.1 have shown the need to incoperate leaf

.1
introduction

Ecosystems in tropical
regions can function as carbon sinks and therefore moderating the unforeseen
effects of the predicted climate change. The response of terrestrial ecosystem
to the changing climate is an area of intense concern. Recent models have with
success been able to accurately predict the various responses of ecosystems to
climate change in order to have accurate and reliable predictions, the
dependence of carbon exchange should be evaluated for any environmental
variable at leaf level studies. The carbon and water exchange by the plant
leaves show a balance between the depression caused by high VPD and the
stimulation from irradiance exposure. Recent studies have shown the need to
incoperate leaf level stomatal regulation into models for ecosystem gaseous
exchange in forests ,it is therefore necessary improving the stomatal
regulation with models for herbaciuous communities.

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Regional
and global environmental changes have stimulated much interest in investigating
the control mechanisms on potential shifts of carbon (C) exchange over
different 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 or
approximately 21% of the total C storage in the terrestrial biosphere
(Gorham1991, Maltbyand Immirzi 1993). A recent report by Intergovernmental
Panel on Climate Change stated that wetlands are highly sensitive to climate
change.

Wetland
ecosystem carbon dynamics is considered to be potentially very sensitive to
globally observed climate changes (Adhikari, 2009; Saunders et al 2012).
Wetlands can be permanently or seasonally wet and for some wetness changes from
year to year depending on precipitation received .Variation in precipitation
causes variation in inundation in wetlands and is often accompanied by shifts
in vegetation patterns.  This variation
may have a direct influence on the adaptability of wetland plants.

Cyperus
papyrus is a large perennial grass that is one of the most widespread plants in
wetlands in tropical wetland regions worldwide. Environmental factors and the
physiological and biochemical characteristics of the plant affect
photosynthetic characteristics. Therefore, understanding the photosynthetic
characteristics of cyperus requires detailed observations of photosynthesis at
different growth stages, under different climatic conditions, and at different
vertical leaf positions on the plant.

The
high rates of NPP and low rates of decomposition characteristic of wetland
ecosystems make them ideal terrestrial carbon sinks (Adhikari, et al., 2009;
Jones and Muthuri, 1997). The two main ecosystem functions in relation to
greenhouse gas fluxes are in?uenced strongly by the presence of C4
characteristics of the  Cyperus papyrus
are the CO2 balance between carbon gains in photosynthesis and
losses in respiration, and H2O vapour losses in evapotranspiration
(Jones & Muthuri, 1997). Generally wetland plants grow at a faster rate
than they decompose, contributing to a net annual carbon sink. Vegetation
affects CO2 ?uxes primarily through photosynthesis and by increasing
the total ecosystem respiration. The high rates of net primary productivity
(NPP) by the wetland macrophytes, as well as anaerobic soil conditions that
limit decomposition make carbon stocks in wetlands among the highest as
compared to other global ecosystems. (Jones & Muthuri, 1997, Brix, et al.,
2001).

Photosynthesis
is a complex biochemical process that converts light energy into chemical
energy and useful organic compounds. It is the most important metabolic process
in plants. Leaf photosynthetic capacity is the basis and the direct driving
force for the formation of plant PRODUCTIVITY. Studies on photosynthesis in P. australis.
Stomata is identified as the point of exchange/regulation for water and CO2Stomatal
pathway and its corresponding resistances to transfer however is only one
component of the total leaf resistance.

Measurements
assess stomatal conductance/resistance = A measure of rate of passage of CO2
(g) or H2O (g) through the stomata. To measure the fluxes of water

 

Stomatal
opening, solar radiation, soil water availability, atmospheric vapour pressure
deficit and temperature are known to be important among the environmental
factors affecting stomatal conductance. Jones (1992) found that the
boundary-line response between conductance and temperature suggests an increase
in conductance from low to moderate temperature followed by a decrease in
conductance as temperature increases above an optimum level. This optimum
temperature ranges between approximately 18 and 22 °C, the highest conductance
values for this range being found at the more humid site.

In
different species, the increase in VPD leads 
a response of stomatal closure (Schulze, 1986; Turner, 1991) which may
either be  linear or nonlinear (Jarvis,
1976; Winkel and Rambal, 1990) depending on the type of control
mechanism.Jarvis (1976) argued that interpretation of the response to
environmental variables is useful as this parameters can be used to make
predictions on various parameters. Due to the functional relationships, these
predictions are only useful at the original site.

vapor
pressure deficit (VPD) is an important environmental factor that affect
stomatal functioning in higher plants.there have  been different views on the stomatal response
to VPD in higher plants and the possible mechanisms that  proposed to explain such response. There are
conflicting results about whether stomata respond to VPD or not and
concequently on how this affects the conductance and resistance of the
stomata.Soil water stress and leaf position are factors that may affect the
stomatal response to VPD and can help to explain these conflicting results.
When stomata do respond to VPD, the mechanism causing such response is not well
understood, and two contrasting hypotheses have been proposed.

With
regard to the response of the stomata to VPD, mechanism causing such response
is not well understood otherss have proposed the feed forward hypothesis  which states that an increase in VPD leads to
a decrease in stomatal cnductance 
.VIEWS  that gs decreases as vpd
increases because of an increase in transpiration (e) that lowers the leaf
water potential. these two mechanisms have been the subject of vigorous debates
as there are published results CONFLICTON THE SUBJECT.

Exchange
of CO2 and water vapour between  the  leaves and the ambient air are important
plant processes through where heat is dissipated through transpiration and a
primary substrate for photosynthesis is taken up.  This exchange primarily takes place in
stomata which are  openings at the leaf
surface that enables the control of water efflux and CO2 influx between the
leaf and the ambient air. Stomata, with regard to external factors, may respond
to many environmental factors such as light (quality and intensity),
ambientconcentration of CO2, leaf temperature, soil water status and vapor
pressure deficit (VPD.The exchange is primarily by diffusion, but the
concentration gradients and their associated fluxes are in the opposite
direction. The leaf is covered with the cuticle on the epidermis. It is a waxy
outer 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 dehydration
especially on low water levels. Therefore, stomata must function in a way to
optimize 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 a
decrease in stomatal conductance that 
may prevents CO2 from entering the leaf (stomatal limitation) and
inhibition of photosynthesis in mesophyll cells that decreases the use of CO2
(non-stomatal limitation). The former causes a decrease in intercellular CO2
concentration, whereas the latter increases the intercellular CO2 concentration
(Xu 1997; Qi et al. 2016).

Chpice
of leaves

With
increasing proximity to the base of P. australis plants, the net photosynthetic
rate and stomatal conductance gradually decreased and the intercellular CO2
concentration increased. These results indicated that non-stomatal limitation
caused 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 that
exhibited midday depression, the CO2 concentration declined with decreasing
stomatal conductance, indicating that stomatal limitation was the main reason
for the midday depression in these layers. However, previous studies have
demonstrated that midday depression in P. australis can be caused by
non-stomatal factors including salinization, drought, and high-water levels.

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