.1introductionEcosystems in tropicalregions can function as carbon sinks and therefore moderating the unforeseeneffects of the predicted climate change. The response of terrestrial ecosystemto the changing climate is an area of intense concern. Recent models have withsuccess been able to accurately predict the various responses of ecosystems toclimate change in order to have accurate and reliable predictions, thedependence of carbon exchange should be evaluated for any environmentalvariable at leaf level studies. The carbon and water exchange by the plantleaves show a balance between the depression caused by high VPD and thestimulation from irradiance exposure. Recent studies have shown the need toincoperate leaf level stomatal regulation into models for ecosystem gaseousexchange in forests ,it is therefore necessary improving the stomatalregulation with models for herbaciuous communities.Regionaland global environmental changes have stimulated much interest in investigatingthe control mechanisms on potential shifts of carbon (C) exchange overdifferent ecosystems. Wetlands cover only about 7% of the Earth’s surface(Lehner and Döll 2004), but the C storage is estimated to be up to 400 GtC orapproximately 21% of the total C storage in the terrestrial biosphere(Gorham1991, Maltbyand Immirzi 1993).
A recent report by IntergovernmentalPanel on Climate Change stated that wetlands are highly sensitive to climatechange.Wetlandecosystem carbon dynamics is considered to be potentially very sensitive toglobally observed climate changes (Adhikari, 2009; Saunders et al 2012).Wetlands can be permanently or seasonally wet and for some wetness changes fromyear to year depending on precipitation received .
Variation in precipitationcauses variation in inundation in wetlands and is often accompanied by shiftsin vegetation patterns. This variationmay have a direct influence on the adaptability of wetland plants.Cyperuspapyrus is a large perennial grass that is one of the most widespread plants inwetlands in tropical wetland regions worldwide. Environmental factors and thephysiological and biochemical characteristics of the plant affectphotosynthetic characteristics.
Therefore, understanding the photosyntheticcharacteristics of cyperus requires detailed observations of photosynthesis atdifferent growth stages, under different climatic conditions, and at differentvertical leaf positions on the plant.Thehigh rates of NPP and low rates of decomposition characteristic of wetlandecosystems make them ideal terrestrial carbon sinks (Adhikari, et al., 2009;Jones and Muthuri, 1997). The two main ecosystem functions in relation togreenhouse gas fluxes are in?uenced strongly by the presence of C4characteristics of the Cyperus papyrusare the CO2 balance between carbon gains in photosynthesis andlosses in respiration, and H2O vapour losses in evapotranspiration(Jones & Muthuri, 1997). Generally wetland plants grow at a faster ratethan they decompose, contributing to a net annual carbon sink. Vegetationaffects CO2 ?uxes primarily through photosynthesis and by increasingthe total ecosystem respiration. The high rates of net primary productivity(NPP) by the wetland macrophytes, as well as anaerobic soil conditions thatlimit decomposition make carbon stocks in wetlands among the highest ascompared to other global ecosystems. (Jones & Muthuri, 1997, Brix, et al.
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,2001).Photosynthesisis a complex biochemical process that converts light energy into chemicalenergy and useful organic compounds. It is the most important metabolic processin plants. Leaf photosynthetic capacity is the basis and the direct drivingforce for the formation of plant PRODUCTIVITY. Studies on photosynthesis in P. australis.Stomata is identified as the point of exchange/regulation for water and CO2Stomatalpathway and its corresponding resistances to transfer however is only onecomponent of the total leaf resistance.
Measurementsassess stomatal conductance/resistance = A measure of rate of passage of CO2(g) or H2O (g) through the stomata. To measure the fluxes of water Stomatalopening, solar radiation, soil water availability, atmospheric vapour pressuredeficit and temperature are known to be important among the environmentalfactors affecting stomatal conductance. Jones (1992) found that theboundary-line response between conductance and temperature suggests an increasein conductance from low to moderate temperature followed by a decrease inconductance as temperature increases above an optimum level.
This optimumtemperature ranges between approximately 18 and 22 °C, the highest conductancevalues for this range being found at the more humid site.Indifferent species, the increase in VPD leads a response of stomatal closure (Schulze, 1986; Turner, 1991) which mayeither be linear or nonlinear (Jarvis,1976; Winkel and Rambal, 1990) depending on the type of controlmechanism.Jarvis (1976) argued that interpretation of the response toenvironmental variables is useful as this parameters can be used to makepredictions on various parameters. Due to the functional relationships, thesepredictions are only useful at the original site.vaporpressure deficit (VPD) is an important environmental factor that affectstomatal functioning in higher plants.there have been different views on the stomatal responseto VPD in higher plants and the possible mechanisms that proposed to explain such response.
There areconflicting results about whether stomata respond to VPD or not andconcequently on how this affects the conductance and resistance of thestomata.Soil water stress and leaf position are factors that may affect thestomatal response to VPD and can help to explain these conflicting results.When stomata do respond to VPD, the mechanism causing such response is not wellunderstood, and two contrasting hypotheses have been proposed.Withregard to the response of the stomata to VPD, mechanism causing such responseis not well understood otherss have proposed the feed forward hypothesis which states that an increase in VPD leads toa decrease in stomatal cnductance .VIEWS that gs decreases as vpdincreases because of an increase in transpiration (e) that lowers the leafwater potential. these two mechanisms have been the subject of vigorous debatesas there are published results CONFLICTON THE SUBJECT.Exchangeof CO2 and water vapour between the leaves and the ambient air are importantplant processes through where heat is dissipated through transpiration and aprimary substrate for photosynthesis is taken up.
This exchange primarily takes place instomata which are openings at the leafsurface that enables the control of water efflux and CO2 influx between theleaf and the ambient air. Stomata, with regard to external factors, may respondto many environmental factors such as light (quality and intensity),ambientconcentration of CO2, leaf temperature, soil water status and vaporpressure deficit (VPD.The exchange is primarily by diffusion, but theconcentration gradients and their associated fluxes are in the oppositedirection.
The leaf is covered with the cuticle on the epidermis. It is a waxyouter layer, which is an effective barrier to both water and CO2 diffusion.Because the diffusion of water and CO2 occurs through the stomata, plants are faced with a constant problem.Allowing the maximal influx of CO2 for photosynthesis which is advantageous but can lead to dehydrationespecially on low water levels. Therefore, stomata must function in a way tooptimize dry matter production by balancing photosynthesis and transpiration.Therefore as a result, stomata respond to internal and external (environmental)factors.
A decrease in the net photosynthetic rate may results from two factors due to adecrease in stomatal conductance that may prevents CO2 from entering the leaf (stomatal limitation) andinhibition of photosynthesis in mesophyll cells that decreases the use of CO2(non-stomatal limitation). The former causes a decrease in intercellular CO2concentration, whereas the latter increases the intercellular CO2 concentration(Xu 1997; Qi et al. 2016). Chpiceof leavesWithincreasing proximity to the base of P. australis plants, the net photosyntheticrate and stomatal conductance gradually decreased and the intercellular CO2concentration increased. These results indicated that non-stomatal limitationcaused the decrease in net photosynthetic rate nearer to the plant base,because of the lower photosynthetic activity of the mesophyll cells.
In upper-or middle-layer leaves of P. australis in the Liaohe Delta wetland thatexhibited midday depression, the CO2 concentration declined with decreasingstomatal conductance, indicating that stomatal limitation was the main reasonfor the midday depression in these layers. However, previous studies havedemonstrated that midday depression in P. australis can be caused bynon-stomatal factors including salinization, drought, and high-water levels.
