Xerophytic plants and their adaptationsPresented by: Jean Francois Alex Nitusha Gaya Madina Molohoo Bhavna Kandhai Mohseena Pawtoo Mary Jane Ravanne Christopher Syed

ContentsIntroduction What are xerophytes? Where are they found? Types of soils that xerophytes grow Types of xerophytes Morphological and anatomical adaptations Physiological adaptations C4 plants C4 photosynthesis CAM plants CAM pathway Conclusion

Introduction What are xerophytes??

Plants which grow in xeric (dry) habitats Xeric environments, e.g. deserts Many adaptations have allowed xerophytes to live there Developed mechanisms to minimize water loss and hence prevent dessication

Where are xerophytes found?? Found in arid areas but as well as in salt marshes, saline soil, or in acid bogs Adapted to chemically hostile, wet environments American Southwest, which includes the Sonoran Desert, the Chihuahuan Desert, the Mojave Desert, and the Great Basin Desert, are habitats for a rich variety of xerophytes. Also occur in Central America and Mexico, in the deserts of Idaho and Oregon, southern Africa and on the island of Madagascar. South Western Australia also contains some species of xerophytes. xerophytes.

In what type of soil do xerophytes survive?

Type of soil = desert soil Thin topmost ayer Low in moisture and nutrient poor Can range from sandy and finefinetextured to loose rock fragments, gravel or sand No subsurface water

Types of Xerophytes

Based on the nature of soil and cause of unavailability of water 1. Physical xerophytes: grow in dry soilE.g.. Opuntia, Casuarina, Ruscus 2.

Physiological xerophytes: grow in soil

having sufficient water but unavailable. E.g.. Mangrove

Based on life cycle and water storage

Succulents Succulents: Have fleshy character. Grow in droughtprone arid regions, dry soil. Absorb large quantities of water in leaves, stem or roots. Stem succelent: E.g. Opundia Leaf succulent: E.g. Aloe Root succulent: E.g. Asparagus.

Aloe

Asparagus Asparagus

Aloe

Non succulent they can tolerate long succulent:drought period. E.g. Casuarina, Zizyphus, Acacia

Short-lived annuals germinate following Shortannuals:rainfall. E.g. California poppy. The seeds lie dormant during drought and then, flower and form seed.

A xerophytic plant is one capable of surviving in an environment with little available water or moisture Plants like the cacti, bromeliads, Euphorbia virosa, pine are examples of xerophytic plants with various adaptations to survive in their respective dry environments

Xerophytes show various morphological & anatomical adaptations. Some or all the adaptations are especially to prevent water loss by the plant.

Sunken stomata Presence of trichome s Thick waxy cuticle

Extensive roots

Small leaf surface area

Adaptation to dry environmentAdaptations LEAF Thick cuticle with low surface area to volume ratio Rolled or reduced leaves Sunken stomata often with hairs. Large numbers of stomata Well developed sclerenchyma. sclerenchyma. ExplanationsDecrease loss of water & light intensity reaching mesophyll tissues. Reduced to spines e.g.: cacti to lower transpiration rate. Creates a chamber with high humidity making evaporation of moisture less likely. Allow rapid uptake of carbon dioxide during wet region. Mechanical strengthening to cell walls.

Adaptation to dry environment cont.Adaptations ROOTS Well developed Thin cortex Well developed xylem STEM Succulent with thick waxy cuticle Explanations To take advantage of superficial rainfall or deep water reserves. Small distance between soil water & vascular tissue. Rapid transport of water. To reduce water loss; succulent parts store water.

Xerophyte adaptations summaryAdaptation thick cuticle small leaf surface area low stomata density sunken stomata stomatal hairs (trichores) rolled leaves extensive roots How it worksstops uncontrolled evaporation through leaf cells

Example

less surface area for evaporation smaller surface area for diffusion maintains humid air around stomata maintains humid air around stomata maintains humid air around stomata maximise water uptake

conifer needles, cactus spines

marram grass, cacti marram grass, couch grass marram grass, cacti

Transverse Section Through Leaf of Xerophytic Plant

.

