SUDEEP KUMAR and I). 1~. SEN

Laboratory of Plant Ecology, Department of Botany, University of Jodhpur, Jodhpur -- 342 001, India

Survival Adaptations of three Euphorbia spp. in Arid Ecosystem

Keywords

Euphorbia granulata, E. prostrata, E. hirta, Leaf area, Number of hairs, Thickness of cuticle, Stomatal index, Relative water content, Water deficit, Bound water, Plant water content aud Soil moiuture content

Abstract

KU.~IA~ S. et SEN D. N. (1985): Survival adaptations of three Euphorbia spp. in arid eco- system. - -Fol ia Geobot. Phytotax., Praha, 20: 5 7 - - 6 6 . - Thr~ species of Euphorbia (E. gra- nulata, E. prostrata and E. hirta) exhibited both morphological and physiological adaptations in dry environmental conditions. High bound water, thick cuticle, dense hair covering and low stomatal index are some of the survival adaptations they developed under water stress condi- tions of the Indian arid zone.

INTRODUCTION

The plants exist in a wide range of habitats sprearling from arid regions to polar ice tips. The story behind this is their potentiality of adaptation to diverse environ- ments. The natural vegetation of the Indian arid zone belongs to the Saharo- Sindian phytogeographical zone, which is a typical desert. Studies on water relations of Australian desert plants with special reference to their ecological amplitude have been conducted by BA~UST~B (1964a,b). There is no other factor tha t determines the vegetation aspect of the earth as strongly as water (La~oE et al. 1976). S~,N (1973) had also opined that water relation study of a plant attributes to the understanding of adaptability and survival strategies of any species.

The planes growing in arid zones are commonly xerophytes. Arid zones are those regions where water is not present in adequate quanti~y. In an arid zone physically dry habitats occur, which prevail in dry climate, when soil has a low

58 FOLIA GEOBOTANICA ET PHYTOTAX0~O~fICA 20, 1985

water re ta in ing capac i ty (Hsmo 1973). To avoid harsh condi t ions p l an t s crcate no~able morphological , ana tomica l and ecophysiological changes. A m o n g Euphor- bias of the I n d i a n ar id zone an ex t remely in t e res t ing example is t ha t of Euphorbia caducifolia which remains character is t ical ly leafless for more t h a n ha l f the year. I t is one of those species in the I n d i a n deser t t h a t possess the highest a m o u n t of latex. I n fact, i t is because of this la tex t h a t this species does no t suffer a t all unde r high e n v i r o n m e n t a l desiccation. S~.~ * et CHAW~" (1972) s ta ted t h a t a com- par ison of ind iv idua l p l an t s po in ts to the unmis t akab le fact t h a t this species exhib i ted d ivers i ty of a remarkable magni tude . This d ivers i ty is reflected in the morphological and eco-physiological behav iour of the p l an t s a nd their water t u rnove r never reaches a p e r m a n e n t deficiency. The behav iour of the p l a n t was control led p r imar i ly b y an endogenous rhy thmic cycle and remained unaffected b y the prevai l ing ex t reme ecological condi t ions.

The presen t s t udy is a imed at u n d e r s t a n d i n g the surv iva l s t rategies of three species of Euphorb ias viz. E. hirta, E. granulata and E. pro~trata which flourish well under the ex t reme env i ronmen ta l hazards of the I n d i a n arid zone. They were s tudied in re la t ion to the leaf area, th ickness of cuticle, opening of s tomata , n u m b e r of hairs, s t o m a t a l index (SI), re la t ive water con ten t (RWC), water deficit (WD), b o u n d water (BW), p l an t water c o n t e n t (PWC) and soil moisture.

