Proc. lndian Acad. Sci. (Plant Sci.), Vol. 92, Number 3, .lune 1983, pp. 297--301. �9 P¡ in India.

Effect o f water stress on n o d u l a t i o n and n i trogenase activity o f guar (Cyamopsis tetragonoloba (L . ) T a u b . )

B VENKATESWARLU, A V RAO and A N LAHIRI Div, sion of SoiI-Water-Plant Relationship, Central Arid Zone Research Institute, Jodhpur 342 003. India

M S received 12 Augus t 1982~ revised 10 May 1983

Abstrmct. The effect of water stress on nodulation and nitrogenase activity of guar (Cya- mopsi5 tetragonoloba IL.) Taub.) was studied at both vegetative and flowering stages. Stress did not induce any change in the nodule number but caused a stgnificant reduction in nodule fresh weight. Drought-induced reduction in nitrogenase activity was observed at both the stages. Upon rewatering, rapid recovery in the activity was noticed which seems to be related to the adaptation of this legume to artd regions.

Keywords. Water stress; nitrogenase activity; guar; nodulation; Cyamopsis tetragonoloba.

1. lntroduction

Nitrogen fixation process is sensitive to water stress in a number of legumes (Sprent 1971, 1972; Engin and Sprent 1973; Pankhrust and Sprent 1975). Water stress is known to induce structural and physiological atterations in the root nodules and affect their nitrogen-fixing ability, h has been found that there was an irreversible loss of nitrogenase activity wben the water content of the nodules falls below 80% (Sprent 1976). However+ studies so far have been on temperate legumes and little information is available on the tropical species particularly those adapted to arid and semi-arid regions. Hence the present study on C. tetragonoloba (L.) Taub., widely grown in the low rainfall regions of western Rajasthan was undertaken.

2. Material and methods

Seeds of guar (D P Safed) inoculated with an efficient locaIly isolated strain of Rhizobium were sown in pots containing about 10 kg of loamy sand soil (pH8, organic carbon 0.25%, total N0.03% and available P7-8 ppm). AII the pots were maintained close to field capacity (10% soil moisture) for 30 days. At this stage (vegetative) and at the flowering stage (50 days from sowing), nodulation and nitrogenase activity were studied at 0, 2, 4, 6 and 8 days of withholding of water in treated pots. Every time five pots containing four plants each were sampled from treated as well as control pots maintained close to field capacity. Rewatering was done on the eighth day and observations recorded 5 hr later, on the first and second day after rewate¡

The nitrogenase activity was assayed by keeping the excised roots from each pot in two glass bottles (180 mi size) of 2 plants each. These were closed with suba seals and tightened by metallic screw caps. Part of the air (10%) inside the bottles was replaced by acetylene and these were incubated at 30+_ I~ for 1 hr. The ethylene produced was estimated by an AIMIL NUCON gas chromatograph employing a

297

',D

Oo

Tab

le 1

. C

hang

es i

n nu

mbe

r an

d fr

esh

wei

ght

of n

odul

es i

n gu

ar d

urin

g w

ater

str

ess

and

reco

very

Day

s af

ter

wit

hhol

ding

w

ater

Veg

etat

ive

stag

e (3

0 da

ys f

rom

sow

ing)

Nod

ule

Con

trol

FlO

wer

ing

stag

e (5

0 da

ys f

rom

sow

ing)

nu

mb

er/p

lan

t N

odul

e fr

esh

wei

ght.

N

odul

e n

um

ber

/pla

nt

Nod

ule

fres

h w

eigh

t (m

g/pl

ant)

(m

g/pl

ant)

S

tres

sed

Con

trol

S

tres

sed

Con

trol

S

tres

sed

Con

trol

S

tres

sed

0 20

--

43

2 --

29

--

2

23

19

452

335

34

30

4 26

20

49

8 25

8 29

28

6

21

20

485

205

32

28

8 24

18

51

4 18

5 28

29

Day

s a~

er

rew

ater

ing

5 hr

26

19

54

2 26

~ 31

26

1

23

18

585

344

33

23

2 28

21

60

2 38

2 30

25

C.D

. at

5%

le

vel

NS

N

S

54

78

NS

N

S

620

602

534

640

435

586

355

636

290

610

596

649

NS

382

474

519

125

ga

Water stress on Nitrogenase activity 299

poropak-N column (2)<0.003m) with a N2 flow rate of 30 mi min -1. After the assay, the same plants were utilized for recording nodule number, fresh weight and dry weight.

3. Results and discussion

Water deprivation for 0, 2, 4, 6 and 8 days brought the soil moisture to 10.5 (0.03 mPa), 6.5 (0.19mPa), 3.4 ( l .41mPa), 2.4 ( > l . 5 2 m P a ) and 1.8 (>l .52mPa) per cent respectively at the vegetative stage and to 10.5 (0.03mPa), 3.8 (1.31mPa), 2.8 ( > 1.52 mPa) and 1.9 (> 1.52 mPa) per cent respectively at flowering.