Thick sclerenchyma

Leaf size and angle of orientation help reduce heat loading

Smaller leaves cool faster than large leaves (thinner boundary layers) Vertical leaves have highest irradiation in early morning and evening

Physiological adaptations of xerophytesStomata of these plant open during night and closed during the day Chemical compounds of cell sap are converted to into wall forming compound( cellulose & suberin) suberin) Enzymes such as catalases, peroxidases are more active in xerophytes than mesophytes Hardened protoplasm ,that provide more resistance to heat and desiccation High osmotic pressure that increases turgity of sap.

Number of succulents found in some familiesFamily Agavaceae Cactaceae Crassulaceae Aizoaceae Apocynaceae Didiereaceae Euphorbiaceae Succulent no. 300 1600 1300 2000 500 11 1000 Modified parts Leaf Stem (root, leaf) Leaf (root) Leaf Stem Stem Stem and/or leaf and/or root Leaf Leaf and stem Distribution North and Central America The Americas Worldwide Southern Africa, Australia Africa, Arabia, India, Australia Madagascar (endemic) Australia, Africa, Madagascar, Asia, the Americas, Europe Africa, Madagascar, Australia The Americas, Australia, Africa

Asphodelaceae Portulacaceae

500 -

Examples of plants showing xerophytic adaptationsMechanism Limit water loss Adaptation Waxy stomata Few stomata Sunken stomata Stomata open at night CAM photosynthesis Large hairs on surface Curled leaves Storage of water Succulent leaves Succulent plant stem Fleshy tuber Water uptake Deep root system Below water table Absorbing surface moisture from leaf hairs or trichomes Acacia Cactus Marram grass, couch grass Marram grass Kalanchoe Euphorbia Marram grass, cacti Prickly pear Example

Transverse section through the leaf of a xerophytic plant

C4 plants- What are plantsthey??

Among the world's most important crops and noxious weeds Maize, sorghum and millet are staple foods throughout the tropics, sugarcane is traded globally, and 14 out of the world's 18 worst weeds are C4 plants.

How did C4 plants evolve?New geological evidence suggests that the early success of C4 species occurred against a background of relative constant atmospheric CO2, suggesting that CO2 was not the only trigger for the rise of these plants. In support of this finding, recent experiments suggest that life history and water balance may be as important as photosynthetic rate in mediating the effects of CO2 on plant fitness. What determines the modern distribution of C4 plants? The global distribution of C4 plants in today's world is mathematically modelled using current understanding of plant relationships with CO2, climate and soils. C4 grasslands (in orange) have evolved in the tropics and warm temperate regions where forests (in green) are excluded by seasonal drought or fire. C3 plants (in yellow) remain dominant in cool temperate grasslands because C4 grasses are less productive at low temperatures.

C4 photosynthesis in xerophytesThis type of photosynthesis is so-called because the carbon dioxide is first soincorporated into a 4-carbon compound. It uses PEP carboxylases as the 4enzyme involved in the uptake of carbon dioxide. This enzyme allows CO2 to be taken into the plant very quickly, and then it transfers the CO2 directly to RUBISCO for photosynthesis With C4 pathway, Xerophytes are able to overcome the tendency of enzyme RUBISCO to wastefully fix oxygen rather than CO2 This is achieved by the use of another enzyme to fix carbon dioxide in mesophyll cells resulting in the formation of Malate or Oxaloacetate; and also by the isolation of RUBISCO from oxygen The leaves of xerophytes are believed to established spatial separation between C4 pathway and Calvin Cycle C4 carbon fixation actually takes place in the mesophyll cells whereas the Calvin Cycle occurs in the Bundle sheath cells