M A T E R I A L S AND METHODS

For the present study plants were marked in the field of New Campus of Jodhpur University and observations were taken in the rainy season (in-season) and the post-n~nsoon season (off- season). The following parameters and formulae were taken into account. For the measurement of fresh weight, a plant sample was immediately weighed at noon and then it was kept in an oven at 80 ~ for 24 h for assessing its dry weight. Air dry weight was taken after keeping the material in the open air for 24 h and for saturated weight, plant material was kept on a wet cotton pad for 6 h and then weighed, water saturation measured at 100 % relative air humidity. The other parameters measured are as follows:

Ca) Leaf area: Leaf from a fixed node (fifth) was collected and area (ram 2) was calculated, by plotting the leaf on the graph paper.

(b) Number of hairs: Hair density was calculated by counting hairs in the isolated epidermal strips from the leaf and stem with a precalibrated microscope. The observations were taken from six different places.

(c) Cuticle: Stem samples were collected from six different places and transverse sections of the same were cut, and the thickness of cuticle was measured with a precalibrated microscope.

(d) Stomatal Index (SI): The stomatal index was determined according to the following formula.

SI = S(S q- E)- ' • 100

where S is the number of stomata per mm 2 and E is the number of epidermal cells per mm 2

(e) Relative water content (RWC): RWC was calculated by WEATSERI~Y'S (1950} method.

RWC ~ Fresh weight -- dry weight • 100 Saturated weight -- dry weight

(f) Water deficit (WD): WD was calculated by the method of STOCXZR (1929) as follows:

WD = Saturated weight -- fresh weight x 100 Saturated weight -- dry weight

KU3[AR AND SE~': ADAPTATIONS OF THREE :EUPHORBIA SPP. 59

(g) Bound water (BW): I t was calculated as follows after SC~ONBZcx et No~To~ (1979).

BW ~ Air dry weight -- dry weight • 100 Fresh weight -- dry weight

(h) Plant water content (PWC): This was measured by the following formula.

PWC ---- Fresh weight -- dry weight (Dry weight) -1 X 100

(i) Soil moisture content: Soil in the vicinity of the plants was taken from four depths, i.e. at soil surface and at 10, 25, and 35 cm depths. Soil moisture percentage was calculated as follows.

Percentage soil moisture ~ Fresh wt. of soil -- dry wt. of soil Dry wt. of soil X 100

r[1 I n - s o o s o n

[ l ]O~Lseoson

I .--

z 100 I l l l-,- Z O L.)

8O W - - �9 ~

LI.

~ 6 0 w lal 121

< w i--

mZ, O IX:

Z Z w 2C m L.) r

L,O I11 CL 0..

(] Eh Eg Ep

Fig.1 Fig. 1. Seasonal changes in relative water t e n a n t (%) of Euphorbia hirta (Eh), E. granulata (Eg) and E. prostrata (Ep).

50

40

30

20

10

0 Eh E9

Fig. 2

Ep

F i g . 2 . Seasonal changes in water deficit (%) of Euphorbia hirta (Eh), $I. granulata (Eg) and E. prostrata (Ep).

60 F O I , IA G E O B O T A . ~ ' I C A ET P H Y T O T A X O N O M I C A 20, 1985

RESULTS

The results obtained are summarised in the following paragraphs and in table 1 and in figs. 1 to 5.

(a) Leaf area (Table 1): Leaf area showed a sharp reduction with change in season in all three species. Maximum leaf area was noted in E. hirta 280.8 + 4- 1.6 m m 2 in in-season. In E. granular, the off-season leaf area was 9.6 + 4- 0.50 m m 2, which was r smallest leaf area recorded.

(b) Number of hairs (Table 1): The number of hairs was higher in off-season as compared with the in-season samples while hairs were total ly absent in E. prostrata.

(c) Cuticle thickness (Table 1): Is increased in plants f rom in-season to off- season but the increase in E. hirta and E. prostrata was not significant. Among the three Euphorbia species studied, E. granulata had the thickest layer of cuticle.

