Stomatal closure conductance

Unfavorable temperatures, especially when suddenly imposed, have been shown to cause leakage of ions and metabolites from the root to the surrounding medium (188). In addition, sudden changes of temperature cause a lowering of conductivity to water of roots and inhibition of ion transport (188). This can indirectly affect the carbon economy of the whole plant by leading to more negative leaf water potentials, partial stomatal closure, and hence a slowing of net CO2 assimilation. 

As rubber plantations are currently expanding to new areas under drier climatic conditions than traditional ones (north-eastern Thailand, Mato-Grosso in Brazil, north-western India), the performance of rubber trees under water stress has become a major issue. Two main traits are to be considered performance (the ability to transfer water and grow under water stress) and resistance (the ability to avoid damage and survive under water stress). Performance mainly relies on the conductivity of the hydraulic system from root to leaf, whereas resistance mainly relies on susceptibility to cavitation, leading to embolism, within xylem vessels. Stomatal closure occurs to avoid severe cavitation. Hence, differences in susceptibility to cavitation among rubber clones are likely to induce differences in resistance to severe water stress. 

A limitation in CO2 uptake during photosynthesis by plants subjected to water stress and salinity is a generally accepted phenomenon. Both environmental stress conditions result in a distinct decrease of leaf water potential, stomatal closure and a drop in leaf diffusive conductance, leading to a decrease in CO2 uptake. It was, however, shown that in spite of the decrease in intercellular C02 concentration under water stress, this was not the cause for the decrease in photosynthesis (1). The response of intact leaf photosynthesis to water and salt stresses were, however, found to be not so similar (2). Water stress inhibited CO2 fixation more drastically than an equivalent intensity of salt stress. 

Ward, D.A. Drake, B.G. (1988). Osmotic stress temporarily reverses the inhibitions of photosynthesis and stomatal conductance by abscisic acid - evidence that abscisic acid induces a localized closure of stomata in intact, detached leaves. Journal of Experimental Botany, 39, 147-55. 

Adaptive features of stomata - rapid closure in response to turgor loss (and perhaps other stimuli) - which reduce transpiration when plants become stressed. Alternatively, for ephemer-als, maintenance of high stomatal conductance and photosynthetic capacity to permit maximum growth rate during the limited time that water is available. 

Fig. 2. Reduction of stomatal conductance for gases by a factor of 0.5, and its effect on photosynthesis, accomplished either through proportional responses of all stomata left), or by closure of one-half of all stomata (right). A C0.2-assimilation rate, E transpiration rate g conductance for COo or H.>0 intercellular partial pressure of CO subscripts o initial p after proportional response b after binary response...

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