Stepwise events in C4 pathwayIn the cytoplasm of mesophyll cells Step1: Step1: CO2 is fixed to Phosphoenolpyruvate (PEP) to give oxaloacetate; reaction catalysed by PEP Carboxylase Step2: Step2: oxaloacetate is then converted into malate or in certain species is converted to aspartate (by addition of amino group) Step3: Step3: Malate move from mesophyll cells to bundle sheath cells In the bundle sheath cells 4: Step 4: Malate is decarboxylated into CO2 and pyruvate Step5: Step5: Pyruvate returns to mesophyll cells where with the help of ATP it is regenerate into PEP Step6: Step6: The CO2 produced is used in Calvin Cycle where it reacts with RUBP to form 3-phosphoglycerate. 3-

Summary

It has been seen that the release of CO2 in the bundle sheath occurs in three different ways. In most C4 species, decarboxylation of malate with an accompanying oxidation to pyruvate is catalyzed by malic enzyme. The three different pathways are as follows: 1. C4 photosynthesis of NADP-malic enzyme type plants NADP2. C4 photosynthesis of NAD malic enzyme type plants 3. C4 photosynthesis of PEP carboxykinase type plant

C4 photosynthesis of NADP-malic enzyme type NADPplants

Step 1: CO2 is first fixed by 1: phosphoenolpyruvate with the formation of an oxaloacetate and release of an inorganic phosphate. Reaction is catalyzed by the phosphoenolpyruvate carboxylase.Step 2: carboxylase.Step Oxaloacetate is transported to the chloroplast where it is reduced by the enzyme NADP malate dehydrogenase to form malate. 3: Step 3: Malate moves to the bundle sheath cells via the plasmodesmata into the chloroplast of the bundle sheath cells where it is oxidatively decarboxylated to form pyruvate. The reaction is catalysed by the NADP malic enzyme. 4: Step 4: The pyruvate is transported to the chloroplast of the mesophyll cell. Pyruvate is converted to phosphoenolpyruvate (PEP).Reaction is catalyzed by the enzyme pyruvate phosphate kinase. kinase.

C4 photosynthesis of NAD malic enzyme type plantsStep 1:Oxaloacetate is converted to aspartate 1:Oxaloacetate instead of malate. 2: Step 2: Aspartate diffuses into the bundle sheath cells where it is transported into the mitochondria across specific transporters. Aspartate is reconverted to oxaloacetate by transamination involving a glutamate-aspartate amino transferase glutamatewhich is regenerated. 3: Step 3: Oxaloacetate is reduced to malate. Malate is decarboxylated to form pyruvate and CO2. Pyruvate is converted to alanine. CO2 goes into the alanine. chloroplast for the Calvin-Benson cycle. CalvinReaction catalyzed by: NAD-malate dehydrogenase NAD4: Step 4: Alanine is transported outside the mitochondria by specific transporters then to the cytosol of the mesophyll cell via plasmodesmata. Alanine is transaminated to pyruvate by the enzyme glutamate aspartate amino trasnferase. trasnferase. 5: Step 5: Pyruvate is then transported to the chloroplast of the mesophyll cell where it is converted to phosphoenol pyruvate (PEP).

C4 photosynthesis of PEP carboxykinase type plantsThis type of C4 photosynthesis is found in several fast growing tropical grasses used as forage crops. Step 1: Oxaloacetate is converted to aspartate in the mesophyll cells. Step 2: Here also aspartate diffuses into the bundle sheath cells where it is transported to the mitochondria. Aspartate is reconverted to oxaloacetate by transamination involving a glutamate-aspartate glutamateamino transferase. transferase. Step 3: Oxaloacetate is decarboxylated. CO2 is formed decarboxylated. and there is formation of phosphoenolpyruvate. The reaction is catalyzed by phosphoenolpyruvate carboxylase. The CO2 formed moves into the chloroplast of the bundle sheath cells. Simultaneously, some malate is formed in the mesophyll cells from oxaloacetate. Step 4: Malate is converted to pyruvate. Pyruvate is converted to alanine which moves from the mitochondria of the bundle sheath cells to the mesophyll cells through the plasmodesmata. Step 5: Alanine is reconverted to pyruvate in the bundle sheath cells then to phosphoenol pyruvate andoxaloacetate.