(d) S tomata l index (SI) (Table 1): A continuous decrease in s tomata] index was observed from in - se~on r off-season plants. E. hirta exhibited the highest SI

~_10 Z i .d l-.- Z o 8 (D

13d laJ I.--

t '~ Z

0

: 7 ~ 2 (..) 13d I.,i.1 Q.

0 Eh Eg Ep

Fig. 3

i,,~

IT] In-season

~] Off-season

Eh Eg

F/g, 4

1 Ep

Fig. 3. Seasoned changes in bound water content (%) of !Euphorbia hirta (Eh), E. grantdata (Eg) and E. prostrata (Ep).

Fig. 4. Seasonal changes in plant water content (%) of Euphorbia hirta (Eh), E. granulata (Eg) and E. prostrata (Ep).

K U M A R A N D S E N : A D A P T A T I O N S O F T H R E E E U P H O R B I A S P P . 61

o

o

C)

0

o

0 " 0

v

Y~

~ o o

-Mqq-H ~ o o c O

oo ~ ,-~

oo

q q ~ l

-Hqq-H

C

-H-H-H

II -H

62 FOLIA GEOBOTA-NICA ET PHYTOTAXON0~CIICA 20, 1985

(20) in in-season which decreased to 15 in off-season plants. E. granulata possessed the lowest SI both in in-season and off-season among the three species of Euphorbia.

(e) Relative water content (RWC) (Fig. 1): I t decreases from in-season to off- season in all three species. Maximum RWC was recorded in E. prosSrata (97 %) which decreased to 75 % in off-season. E. granulata was found to possess the lowest (49 %) RWC in off-season.

(f) Water deficit (WD) (Fig. 2): WD showed just the opposite trend to RW0. It increased tremendously from in-season to off-season. E. granulata possessed the maximum in-season (11%) WD which increased to 50 % in the off-season E. prostra~ exhibited the lowest (5 %) WD in in-season and it increased to 28 % in off-season.

10

" ' 8 Oc

~ 6

Z L~J ( J

2 LIJ Q.

o--o In-seoson : ~, Off-seoson

I I I I

0 10 25 35 DEPTH (cm)

Ffg.5

Fig. 5. Seasonal changes in soil moisture (%) during in-season (rainy) and off-season (post monsoon).

(g) Bound water (BW) (Fig. 3): BW showed only slight changes from in-season to off-season.

(h) Plant water content (PWC) (Fig. 4): PWC showed the same pattern as RW i.e. it decreased from in-season to off-season. E. hirta contained the most water while E. granulata the least both in in-season and off-season plants.

(i) Soil moisture content (Fig. 5): Its pattern reveals that it increased with depth in both in-season and off-season, being higher in the whole soil profile in in-season than in off-season.

K U I ~ A R A N D S E N : A D A P T A T I O N S OF T H R E E E U P H O R B I A SPP . 63

DISCUSSION

Ecophysiology of water relations is one of the important parameters to judge the adaptability of a particular plant species to prevailing harsh climatic and edaphic conditions. Hot winds cause considerable damage to the aerial parts of many plants in the Indian arid zone. Most of the plants show severe leaf scorching and subsequent defoliation. Inevitably the survival of plant populations depends on their water regimes and water economy and on the biochemical and physico- chemical properties of their cells. Drought resistance is the result of anatomical, morphological and eco-physiological adaptations by which plants avoid water stress. Drought resistance of plants has been studied by a number of workers (MAxI•ov 1929; ILJn~ 1957; SEN et al. 1979). Reduction of water loss by plants has been sought to conserve soil moisture and to increase survival of plants during drought.

