GENERALARTICLES
ARTICLES
GENERAL
Should plants keep their (canopy) ‘cool’ or
allow themselves to grow ‘warm’ under stress:
It is a Hobson’s choice and plants survive by
doing a balancing act
V. R. Sashidhar*, S. J. Ankegowda, Mahesh J. Kulkarni, M. N. Srinivas, T. G. Prasad,
U. Nagalakshmi and R. Devendra
Two drought postponement strategies are put into operation in plants when the soil starts to dry.
Ironically, one results in ‘cooler’ canopies (maintains transpiration (T) by keeping stomata open) and
the other initiates a cascade of events leading to closure of stomata and the consequent ‘warming’ of
canopies. The ‘dilemma’ of plants is which of these two strategies to operate at what time and for how
long. Our studies suggest that it is a Hobson’s choice for plants as both processes have distinct advantages and disadvantages. In this paper we discuss why, how and when the plant makes the choice between these two strategies when it is undergoing a long drought in the field. We also briefly discuss the
metabolic costs involved in adapting either of these strategies and the balancing act a plant does, not
to just survive, but maximize carbon gain in a difficult catch 22 situation.
THE well-known stomatal physiologist Rascke said: ‘Land
plants are in a perpetual dilemma throughout their lives.
Assimilation of CO2 from the atmosphere requires intensive
gas exchange and the prevention of excessive water demands that the gas exchange be kept low.’ Transpiration (T)
has often been called as a ‘necessary evil’, ‘necessary’ because it ‘cools’ the leaf below air temperature and ‘evil’ because it accelerates the rate of loss of soil water; which is
often limiting particularly in the semi-arid tropics1. If we accept the fact that the process of transpiration keeps leaf
temperatures below air temperatures, it is also necessary to
ascertain by what degree the leaves become warmer or hotter if this process is curtailed. In fact plants could well be
regulating the process of transpiration in concert with their
ability to tolerate different temperatures. For example, plants
growing in deserts where the temperatures often reach 50°C
have the option of either curtailing T or maintaining a moderate level of T (ref. 1). The former strategy would result in
shoot temperatures rising perilously close to lethal temperatures, while the latter would still keep shoot temperatures at
least a few degrees below the air temperature. We present
evidence to show that plants adopt either of these strategies, presumably in tune with their own adaptation to high
temperatures.
Strategy I: Programmed to keep their cool under
stress
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In a field study which we conducted for four consecutive
years (1993–96), with two hundred genotypes of finger millet, it was observed that the plants stayed ‘cool’ amazingly
even 16 days after imposing a long duration of drought in
the field (Figure 1). This suggested that the plants had developed a strategy to maintain a certain level of transpiration. This is possible only by elongating their roots further
to tap water available in deeper layers. Data on a few selected genotypes in separate studies showed that these
‘stay cool’ genotypes tend to produce significantly longer
roots when a drought stress was imposed (Figure 2). What
is the adaptive strategy involved here and how common is
it? Data from the literature show that the first option
whether it be the millets or many desert species, is to stay
cool, i.e. the Crop Canopy Air Temperature Difference
(CCATD) is negative (Table 1). This means that the plant
decides to expand or divert its metabolic energy to maintain
T. The metabolic costs involved are enormous because the
only way it can do so is to send its roots deeper. This
means diversion of energy meant for shoot growth to produce longer roots (Figure 3). Separate studies in selected
pot-grown plants differing in CCATD in the field have confirmed this observation. This strategy to maintain a high
water uptake through a better root system is considered the
The authors are in the Department of Crop Physiology, University
of Agricultural Sciences, GKVK, Bangalore 560 065, India.
*For correspondence. (e-mail: vrsashi@yahoo.com)
CURRENT SCIENCE, VOL. 78, NO. 7, 10 APRIL 2000
GENERAL ARTICLES
most effective dehydration postponement trait2,3.
Strategy II: Programmed to grow ‘warm’ under
stress
Stomatal closure by ABA-mediated root to shoot signalling
leads to warming of canopies. Root to shoot signalling is
now widely accepted as another dehydration postponement
trait4,5. Inherent in this strategy is a system of communication which will signal the stomata to restrict opening to
avoid excessive water loss. In this system a message synthesized by the roots which are the first to sense the water
deficit, leaves the roots, uses the xylem as a conduit,
reaches the stomata and restricts the water loss as classical
‘first line of defence’6. We had in an earlier paper reported
the advantages and disadvantages of two types of root to
shoot signal communication; the electrical versus the
chemical signal7; ABA with its specific mode of action on
the stomata is widely accepted as the predominant signal.
