Plant Physiol Biotech 3470 Lecture 16 Chapter 9 Thurs 23 March 2006 C assimilation and plant productivity Stern, “Introductory plant biology,” 10 th 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 [CO 2 ] –more available CO 2 → 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 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 CO 2 levels) Light fluence rate –At low light, respiration > photosynthesis At light compensation point, net CO 2 exchange is zero Saturation rarely occurs in natural conditions Fig. 9.3 –Here, respiration rate = photosynthetic rate –10-40 μmol photons / m 2 / s C3 plant photosynthesis becomes light-saturated- usually due to other photosynthetic limitations (e.g., CO 2 availability) C4 plants do not light saturate –Continue photosynthesis even at low internal CO 2 levels thanks to Kranz anatomy

Productivity also depends on CO 2 availability [CO 2 ] in the atmosphere = 0.035% (v/v) = 350 ppm –Below CO 2 saturation levels for C3 plants –Therefore CO 2 often limiting –Except in C4 plants → saturate at ambient Photosynthesis more dependent on intracellular [CO 2 ] rather than ambient [CO 2 ] –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 → ↑[CO 2 ] causes upregulation of CO 2 fixation (PCR cycle) enzymes –Too high [CO 2 ] feedback limits photosynthesis (nutrient limitations) –Also: limitations of source (leaf) tissues to store photoassimilate prior to transport to sinks Little CO 2, lots of RuBP Lots of CO 2, little RuBP Fig. 9.5 At high CO2 levels, photosynthetic rate is limited by low RuBP levels –When there is lots of CO 2, rubisco activity is saturated –Availability of RuBP here is limiting for photosynthesis CO 2 assimilation walks a line between these 2 limitations Little CO 2, 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 NH 3 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 [CO 2 ] atm to reach our Kyoto agreements! One solution - now intensive research efforts, including in Canada (BIOCAP)