Global climatic change, with increased ambient CO
2 concentration and temperature of the planet, is predicted to alter precipitation patterns significantly, causing an increase in aridity in the semi-arid areas of the world (
Lawlor, 2001;
IPCC, 2007). High temperatures and dry conditions may be important factors in changing the success and regeneration of plant species (
Wei et al., 2009). Understanding how plants respond to drought can play a major role in the protection of natural vegetation in these areas. In most terrestrial ecosystems, water availability is the main environmental factor limiting photosynthesis, growth and productivity (
Schulze et al., 1987;
Wullschleger et al., 2002) even in plants well adapted to arid conditions, and influences the distribution and the abundance of many species of plants (
Schulze et al., 1987).
Seasonal loss of leaves, smaller photosynthetic leaf area and increased importance of photosynthetic stems, together with high trichome density, succulence, development of a deep root system, low osmotic potential and high water use efficiency (WUE), are, among others, the main adaptations of genera and of species within genera to dry zones (
Solbrig and Orians, 1977). For example, within a genus,
Eucalyptus cloeziana maintains its water status under drought due to its deeper root system and a greater capacity to extract water from soil than
Eucalyptus argophloia (
Ngugi et al., 2003). Also, photoprotective mechanisms involve physiological leaf adjustments in biochemistry, photochemistry, leaf morphology and anatomy (
Havaux and Niyogi, 1999). Leaf pubescence has been reported to be an adaptation to arid environments by reducing the radiant energy absorbed by leaves, affecting their energy balance (
Ehleringer, 1983), reducing transpiration whilst maintaining a favourable leaf temperature and so helping to keep a favourable water balance (
Savé et al., 2000;
Galmés et al., 2007). Variations between species within a genus have been shown in photosynthetic parameters of
Mosla and
Salix (
Liu et al., 2003;
Ge et al., 2004); transpiration rate (
E) in shrubs of
Caragana (
Ma et al., 2004) and morphology and photosynthesis in two varieties of
Digitalis minor growing in the Balearic Islands (
Galmés et al., 2007).
To analyse differences between genera or species which may be responsible for their ecological behaviour in relation to drought it is necessary to consider a number of potential mechanisms. Drought causes reductions in stomatal conductance (
gs) which may limit the photosynthetic rate (
A) due to a direct effect on CO
2 availability to chloroplasts by limiting diffusion through stomata (
Cornic, 2000) and/or reduction of the mesophyll conductance,
gm (
Flexas et al., 2008;
Hassiotou et al., 2009), or diminishing metabolic processes (
Tezara et al., 1999,
2008;
Lawlor, 2002;
Nunes et al., 2008;
Lawlor and Tezara, 2009) and/or photosystem II (PSII) activity and electron transport (
Tezara et al., 2003). To identify differences between species, an increase in the relative stomatal limitation (
Ls) may be calculated from the response curve of
A to the intercellular CO
2 concentration (
Ci) to determine whether
A is reduced only because of decreased
gs or if there as an increase in non-stomatal limitation (
Lm;
Farquhar and Sharkey, 1982).
Jacob and Lawlor (1991) defined
Lm as the proportional reduction in
Ci-saturated
A (
ACO2-sat) of plants subjected to stress relative to unstressed plants. Changes in
Lm due to water deficit may reflect changes in photosynthetic processes such as Rubisco activity, ribulose bisphosphate (RuBP) production, ATP supply, electron transport rate (
J) and light capture efficiency (
Lawlor and Cornic, 2002;
Tezara et al., 2005;
Lawlor and Tezara, 2009). Therefore,
Lm is also affected by parameters such as chlorophyll
a fluorescence and carboxylation efficiency (CE), among others. Measurement of some of these plant characteristics using such techniques may be used to identify the physiological and other factors responsible for the ecological differences between species.
They have been applied to analyse the response of two species of
Calotropis (Apocynaceae), known as milkweed, to seasonally dry conditions.
Calotropis is a wasteland weed of world-wide distribution but most abundant in the sub-tropics and tropics, and rare in cold countries (
Singh et al., 1996).
Calotropis procera (Aiton) WT Aiton is native to Asia, Arabia and tropical Africa, and
C. gigantea (L.) WT Aiton is native to southeast Asia (Malaysia, the Philippines, Sri Lanka, Thailand, India and China), and both were introduced into South America and the islands of the Caribbean (
Lebrun, 1998).
Calotropis procera grows up to an altitude of 1200 m (
Sayed and Mohamed, 2000), while
C. gigantea reaches 900 m (
Usmani and Kushwaha, 2010). The genus
Calotropis consists of common weeds which occur in arid ecosystems but have become naturalized in warm climates, where they grow commonly in disturbed areas. In Venezuela,
C. procera shows a wider geographical distribution than
C. gigantea. Both species are commonly found growing in hot and dry zones, generally next to the sea or in open and sunny places (
Steyermark, 1994). Both are successful invaders, particularly in old fields. The two species of
Calotropis studied here are perennial evergreen shrubs or small trees reaching 3–5 m tall, with large silver-green leaves, opposite, sub-sessile, clusters of waxy purple-tipped flowers, and inflated pale green seed pods (
Kleinschmidt and Johnson, 1977;
Nicholson, 1991). These species differ in characteristics, particularly in leaf pubescence which may be important, as mentioned above:
C. procera is less pubescent (
Colombo et al., 2007).
Most research regarding
Calotropis has been concentrated on its medicinal properties (
Lewis and Elvin-Lewis, 1977;
Oudhia and Dixit, 1994;
Oudhia, 1999a,
b) as it is an important source of pharmaceutical compounds (
Longanga et al., 2000;
Ahmed et al., 2005;
Usmani and Kushwaha, 2010). Also,
Calotropis yields a durable fibre useful for ropes, carpets, fishing nets and sewing thread (
Pérez-Arbeláez, 1978;
Tuntawiroon and Samootsakorn, 1984). The effect of water deficit on the growth of
C. procera in greenhouse-grown and cultivated plants has been evaluated (
Boutraa, 2010), but there are few ecophysiological data on natural populations of species of this genus (
Colombo et al., 2007;
Boutraa, 2010). Both species show high
A throughout the year even during the dry season (DS), suggesting the occurrence of particular strategies of drought resistance (
Khan and Beena, 2002;
Colombo et al., 2007;
Khan et al., 2007;
Boutraa, 2010).
In order to gain insight into the possible mechanisms of tolerance to drought and assess differences in physiological responses to environmental variables of these two species of
Calotropis, the seasonal changes in gas exchange, water relations and chlorophyll
a fluorescence parameters of plants growing in their natural habitat were measured. The climate in the study area is typical of a tropical semi-arid ecosystem, characterized by high radiation, high temperatures, annual rainfall of ≤800 mm, with evaporation exceeding precipitation and so limiting water availability for vegetation growth during seasonal drought (
Wilson, 1989). The following four hypotheses were tested: (1) that differences in ecophysiological traits related to maintenance of water balance and photosynthetic capacity contribute to the greater success of
C. procera than
C. gigantea in seasonally dry areas; (2) that the higher density of trichomes in
C. gigantea provides a photoprotective mechanism that improves its photosynthetic performance compared with
C. procera; (3) that
C. procera is more widely adapted to short-term changes in temperature than
C. gigantea; and (4) that limitation of photosynthesis to drought in xerophytic, successfully adapted species is caused by stomatal closure and not by metabolic limitation.
![[var phi]](/corehtml/pmc/pmcents/x03C6.gif)
PSII) and in the photochemical quenching coefficient (qP) was more pronounced in C. procera than in C. gigantea.