The level of this hormone increases up to 50 times in the
xylem sap of stressed plants4–7.
Root to shoot signalling leading to an initial restriction of
the stomatal opening and finally in the stomatal closure
should therefore result in positive CCATD or warmer canopies. In opuntia shoot temperatures are often 20–25°C
higher than the air temperature1. In the same study and under similar environmental conditions, six other desert species still maintained slightly cooler canopies although the
soil only a few centimeters away was 12 to 28°C higher than
the air temperature. Long-duration droughts of forty days in
rainfed
cotton
resulted
in
canopy
temperatures of at least 10°C higher than the irrigated control8. However, we know that a negative CCATD is maintained in a majority of field-grown crops well into drought
even up to the 16th day after drought (Figure 1, Table 1).
How does this happen? This is where the second part of
dehydration postponement comes in, i.e. maintenance of
root elongation under stress to tap water available in deeper
layers. This mechanism is as complex as it is intriguing.
There is good evidence from a wide range of crops to show
that root elongation is indeed maintained at low water potentials9,10. Root osmotic adjustment is the central mecha-
Table 1.
Strategy I. Field studies involving different species
where the plants adopted to ‘stay cool’
Crop
Duration of drought
Reference
Long duration
Long duration
Short duration
Long duration
1
12
13
14
Six desert species
Sorghum
Wheat
Soybean
In these types, the crop canopy air temperature difference
was negative. This means that the canopy temperature is
several degrees lower than the air temperature, thereby giving a negative CCATD value. However, the soil temperature
is much higher than air. In the desert study, soil temperatures
were more than 20°C higher. Astonishingly the leaf temperatures were a few degrees cooler than air.
40
600
Figure 1. Average crop canopy air temperature difference of 200
genotypes of finger millet in the field which maintained a negative
CCATD (canopy temperature lower than air, i.e. cooler canopy) on
all days during a 16-day drought imposed in the field. Plant canopies
can be at temperatures ranging from 7 to 8°C (– 7 to – 8°C display
of CCATD on the infra-red gun; Figure 5). CCATD values decrease
from more negative to less negative values, eventually reaching 0°C,
the threshold at which the stomata close. Then canopies become
warmer. Finger millet genotypes were a few degrees cooler even on
16th day, i.e. the stomata were partially open.
CURRENT SCIENCE, VOL. 78, NO. 7, 10 APRIL 2000
Cooler type
Normal type
Figure 2. Average root length and total root length of cooler and
normal type finger millet genotypes under stress. Total root length
was calculated by multiplying average root length and total number
of roots.
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Figure 5. The infra-red gun measures canopy temperatures in the
field. It also gives a measurement of crop canopy air temperature
difference.
Figure 3. Relationship between root to shoot ratio and shoot dry
weight under stress.
Drought
Root osmotic
adjustment
Root to shoot
signalling
Root elongation
Stomatal closure
Water trapped from
deeper layers
Decreased
transpiration
Transpiration
maintained
Cooler canopy
Warmer canopy
The paradox, however, is that both the ‘strategies’
that a plant could adopt – root to shoot signalling or
maintenance of water uptake by increased root elongation,
are called dehydration postponement mechanisms. While
the former results in ‘warming’ of the canopy, the latter
helps the plant maintain a cooler canopy for a while under
drought (Figure 4).
The Hobson’s choice for the plant is which mechanism
should be put forth first when a drought occurs. The plant
has resolved this by doing a balancing act. The roots send
an early warning signal in the form of ABA by the help of
which the plant is able to restrict the stomatal opening
within a few days after the onset of a drought. This restriction results in a drop in the stomatal conductance by at least
25 to 30%. However, uptake of water by lower roots in contact with moist soil, still maintains a reasonably high transpiration rate as the stomata are still partially open. This results
in cooler canopies or a negative CCATD. Around this time
the ABA accumulated in the top roots which have undergone dehydration helps the plants to maintain root turgour
and elongation possibly through osmotic adjustment. Eventually, as the drought progresses the ‘second line of defence mechanisms’ operate which result in increased
production of ABA both in the roots and the leaves. Root
elongation ceases and stomatal closure occurs. The canopy
becomes warmer.
Which of these two mechanisms is more
cost effective
Dehydration postponement
Figure 4. Flowchart showing the two divergent dehydration
mechanisms that a plant can adopt. Ironically, one results in a
warmer canopy and the other in a cooler canopy.
nism responsible for this phenomenon14.