MARTIN (1943) and SLATYER (1964) emphasised the role of leaf size by stating that smaller leaf size during the stress period would reduce the surface area. They also pointed to transpiration as being of importance in reducing overheating. In the present studies, the largest leaf area was recorded in E. hirta and the smallest in E. granulata. The leaf area gradually decreased in all species from in-season to off-season which reveals chat availability of water is one of the factors governing the survival of Euphorbia spp. in the arid zone. Ic is also curious that E. granulata which has smaller leaves is less resistant to drought than E. hirta which has larger leaf blades, which gets support from VoGwn (1968), who claimed that the larger the blade, the thicker will be the film of still air that envelops it and retards the outward diffusion of water vapour.

Presence of hairs on E. hirta and E. granulata is another additional structural adaptation that helps them in their survival. Except for a few fleshy plants, desert species are characterised by a dense clothing of hairs which forms a non-conducting screen against heat and strong insolation as well as sunlight reflected from the sand (SEN 1977). SS and CP forms (SS-spreading suberect; CP-compaet prostrate) of Convolvulus microphyllus were found to possess 3684 and 14210 hairs per em z, respectively, on the upper leaf surface (SEN 1972). YAPP (1909) suggested that the presence of hairs on Stachys lanata save the water loss by transpiration; his view gets support from the present observation, as the number of hairs increases in off-se~son. The role of the cuticle in reducing water loss is now well recognised and plants growing in arid environments possess chiek]y cuticularised leaves (SEN 1978). The present study adds to the information chat cuticularisation of leaves increases when plants survive in off-season. In the three species of Euphor- bia, the cuticle was thinnest in the in-season plants and became graddally thicker in the off-season. Increase in cuticle thickness during water-stress conditions appaxent]y played a significant role in survival of the plants, most probably by reducing the rate of their desiccation.

SHaEv~. (1934) claims that physiological adaptations are not all matched by structural ones and so each plant type must be analysed physiologically and struc- turally throughout its developments] cycle before the complex of its adaptations can be defined. The water relation study of a plant attributes to the understanding of adaptability and survival strategies of any species (SEN 1973). In the present

64 F O L I A G E O B O T A . N I C A ET P H Y T O T A X O ~ N O M I C A 20, 1985

investigations the internal water balance was found to be associated with soil moisture content. Maximum RWC was obtained in in-season plants when there was ample moisture in the soil. I t decreased gradually in off-season with the depletion of soil moisture. The same trend was reported in Ga/otrop/s procera, Prosopi~]uliflora, P. cineraria and Tephrosia purpurea by MATHUR et SEN (1972). E. hirta and E. granulata maintained higher RWC than E. prostrata, maybe because of greater surface resistance to transpiration due to the presence of thick cuticle and dense hairs.

In all the three species, the water deficit was lower in in-season and higher in off-season plants. Thus, direct correlation is noted here between water deficit and soil moisture content. The highest (50 %) water deficit was found in E. granulata and the lowest (25 %) in E. hirta. SE~ (1973) reported 8.6 ~/o water deficit in E. h/rta. He did not correlate WI) with soil moisture content, but its low value may be due to high moisture content of the soft when his s tudy was conducted. In Citrullus colocynthis and C. lanatus, water deficit decreased soon after rainfall and it increased slowly thereafter (SE~ et BHANDARI 1975, 1978). The pat tern of plant water content found in the present s tudy was associated with soil moisture content. Stomatal index was also found to be associated with internal water balance of the plants as it decreased in off-season plants. In all three species, the SI was higher in in-season and lower in off-season. This appears to be an adaptation reducing transpiration losses.

Bound water normally does not participate in physiological activities. MmA.gn> (1944} reported the bound water content to be higher in xerophytes than mesophytes. VADIA et al. (1961) cot. related plant hardiness to the relative amounts of bound water; the more bound water, the hardier the plant. JHA (1982) obtained more bound water under water stress conditions in Cyper~ rotun- dus. During the present investigation higher bound water content was obtained in in-season than in off-season but it is evident that E. hirta which had a high bound water (1.9--6 %) wa~ more tolerant to water stress as compared with observations of JHA et SE~r (1980). Thus, the present findings support the conclusions of MmAHID (1944), LEVII"r (1956), V.CD~ et al. (1961} and J ~ ct SF~" (1982). JHA et S~.~r (1980) reported 3.6 % bound water in E. hirta, 6.43 in Prosopis iuli- flora and 3.36 % in Cyperus rotundus in July when the plants were in juvenile stage.

