Plant Physiol Biotech 3470Lecture 16Chapter 9

Thurs 23 March 2006

C assimilation and plant productivity

Stern, “Introductory plant biology,” 10th edn.

Plants provide the carbon needed by all organisms

• We have seen that plants reduce C from the atmosphere• This reduced C becomes biomass which includes

– Crops– Trees and other forest species– Grasses

• Human activity alters the biosphere• This affects plant growth via increased atmospheric [CO2]

– more available CO2 → more productivity (plant growth)? ≡ higher yield?

• Yield concerns important to agriculture but also ecologically– Energy and nutrient flow within communities– How plants respond to stress– Upper limits to productivity: need to understand to feed the world

• Productivity is usually expressed as a rate – e.g., 300 kg per ha per year

Productivity is dependent on C fixation• Primary productivity (PP) is the

conversion of solar energy to organic matter by plants

• Gross PP is total carbon assimilation by plants

• But, some C is respired (30-60%!)• Therefore, we can define

Net PP = Gross PP – respiration = biomass available to animals

• Respiration rate is a significant limitation to plant growth

• We can distinguish two types of respiration that depend on its purpose

Mature leaf (not actively growing)

Very young, actively growing leaf

Plants having a higher growth rate will have a higher growth respiration rate

Fig. 9.1

1. Growth respiration → the carbon cost of growth; the amount of fixed C required in respiration to power growth via ATP synthesis (mitosis of rapidly dividing cells)

2. Maintenance respiration → the amount of fixed C allocated to providing energy for processes not resulting in growth (normal metabolism)

High respiration rates limit productivity• Can in theory improve productivity by

lowering respiration rate• In ryegrass, high growth rates found

in ecotypes having low respiration rates

– Therefore, more C available for growth • Can also manipulate components of

respiration– knock out the alternative oxidase to

increase yield (much less ATP synthesis!)

• Caution! Many enzymes may be required in the field under stress conditions! (as in maintenance respiration)

• Complicated processes, many enzymes → to manipulate efficiently via genetic engineering requires a thorough knowledge of pathway biochemistry– Identify regulatory steps– Changing one pathway will likely impact others! (e.g. hexose-P metabolism)

Total respiration

Fig. 9.2

In ryegrass, higher growth rate at lower respiration rate

Many environmental factors limit productivity• These include

– Nutrients– Water– Temperature

• Let’s examine a few others in detail (light and CO2 levels)

Light fluence rate– At low light, respiration >

photosynthesis• At light compensation

point, net CO2 exchange is zero

Saturation rarely occurs in natural conditions

Fig. 9.3

– Here, respiration rate = photosynthetic rate– 10-40 μmol photons / m2 / s

• C3 plant photosynthesis becomes light-saturated- usually due to other photosynthetic limitations (e.g., CO2 availability)

• C4 plants do not light saturate– Continue photosynthesis even at low internal CO2 levels thanks to

Kranz anatomy

Productivity also depends on CO2 availability• [CO2] in the atmosphere = 0.035%

(v/v) = 350 ppm– Below CO2 saturation levels for C3

plants– Therefore CO2 often limiting– Except in C4 plants → saturate at

ambient• Photosynthesis more dependent on

intracellular [CO2] rather than ambient [CO2]– But, ambient ≈ intracellular if the

stomata are open in C3 plants

atm

C3 plants increase max p’syn rate and CO2 sat’n level at high fluence

Fig. 9.4

• Photosynthetic capacity is determined by the balance of CO2 fixation capacity by rubisco and e- transport capacity

At low CO2 levels, photosynthetic rate is limited• Not enough CO2 to operate the PCR cycle quickly• The cycle backs up with lots of the other rubisco substrate

( ______ ) present

Photosynthetic capacity limits the C assimilation rate

– Regulating the size of the stomatakeeps photosynthesis in transition zone where neither RuBP levels or CO2 levels are limited

– BUT… stomata are supposed to regulate water loss (transpiration rate)! –different theory!

• CO2 enrichment used to increase productivity in greenhouses → ↑[CO2] causes upregulation of CO2 fixation (PCR cycle) enzymes– Too high [CO2] feedback limits photosynthesis (nutrient limitations)– Also: limitations of source (leaf) tissues to store photoassimilate prior

to transport to sinks

Little CO2, lots of RuBP

Lots of CO2, little RuBP

Fig. 9.5

At high CO2 levels, photosynthetic rate is limited by low RuBP levels– When there is lots of CO2, rubisco

activity is saturated– Availability of RuBP here is limiting for

photosynthesis

• CO2 assimilation walks a line between these 2 limitations

Little CO2, lots of RuBP

Maximizing biomass production requires integration of complex processes

• Intimate metabolic connections exist between these processes!

• Recall that photosynthesis-derived energy powers C skeleton biosynthesis for anabolism via the hexose-P pool

• The integration of metabolism makes manipulating it to increase productivity tricky!

Powers growth

Amino acids for enzymes

NH3 reduction

Amino acids for protein

How does primary production work on a global scale?

• The Earth produces 172 billion tonnes of biomass per year

• 68% from terrestrial ecosystems ~ 30% area• 32 % from marine ecosystems ~ 70% area• Therefore, terrestrial productivity ~ 5X marine

– Due to differences in nutrient supply– Water: nutrients sink out of photosynthetic active zone– Land: plants retain more available nutrients in litter

• Most productive: tropical forests– ~21% of total biomass in rainforests alone! – long growing season

• Only ~5% produced via agriculture– limited suitable lands available

Improving productivity on marginal land is a key goal

• “Green revolution”- new cereal crop strains BUT mostly improve productivity on land already suitable for agriculture

• A much bigger challenge is to improve productivity on marginal land– Salt-stressed– Drought-stressed– Unsuitable temperatures– Being addressed through biotechnological approaches– Needed to feed growing population

• Most biomass produced by forests → but extensive deforestation constantly occurring!

• Causes declines in forest biomass and world biomass just at the time we need to reduce [CO2]atm to reach our Kyoto agreements!

• One solution - now intensive research efforts, including in Canada (BIOCAP)