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The ‘early warning defence’ (strategy II) results in a
penalty on growth because decreased gas exchange also
means decreased rates of photosynthesis, which essentially
means less growth. The latter mechanism, i.e. root elongaCURRENT SCIENCE, VOL. 78, NO. 7, 10 APRIL 2000
GENERAL ARTICLES
tion to tap water in deep layers, has two disadvantages: (a)
there are structural costs involved, and (b) there is always
the danger of a limited supply of water getting exhausted
too quickly.
There is some evidence to show also that more metabolic
energy is required to produce 1 g of root than 1 g of shoot 2.
Ultimately the plant tries to survive by adopting two divergent dehydration postponement mechanisms; one resulting
in a ‘warmer canopy’ and the other maintaining a ‘cooler
canopy’. A plant surviving under a twenty-day drought in
the field would probably have done a balancing act maintaining these two strategies, simultaneously or
at different rates separated by a narrow time frame. The efficiency of the balancing act decides whether the plant
survives or succumbs to the soil–plant dehydration syndrome.
1. Gates, D. M., Alderfer, R. and Taylor, E., Science, 1968, 159,
994–995.
2. Ludlow, M. M. and Muchow, R. C., in Drought Research Priorities for the Dryland Tropic, ICRISAT, India, 1988, pp. 179–213.
3. Turner, N. C., Adv. Agron., 1986, 39, 1–43.
4. Davies, W. J. and Zhang, J., Annu. Rev. Plant Physiol. Mol.
Biol., 1991, 42, 55–76.
5. Davies, W. J., Tardieu, F. and Trjo, C. L., Plant Physiol., 1994,
104, 309–314.
6. Sashidhar, V. R., Prasad, T. G. and Sudarshana, L., Ann. Bot.,
1996, 78, 151–155.
7. Rekha, G., Sudarshana, L., Prasad, T. G., Kulkarni J. Mahesh and
Sashidhar, V. R., Curr. Sci., 1996, 71, 284–289.
8. Burke, J. J., Hatfield, J. L., Klein, R. R. and Mullet, J. E., Plant
Physiol., 1985, 78, 394–398.
9. Sharp, R. E. and Davies, W. J., in Plants Under Stress (eds Jones,
H. G., Flowers, T. J. and Jones, M. B.), Society for Experimental
Biology, Seminar Series 39, Cambridge University Press, Cambridge, 1989, pp. 71–94.
10. Saab, I. N., Sharp, R. E., Pritchard, J. and Voetberg, G. S., Plant
Physiol., 1990, 93, 1329–1336.
11. Westgate, M. E. and Boyer, J. S., Planta, 1985, 164, 540–549.
12. Choudhari, U. N., Deaton, M. L., Kanemasu, E. T., Wall, G. W.
and Marcarian, V., Agron. J., 1986, 78, 490–494.
13. Blum, A., Mayer, J. and Gozhan, G., Field Crop Res., 1982, 5,
137–146.
14. Harris, D. S., Schapaugh, W. T. and Kanemasu, E. T., Crop Sci.,
1984, 24, 839–842.
ACKNOWLEDGEMENTS. V.R.S. and T.G.P. thank DST, New
Delhi for funding the ad hoc research project as ‘Regulation of root
elongation in drying soil’.
Received 16 November 1998; revised accepted 25 February 2000
Derivatives of ultramafic rocks as decorative
and dimensional stone in Rajasthan
M. S. Shekhawat
The Precambrian formations of southern Rajasthan host the unique and the largest deposits of ‘green
marble’ in India located mainly around Rikhabdev, Kherwara, and Dungarpur areas. Field study coupled with mineralogic and petrologic studies indicate that the deep green and massive bodies of serpentinite are mainly composed of antigorite with subordinate amounts of carbonates and iron oxides.
These serpentinite bodies occur as large sheet-like masses, emplaced concordantly within the Proterozoic formations of Aravalli Supergroup. These deposits are being utilized by fully mechanized, opencast, block-bench mining methods for their extensive use as decorative and
dimensional stone. About 70% of the recovered ‘green marble’ is being exported to various countries
either in the form of well dressed blocks or in the form of finished products as slabs or tiles of suitable
sizes. The highly fractured deep green serpentinites are also being exploited by small-scale, manual,
open-cast mining methods to manufacture immensely popular flooring mosaic chips for civil engineering works. A very small quantity of steatitized serpentinite (steatite and chlorite schist) is also being
used to manufacture different types of carved items and idols.
