The Physiology of Flowering Plants - KHAM PHA MOI

30 FLOW OF ENERGY AND CARBON THROUGH THE PLANTEconomically CAM plants do not make a great contribution; thepineapple (Ananas comosus), the vanilla orchid (Vanilla fragrans) andagave (Agave americana) are CAM crops. The significant feature of CAMplants is water conservation, and they are found in arid regions. Thesucculent flora of a hot desert may consist nearly entirely of CAMspecies. But CAM plants are found also in water-poor niches in temperateclimates; such are, for example, the stonecrops (Sedum spp.)which grow on stony ground. Another large group of CAM plantsembraces the bromeliads and orchids growing epiphytically on treesin tropical rainforests. One thinks of rainforests as very wet areas; butepiphytes, which have no access to soil water, may be water-limitedeven in a humid climate and at the top of the canopy, humidity levelsduring the day can be very low.There are facultative CAM plants (C 3 –CAM intermediates), whichcan switch from C 3 metabolism to CAM in response to water stress;an example is the ice plant (Mesembryanthemum crystallinum) from theMediterranean region. The aquatic plant Hydrilla verticillata on theother hand develops C 4 characteristics in response to a low level ofCO 2 . In some species the induction of CAM is rapidly reversible, inothers it is permanent. Under certain environmental conditions,some CAM intermediates do not close stomata fully by day and thusfix CO 2 by day and night.It is believed that both C 4 photosynthesis and CAM arose severaltimes independently in evolution, since both types of metabolism arefound in totally unrelated families, including monocotyledons anddicotyledons, which diverged very long ago; CAM is exhibited also bysome spore-bearing plants. One genus can contain representatives ofall three photosynthetic types: Euphorbia corcollata –C 3 ; E. maculata –C 4 ; E. grandidens – CAM. The selection pressure for CAM has obviouslybeen water stress. It has been suggested that the evolution of C 4photosynthesis has been stimulated by the decline in atmosphericCO 2 levels which occurred from 120 to 30 million years ago.Evolution is considered further in Section 2.7.1.2.5 Limiting factors for photosynthesisA leaf is photosynthesizing in a garden. What determines its rate ofphotosynthesis at a given moment? This is not an easy question toanswer.2.5.1 Limitation in the short termThe rate of photosynthesis is affected by the PFD, CO 2 concentration,temperature, wind speed, the plant’s water status and the degree ofstomatal opening. The levels of some inorganic nutrients, too, mayaffect the rate; e.g. an adequate phosphate supply is critical formaintaining the levels of phosphorylated intermediates, and fornormal opening of stomata. The effect of varying any one of thesefactors depends on the value of the others so that, in a sense, the rate