In both the stages, increasing water stress significantly reduced the fresh weight of nodules, rather than their number (table 1). Shedding of nodules was not noticed even after 8 days of stress at either of the stages, although structural damage was caused due to loss of turgidity and shrinkage. Reports of drought-induced reduc- tion in the nodule number of Phaseolus vulgaris (Wilson 1951)and absence of such an effect in Trifolium repense (Engin and Sprent 1973), and in the present case suggest the variability of stress effect on the nodule number of different crops. Among other factors, this may be related to the type of nodules (i.e. spherical or meristematic) as suggested by Sprent (1976).

The decline in the nodule fresh weight of droughted plants was of the order of 64.5 and 54.4% at vegetative and flowering stages respectively, after 8 days of water deprivation, lncrease in nodule fresh weight of control plants during this period, particularly at the vegetative stage, without any significant change in nodule number, suggested a continuation of nodule growth with age. The contrasting situa- tion in the stressed nodules could result from dehydration and inhibition of meriste- matic growth as reported for certain other legumes (Engin and Sprent 1973; Sprent 1976). Restoration of turgidity on rewatering increased the nodule fresh weight of droughted plants, but the original weight could not be regained, presumably due to the irreversible damage of some nodules.

A slight decline in nodule moisture sharply reduced the specific nitrogenase activ- ity (activity/g of nodules) at both the stages (Fig. i). This drop (30-40% with less than 5% loss in nodule moisture) in activity increased to more than 95% when water loss exceeded 10% and became insignificant ( < Ipmol C2H4 g-1 nodules) under acute stress (without water for 8 days). Despite this similar trend at both the stages, the fall of nitrogenase activity was more at the vegetative, as comp'ared to the flow- ering stage, at a comparable degree of water stress. This was due to the preva.lence of markedly higher activity at the former, as compared to the latter stage. Thus, water stress during the vegetative period may be more detrimental for the total nitrogen fixed by plants.

A number of explanations may be provided for the reduction in the nitrogenase activity of the stressed nodules, such as inhibition of respiratory activity of the nodules which affects the ATP supply (Sprent 1976), noiaavailability of photosyn- thates to the nodules (Huang et al 1975) and direct effects on the enzyme itself. Water stress did not cause any change in the number of bacteroids per unit area. Therefore it is likely that reduced efficiency of bacteroids led to the decline of nitrogenase activity.

Rewatering led to a rapid recovery in the nitrogenase activitv in guar. It has generally been found in several legumes that the inhibition of nitrogenase activity

300 B Venkateswarlu, A V Rao and A N Lahiri

~00~

9O

-B g 8 o

2 6 70 E

60

80 T

60

~ 40

~ 2o o Ii'~l

~Ÿ O

�9 Control �9 Stressed

- e ~

Vecjetalive sloge ' ~ " 1 . . . . FIowr s locje \ t

I I I I -0 2 4 6 8 IC)

" , , ~

r- i 0 ~ 2 4 6 8 10

Ocas afler Ÿ

Figure l. Effect of water stress followed by rewatering on the nodule water content and nitrogenase activity in guar. Vertical line at 8th day indicates the time of rewatering.

becomes irreversible if the water content of nodules declines below 80% (Sprent 1976). However, in this case the stress-mediated depression in acetylene reduction was found to be reversible even when the nodule water content dropped to 60 and 70% at the vegetative and reproductive stages respectively. This could be due to the greater drought tolerance of guar as compared to the temperate legumes studied by Sprent (1976). Further meristematic type of nodules, as found in guar, was reported (Engin and Sprent 1973) to have a greater capacity for recovery than the sphe¡ type. This seems to be related to the initiation of fresh growth in meristematic nodules upon rewatering. In guar, fresh growth of stressed nodules was observed after 2 days of rewatering and the recovery may be attributed to both fresh growth and rehydration of existing nodule tissue. After 5 hr of rewatefing 17.1 and 14.5% of activity was restored at vegetative and flowering stages as compared to their respective controls. Again, 50% of the activity was restored after 24 hr at both the stages and after two days the activity was almost fuUy restored at vegetative stage but only about 85% was regained at flowering.

It may be concluded that the N2-fixation process in guar is highly sensitive to soil moisture stress but it has a higher capacity to recover, under favourable moisture condition, as compared to the temperate legumes.

References

Engin M and Sprent J I 1973 Effects of water stress on growth and nitrogen fixing activity of 7~ifolium repenr; New PhytoL "/2 11%126

Water stress on N i t r o g e n a s e ac t iv i ty 301

Huang C Y, Boyer J S and Vanderhoef L N 1975 Limitation of acetylene reduction by photosynthesis in soybean having various leaf and nodule water potentials; Plant Physiol. 56 228-232

Pankhurst C E and Sprent J 1 1975 Effects of water stress on the respiratory and nitrogen fixing activity of soybean root nodules; J. Exp. Bot. 26 287-304

Sprent J 1 1971 Effects of water stress on nitrogen fixation in root nodules; Plant Soil Specl. Vol., 225-228

Sprent J 1 1972 The effects of water stress on nitrogen fixing root nodules. IV Effects on whole plants of Vicia faba and Glycine max; New Phytol. 71 603-611

Sprent J 1 1976 Water deficits and nitrogen fixing root nodules. In Water deficits andplant growth Vol. IV (ed) TT Kozlowski (New York: Academic Press)

Wilson JK 1931 The sheddirtg of nodules by beans; J. Aro. Soc. Agron. 2,3 670-74