ULTRAMAFIC (UM) rocks, specially serpentinites have attracted the attention of geoscientists as well as common
M. S. Shekhawat is in Department of Geology, M.L. Sukhadia University, Udaipur 313 002, India. (e-mail: geo@mlsu.ac.in)
CURRENT SCIENCE, VOL. 78, NO. 7, 10 APRIL 2000
men not only because of their geologic significance but also
because of their economic significance. Geologically, they
reveal valuable information about the composition of the
underlying mantle while, economically, they host a number
of metallic as well as industrial mineral deposits. In southern
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GENERAL ARTICLES
Rajasthan,
unfortunately,
these
rocks
are
totally devoid of metallic deposits but they host large deposits of talc1 and world’s largest deposits of amphibole
decorative and dimensional stone (DDS) all over the world
and is utilized as ‘green marble’, popularly known as ‘Kesariaji green marble’. Geologically, it is serpentinite but has
acquired a name in the marble industry due to its easy
blockability, ability to take excellent polish and low hardness. Some of the other derivatives like highly fractured
green serpentinites; the massive, non-foliated and compact
talc schist (steatite) and chlorite schist have also found a
place in the field of architecture.
Geological setting and metamorphism
In southern Rajasthan, the UM rocks occurring within the
metasediments (phyllite, quartz–mica schist and quartzite) of
Aravalli Supergroup (2500 to 2000 m.y.)4 are exposed mainly
as lenticular bodies or sheet-like masses of varying dimensions and exhibit concordant relationship with them4–7 (Figure 1). UM rocks of this area are considered to be emplaced
within the Aravalli rocks at the end stages of sedimentation
but prior to their deformation8 and are represented as ophiolite9–11 (obducted oceanic crust). They are also considered
asbestos 2,3. Moreover, since the last decade, one of their
metamorphosed derivatives, the massive and green
serpentinite has acquired a significant recognition as a
as derivatives of lherzolite sub-type12 (orogenic ‘root-zone’
peridotite). In this region, they constitute three well-defined
belts, namely Rikhabdev UM belt, Jharol-East UM belt and
Jharol-West UM belt12. The original UM rocks have undergone a varying degree of metamorphism in different areas13.
They are now represented by varied lithologies, viz. antigorite schist, antigorite–talc–chlorite schist, serpentine–carbonate rock, talc–chlorite–antigorite schist and talc–
antigorite–tremolite–actinolite schist in the former two belts
and anthophyllite–talc schist12 in the latter one. On the
basis of mineral assemblages present in these rocks, the
following successive stages of metamorphism have been
deduced.
(i)
Formation
of
antigorite
schist
(serpentinization); (ii) Formation of talc and talc–chlorite
schist
(steatitization);
(iii)
Formation of tremolite–actinolite schist (tremolitization);
(iv) Formation of anthophyllite schist (anthophyllitization12).
The present study reveals that in the Rikhabdev and Jharol-East belts, UM bodies have undergone changes up to
the third stage of alteration. But in the central part where
these rocks occur as comparatively large bodies, they have
suffered changes only up to the stage of serpentinization.
The UM bodies of the Jharol-West belt are highly altered
and have undergone all the four stages of alteration followed by retrograde metamorphism12.
UM rocks as DDS
Figure 1. Geological map of the study area showing the location of
‘green marble’ deposits (modified after ref. 4).
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Figure 2. Field photograph showing diamond wire-sawed blocks at
a ‘green marble’ mine.
CURRENT SCIENCE, VOL. 78, NO. 7, 10 APRIL 2000
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Serpentinites as ‘green marble’
The UM bodies which have undergone metamorphic
changes only up to the stage of serpentinization and occur
as deep green, massive serpentinite are suitable for use as
DDS. They occur at a number of localities (Figure 1), but the
richest and best quality of deposits occur at Odwas and
Masaron-ki-Obri near Rikhabdev, hence they can be defined
as type areas of ‘green marble’. Mining activity in the state
started about a decade and half back but a significant increase in production in the form of well-dressed blocks
(Figure 2) has been achieved since 1990 with the introduction of diamond wire-saws. In addition to its
domestic consumption, a major proportion is being exported
for use as dimensional stone. The ‘green marble’ is also
extensively used for manufacturing a large variety of decorative items and idols (Figure 3) which take an attractive
black colour on being smeared with oil. Incidentally, the
famous
black
idols
of
‘Shri
Eklingji’
and
‘Rikhabdev’ are also made of this rock.
Varieties and industrial characteristics of
‘green marble’
On the basis of texture and colour, four varieties of ‘green
marble’ have been identified. These are: (i) plain deep green
(without a white network structure), (ii) deep green with
white network structure, (iii) parrot green (also known as
apple green) and (iv) greyish-green with a white network
structure. The characteristic features that make the UM
rocks globally popular as DDS are: (i) their occurrence as
massive and compact bodies which can easily be quarried
and cut into blocks, (ii) their favourable hardness (about 4
on Mohs’ scale of hardness), (iii) their ability to take excellent mirror-polish, and (iv) their attractive colour and beautiful designs (green with white network structure and plain
deep green colour).
