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1.  Water relations and photosynthetic capacity of two species of Calotropis in a tropical semi-arid ecosystem 
Annals of Botany  2010;107(3):397-405.
Background and Aims
Calotropis procera and Calotropis gigantea, originally from warm parts of Africa and Asia, are now pan-tropical and in ecological terms considered an indicator of overgrazed, disturbed lands; they grow successfully in dry areas. Variations in water relations, morphology and photosynthesis of the two species growing in the same habitat were studied to assess possible mechanisms of tolerance to drought and how these relate to their ecophysiological success. Also the hypothesis that their photosynthetic rate (A) under drought would be affected by stomatal and non-stomatal limitations was tested.
Methods
Water relations, gas exchange, water use efficiency (WUE), fluorescence parameters, pubescence and specific leaf area (SLA) of Calotropis procera and C. gigantea plants growing in the field were evaluated during the wet (WS) and dry (DS) seasons.
Results
The xylem water potential (ψ) was similar in both species during the WS and DS; drought caused a 28 % decrease of ψ. In C. procera, A, stomatal conductance (gs) and carboxylation efficiency (CE) were higher in the WS with half the values of those during the DS, this species being more affected by drought than C. gigantea. A high δ13C of C. gigantea (–26·2 ‰) in the WS indicated a higher integrated WUE, in agreement with its lower gs. Leaves of C. gigantea were more pubescent than C. procera. Relative stomatal and non-stomatal limitation of A increased with drought in both species; no changes in maximum quantum yield of photosystem II (PSII; Fv/Fm) were observed. The decrease in the relative quantum yield of PSII (φPSII) and in the photochemical quenching coefficient (qP) was more pronounced in C. procera than in C. gigantea.
Conclusions
The photosynthetic capacity of C. procera was higher than that of C. gigantea. During the DS, A was regulated by stomatal and non-stomatal factors in a coordinated manner and drought did not cause chronic photoinhibition. A higher density of trichomes and leaf angle in C. gigantea may contribute to the maintenance of A and confer more efficient protection of photochemical activity in the DS. Ecophysiological traits such as high photosynthetic rate throughout the year even during the DS, and high WUE, highly pubescent leaves and low SLA observed in both species contribute to the establishment and growth of Calotropis in dry conditions.
doi:10.1093/aob/mcq245
PMCID: PMC3043925  PMID: 21149276
Photosynthesis; Calotropis procera; Calotropis gigantea; fluorescence; non-stomatal limitation; stomatal limitation; drought
2.  Causes of decreased photosynthetic rate and metabolic capacity in water-deficient leaf cells: a critical evaluation of mechanisms and integration of processes 
Annals of Botany  2009;103(4):561-579.
Background
Water deficit (WD) decreases photosynthetic rate (A) via decreased stomatal conductance to CO2 (gs) and photosynthetic metabolic potential (Apot). The relative importance of gs and Apot, and how they are affected by WD, are reviewed with respect to light intensity and to experimental approaches.
Scope and Conclusions
With progressive WD, A decreases as gs falls. Under low light during growth and WD, A is stimulated by elevated CO2, showing that metabolism (Apot) is not impaired, but at high light A is not stimulated, showing inhibition. At a given intercellular CO2 concentration (Ci) A decreases, showing impaired metabolism (Apot). The Ci and probably chloroplast CO2 concentration (Cc), decreases and then increases, together with the equilibrium CO2 concentration, with greater WD. Estimation of Cc and internal (mesophyll) conductance (gi) is considered uncertain. Photosystem activity is unaffected until very severe WD, maintaining electron (e−) transport (ET) and reductant content. Low A, together with photorespiration (PR), which is maintained or decreased, provides a smaller sink for e−, causing over-energization of energy transduction. Despite increased non-photochemical quenching (NPQ), excess energy and e− result in generation of reactive oxygen species (ROS). Evidence is considered that ROS damages ATP synthase so that ATP content decreases progressively with WD. Decreased ATP limits RuBP production by the Calvin cycle and thus Apot. Rubisco activity is unlikely to determine Apot. Sucrose synthesis is limited by lack of substrate and impaired enzyme regulation. With WD, PR decreases relative to light respiration (RL), and mitochondria consume reductant and synthesise ATP. With progressing WD at low A, RL increases Ci and Cc. This review emphasises the effects of light intensity, considers techniques, and develops a qualitative model of photosynthetic metabolism under WD that explains many observations: testable hypotheses are suggested.
doi:10.1093/aob/mcn244
PMCID: PMC2707350  PMID: 19155221
Water stress; photosynthesis; photorespiration; stomata; ATP synthase; ATP; photoinhibition; electron transport; Rubisco; fluorescence; sucrose; mesophyll conductance

Results 1-2 (2)