The present study has revealed that Euphorbia app. avoid drought by developing some structural and functional adaptations so that they tolerate dry conditions like other xerophytes of ~he region. I~ is evident from the present study that E. hirta had a more effective survival strategy than E. r and E. prostrata. E. pro~trata appeared least tolerant and is the first to disappear when harsh condi- tions set in.

Two important conclusions can be drawn from the present s tudy: (a) changes in plant waler relations are associated with concurrent changes in

temperavure, soil moisture content and air humidity; and (b) the water status of a plant is governed by interactions within a multiple

system of factors viz. the soil moisture, water-vapour saturation deficit of the atmosphere, leaf area, thickness of cuticle, density of plant haires, s~oma~al behaviour and other internal factors.

Acknowledgements

The authors are grateful to Prof. H. C. AR'ZA for the facilities provided. Merit scholarship awarded to one of the authors (SK) by the University of Jodhpur Is gratefully acknowle~lged.

KU3IAR A.'~D SE.N': ADAPTATI:ONS OF TIIREE EUI'HORBIA SPP. 65

S U M M A R Y

T h e t h r e e spec ies o f Euphorbia (E. hirta, E. granulata, a n d E. prostrata) i n v e s t i - g a t e d a r e a m o n g t h e w e e d s a~lapted b e s t t o a d r y e n v i r o n m e n t . W a t e r r e l a t i o n s o f t h e s e w e e d s h a v e b e e n c o m p a r e d w i t h t h o s e o f species w e l l e s t a b l i s h e d i n a r i d e c o s y s t e m s s u c h as Cyperus rotundus, Prosopis ]uliflora, P. cineraria a n d Calo- tropis procera. T h e t h r e e Euphorbia spec ies s h o w e d s u r v i v a l a d a p t a t i o n s in t e r m s of t h e t h i c k n e s s o f cu t i c l e , n u m b e r o f ha i r s , l e a f a rea , s t o m ~ t a l i n d e x , r e l a t i v e w a t e r c o n t e n t , w a t e r def ic i t , b o u n d w a t e r , a n d p l a n t w a t e r c o n t e n t . T h e t h i c k n e s s o f cu t i c l e and d e n s i t y o f ha i r s i n c r e a s e d f r o m i n - s e a s o n t o o f f - s ea son whi le l e a f a r e a s h o w e d a s h a r p dec rease . E. hirta h a d t h e h i g h e s t r e l a t i v e w a t e r c o n t e n t (95 ~ ) wh i l e E. granulata was c h a r a c t e r i s e d b y t h e l o w e s t c o n t e n t (82 ~ ) . W a t e r def ic i t i n c r e a s e d c o n t i n u o u s l y f r o m i n - s e a s o n t o of f -season. T h e p l a n t w a t e r con- t e n t was a s s o c i a t e d w i t h t h e soi l m o i s t u r e c o n t e n t i.e., i~ d e c r e a s e d w i t h d e p l e t i o n o f soil m o i s t u r e . T h e p r e s e n t s t u d y has f u r t h e r p r o v e d t h a t p l a n t s t e n d t o a v o i d h a r s h c o n d i t i o n s b y r e d u c i n g t h e i r s u r f a c e a rea , i n c r e a s i n g t h e i r b o u n d w a t e r c o n t e n t a n d b y a l o w s t o m a t a l i n d e x , t h i c k c u t i c l e a n d b y o p e n i n g t h e i r s t o m a t a d u r i n g hou r s o f a l o w t r a n s p i r a t i o n d e m a n d .

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�9 Rece ived 30 J u n e 1982