Mineralogic and petrographic characters
Petrographic study reveals that these rocks mainly consist
of variable amounts of antigorite, relict olivine, iron
oxides, carbonates, chrysotile and chlorite. Antigorite, a
re 3. Photograph of a statue of Lord Buddha carved from
ntinite.
Figure 4. Field photograph showing a well developed, fully mechanized ‘green marble’ quarry with diamond wire-sawed vertical faces
and spheroidal boulders (a significant guide for massive serpentinite)
at the top of the hill.
CURRENT SCIENCE, VOL. 78, NO. 7, 10 APRIL 2000
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GENERAL ARTICLES
variety of serpentine, is the chief constituent of these rocks
and is formed by alteration of olivine as evidenced by the
development of antigorite along the boundaries of relict
olivine and also within the irregular fractures of polygonal
olivine grains. The relicts of olivine are also present in variable amounts within the groundmass of antigorite and the
size of its grains vary from microscopic to macroscopic (1 to
5 cm). Unaltered grains of olivine are not present but altered
pseudomorphic grains are commonly observed. Other important constituents of the ‘green marble’ are carbonates
(dolomite and magnesite) found in recrystallized form along
the fractures developed within these rocks by shearing effects. The carbonates act as a cementing medium and impart
an attractive network structure. At a few places, white to
smoky coloured crystals of calcite have also been noticed
along the former fractures. The iron oxides (magnetite and
ilmenite) are present in considerable amounts mostly as
andedral to euhedral grains. Chrysotile frequently occurs in
the
form of cross-fibered, irregularly oriented, thin and long
ribbon-shaped veins which vary in width from 1 to 10 mm.
Guide for locating ‘green marble’ deposits
In greater parts of the area, the serpentinites are highly altered, intensively fractured and majority of outcrops are also
capped by thin soil cover and hence are a hindrance in localizing the underlying deposits of commercial importance.
By field study, spheroidal dark green boulders
lying partially or fully detached with the underlying outcrops of serpentinites (Figure 4) have been deduced for
localizing the potential deposits of ‘green marble’ in the
area. These boulders ranging in diameter from 1 to 10 m
generally rest on the topmost part of the hill/outcrop. They
exhibit massive form (non-fracturing nature) and have the
same mineralogical composition as that of the underlying
outcrops of serpentinite.
Steatite and chlorite schist as decorative stone
Steatitized UM rocks occurring in the area host potential
deposits of steatite (a compact rock rich in talc, also known
as ‘soapstone’ as it is soapy to touch). These
deposits are being extensively quarried for various industrial uses. However, a small quantity of massive and nonfoliated steatite recovered in the form of large-sized lumps
find its use in the manufacturing of various types of decorative items and statues due to ease of sculpting. Similarly, the
massive, non-foliated and fine grained variety of chlorite
792
schist, occurring in association with serpentinites, has also
been used for making various types of carved items and
idols.
Conclusions
Deep green, massive serpentinites occurring in southern
Rajasthan are composed mainly of antigorite with subordinate amount of carbonate minerals. Their massive nature, appealing deep green colour, white network structure
of carbonates, desirable hardness and ability to take excellent polish make them suitable for use as DDS green marble
all over the world. Occurrence of dark green spheroidal
boulders at the topmost part of the hill/outcrop is the key
guide for locating its deposits in this area. On the other
hand, the highly fractured deep green serpentinite has established itself in the domestic market as flooring mosaic
chips. Steatite and massive, fine grained, chlorite schist are
used for carving decorative items.
1. Rakshit, A. M., Indian Miner., 1977, 31, 20–25.
2. Ross, M., in Reviews in Mineralogy (ed. Veblen, D. R.), Mineral
Soc. Am., Washington, 1981, vol. 9A, pp. 279–319.
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pp. 95–108.
12. Shekhawat, M. S., Ph D thesis, University of Rajasthan, 1987,
p. 186.
13. Shekhawat, M. S., in Proc. of 30th IGC, Beijing, China, 1996,
vol. 2, pA 670.
ACKNOWLEDGEMENTS. I thank Dr P. S. Ranawat and Dr P. C.
Avadich, M.L. Sukhadia University, Udaipur, for critically reviewing
the manuscript and for their valuable suggestions. I also thank Dr S.
C. Khosla, former Head of the Department for providing facilities
for
the study and the two anonymous reviewers for their constructive
comments.
Received 14 September 1998; revised accepted 21 January 2000
CURRENT SCIENCE, VOL. 78, NO. 7, 10 APRIL 2000