Adaptation in leaves of XerophytesThere are two majors carboxylating enzymes of photosynthesis present in those leaves: RUBISCO and PEP Carboxylase They differ from each other by the use of different forms of carbon dioxide. RIBUSCO usually use unhydrated CO2 whereas PEP carboxylase use hydrated form of CO2 namely bicarbonate PEP carboxylase has high affinity for bicarbonate and is not affected by the presence or concentration of Oxygen, in contrast to RUBISCO PEP carboxylase operates very efficiently, even when the concentration of its substrate is quite low

Efficiency of C4 carbon fixationFixation of CO2 in C4 plants has a larger energy cost than in C3 plants Total amount of ATP used for the fixation of 1 molecule of CO2 is five whereas for C3 plants it is only three C4 plants have a distinct advantage over C3 plants because fixation of CO2 is not affected by the presence of oxygen

CAM plantsCAM is an acronym for the name of the plants that have this type of photosynthesis and generically describes the type of metabolism. The C stands for the plant family Crassulaceae. Crassulaceae. The A stands for acid since acid is produced at night and M stands for metabolism. Crassulaceae includes many succulents. These succulents evolved in hot areas where water must be conserved. Like many xerophytes CAM plants have adaptations to live in hot and arid areas. These plants fix CO2 during the night, storing it as the four-carbon acid malate. The CO2 is fourreleased during the day, where it is concentrated around the enzyme RuBisCO, increasing the efficiency of photosynthesis. The CAM pathway allows stomata to remain shut during the day, reducing evapotranspiration. Therefore, it is especially common in plants adapted to arid conditions.

Crassula capitella

Crassula lycopodioides

Crassulacean acid metabolism (CAM): a twotwopart cycle Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway present in many plants such as xerophytes growing in very dry and often hot habitats. CAM is a two-part cycle consisting of a night cycle followed twoby a day cycle.

The night cycleDuring the night, carbon dioxide enters the tissue through the open stomata and diffuses into the photosynthetic cells where it dissolves into the aqueous milieu and generates HCO3-. The starch located in the chloroplasts is degraded to triose phosphate which is then exported via the triose phosphatephosphatephosphate translocator and is converted to phosphoenolpyruvate in the cytosol. cytosol. The HCO3- then reacts with phosphoenolpyruvate(PEP) under phosphoenolpyruvate(PEP) catalytic influence of phosphoenol pyruvate carboxylase to form oxaloacetic acid (OAA) .

The OAA is then reduced to malic acid by NADH and NADNADmalate dehydrogenase. This NADH dehydrogenase. is provided by the oxidation of triose phosphate in the cytosol. cytosol. The malic acid is actively transported across the tonoplast membrane and accumulates in the vacuole This keeps the concentration of malic acid low in the cytoplasm which is important because it is asn allosteric inhibitor of PEP carboxylase. carboxylase. Production of malic acid proceeds throughout the night but slackens off as dawn approaches.

A summary of the night cycle

The day cycleDuring the day, malic acid is transported back to the cytosol for the decarboxylation reaction. Calvin cycle overflowed with CO2 Sugars are accumulated Next night stored carbohydrates catabolised provide acceptor molecule for dark reaction

At night malic acid

Sugars

During the day malic acid

Sugars

CAM pathway in daylight

CAM plants are slow-growing. .WHY? slowEnergy lost at night by the use of starch to provide acceptor molecule for dark reaction SlowSlow-growing : Yes, but. advantageous over other plants by keeping guard cells open at night and closed in the daytime to avoid dessication. dessication. Many CAM plants function as both CAM and C3 HOW? In moist and cool conditions they grow as C3 plants In dry and warm conditions they grow as CAM plants Again this avoids dessication! dessication!

ConclusionXerophytes are subjected to harsh environments such as extreme temperatures and dry conditions. Even though they have been able to adapt themselves through their evolution by developing special physiological features as well as biochemical mechanisms that assist them in facing the extreme conditions and increase the chance on surviving. They now are able to thrive in many parts of the world through a large multiplicity of environments whether it may be in dry or even humid locations.