 4, not much has been studied about organic acid liberation through host roots when under the stress of infection by soil-borne pathogen. In the present studies, therefore, shifts brought about in the exudation of hydroxy-di, and tricarboxylic acid spectrmn of root exudates of wheat in relation to its pathogenesis by Helmin- thosporium sativum Pamm., King and Bakke, causing root rot disease are reported.

Materials and methods

Two mexican varieties of wheat viz Kalyan 227 and S-308 were used for root exudate studies. After usual seed sterilization and aseptic plant culture ~, the root exudate samples were collected at four stages of plant growth, viz 7, 14, 21, and 28 day intervals. Root exudates from inoculated series were collected by immersing the washed wheat seedlings in spore suspension of the test organism for five minutes and subsequently transferred to the sterile plant culture tubes containing nutr ient solution 1. The resulting extracts were then analysed for hydroxy-di, and tricarboxylic acids, after desalting the samples by their passage through a cation (Amberlite IR-120H+) and anion (Amberlite IRA 410) exchange resins. The organic acids thus obtained from the fraction adsorbed and eluted from the anion exchanger, were qualita-

426 SHORT COMMUNICATION

t ively analysed by spo t t i ng the f rac t ion samples on W h a t m a n ch romato - graphic paper no. 3 .The ch roma tog rams were developed in n -Butanol - formic ac id-water (4 : 1 : 5) and e t h a n o l - a m m o n i a - w a t e r (90 : 5 : 5) so lvent sys- t ems ; and de tec ted by spray ing 0.5% w/v e thanol ic solut ion of bromocresol green af ter suspend ing the ch roma tog rams in a cur ren t of air (to pe rmi t t he volat i l izat ion of formic ac id /ammonia) for 48 hours. The q u a n t i t a t i v e analysis was done by developing the ch roma tog rams two d imensional ly ; e t h a n o l - a m m o n i a - w a t e r (80 : ~ : 15) so lvent sys tem in the first d i rect ion and n- Butanol - formic ac id-water (7 : 3 : 12) in the second. The spot de tec t ions were done by d ipping the ch roma tog rams in ani l ine-xylose reagen t and dried in an oven ma in t a ined at 105°C unt i l b rown spots appeared on cream-coloured background. The spo t colour was eluted by 70% ethanol , and then absorbances measured by Spect ronic 20, a t 365 m~ wavelength .

Results and discussion

Resul ts ob ta ined f rom hea l thy and inocula ted whea t root exuda tes are p resen ted in Table 1. Seven organic acid spots were de t ec t ed in root exuda tes of hea l t hy as well as diseased w h e a t plants , including one unident i f ied spot in the former and two in the la t ter . In the inocula ted series, however , com- plete suppress ion of lactic acid exuda t ion was of in te res t to observe. Fu r t h e r - more, the exuda t ion g rad ien t of mos t of the organic acids ident i f ied were s l ight ly d iminished in t he inocula ted p lan t s ; only glycolic acid and succinic acid were exuded s igni f icant ly in large amounts .

The exper imen ta l f indings reveal t h a t t he a m o u n t of K r e b ' s cycle acids l ibera ted in t he exuda tes of infec ted roots is lesser t h a n those in the h ea l t h y cont ro ls - which m a y be a t t r i bu t ab l e to the i r u t i l iza t ion by the t e s t o rganism for respira t ion. The presence of a b u n d a n t l ibera t ion of succinic acid and

TABLE 1

Hydroxy-di, and tricarboxylic acids in root exudates of healthy and infected wheat plants at different stages of plant growth

Organic acid Stage of plant growth (days) ([xg/50 plants)

7 14 21 28

H I H I H I H I

Oxalic acid 70.17 61.94 72.81 64.89 79.37 69.73 79.37 74.13 Lactic acid 40.05 -- 47.53 -- 47.53 -- 47.53 -- Citrieacid 59.73 50.73 61.19 50.73 67.29 50.73 74.11 57.69 Malie acid 54,37 -- 59.15 38.83 67.31 47.13 67,31 50.43 Succinic acid 35.55 62.18 39.10 69.74 45.42 78.79 54.62 82.32 Glycolic acid . . . . . 52°03 47.13 69.99 Unidentified- 1 . . . . . . 37.09 49.83 Unidentified-2 . . . . . . . 56.17

H = Healthy series; and I = Inoculated series (inoculated with Helminthosporium satiwm).

SHORT COMMUNICATION 427

glycolic ac id are, however , d i f f icul t to exp la in on th i s basis. However , i t seems poss ible t h a t t h e se lec t ive e x u d a t i o n of these two acids in s ign i f ican t a m o u n t s u n d e r t h e s t ress of roo t r o t in fec t ion is as a r e su l t of de f in i t e b io t ic env i ron - m e n t p r eva i l i ng in t h e zone of roo t inf luence , as was shown b y our l a t t e r i nves t iga t ions . This inc reased e x u d a t i o n m a y also be p a r t l y exp la ined b y t he u n c o u p l i n g of t h e o x i d a t i v e p h o s p h o r y l a t i o n - caus ing increase in t he K r e b ' s r e s p i r a t o r y cycle as a n a t t e m p t to ove rcome the r e s p i r a t o r y insuf f ic iency caused p r e s u m a b l y b y t he tox ic m e t a b o l i t e s p r o d u c e d b y t h e t e s t organism.

Acknowledgements

T he a u t h o r s wish to t h a n k t h e la te Dr. A u r a r K r i s h a n J a l a l i for his u n s t i n t e d he lp d u r i n g t h e course of these i n v e s t i g a t i o n s ; to Dr. S. C. C h a k r a - v o r t y a n d Dr. S. D. V e r m a of D e p a r t m e n t s of Microbio logy a n d Bioche- m i s t r y r e spec t ive ly for offer ing t e c h n i c a l suggest ions .

BUSHAN L. JALALI* a n d D. SURYANARAYANA**

Department of Plant Pathology, Haryana Agricultural University, Hissar

Received September 24, 1973

Re]erences 1 J a l a l i , B. L. and S u r y a n a r a y a n a , D. Plant and Soil 34, 261-267 (1971). 2 J a l a l i , 13. L., Syrup. All Ind. Microbiol. Conf. 29-30 (1970). 3 Rov i ra , A. D., Plant and Soil 7, 178-194 (1956). 4 S e h r o t h , M. N. and H i l d e b r a n d , D. C., Ann. Rev. Phytopathol. 2, 101-132 (1964).

* Nowat: Institut fflrBodenbiologie, Forsehungsanstalt ffirLandwirtschaft, Braunseh- weig, West Germany

** Indian Agricultural Research Institute, New Delhi, India.