Stomatal opening and closing

Plants exchange gases through their stomata

-          CO₂ in for Calvin cycle and Rubisco

-          O₂ byproduct of photosynthesis out

-          Need to open stomata to spongy mesophyll to exchange air in spongy mesophyll

-          BUT, tradeoff – lose water through stomata as we have seen

-          How does a plant decide whether to open stomata

Role of blue light in opening stomata

-          blue light wavelengths are one of the peaks of absorbance for chlorophyll

-          so plants want to do photosynthesis when they sense blue light

-          stomata are flanked by guard cells

o   guard cells open and close stomata by turgor changing shape of cells (Fig. 38.18)

§  when full of water, they open

·         because of orientation of cellulose microfibrils in the cell wall

·         when water leaves, they become flaccid and sag closed

-          How do they open and close? (Fig. 38.19)

o   One way is the blue light sensor

·         A molecule senses blue light and opens stomata

-          Closing stomata in response to abscisic acid (ABA)(a plant hormone)

-          guard cells have receptor for ABA

Phloem and movement of sugar

Phloem is used to move sugars (dissolved in water) in the plant.

-          movement is always from source to sink

o   a source is a tissue where sugar enters the phloem

o   a sink is a tissue where sugar exits the phloem

§  in the summer the leaves are a source and the roots, meristems, developing flowers and seeds are all sinks

§  in the spring, the roots are the source and the young leaves are the sinks

Phloem structure

-          sieve tubes and companion cells (Fig. 38.22)

-          phloem tubes are continuous from top to bottom, from leaves to roots

Pressure-flow hypothesis (Fig. 38.25)

-          at the source

o   sugars are loaded into sieve tubes at sources (such as in a leaf doing lots of photosynthesis) (phloem loading)

o   so right at that point in the tube, there is high concentration of solute (sugar)

o   so water potential is low compared to surrounding tissue and xylem (because solute potential is high (and by convention solute potentials are always negative, so water potential is low)

o   so water moves from xylem into phloem tube

o   this increases turgor pressure (pressure potential)

§  lots of water, no where to go

§  so the sugar-rich fluid is pushed through the tubes

-          at the sink

o   sugars are unloaded into sink cells (phloem unloading)

o   as sugars are unloaded, solute potential of the phloem gets less negative and finally this is less concentrated than fluid in xylem

§  so water potential of fluid is higher than in xylem

§  so water flows into the xylem from the phloem

-          water potential of phloem at the source is high, at the sink it is low

How does phloem loading happen?

-          Some plants do this entirely through symplast using plasmodesmata (Fig. 38.24a)

o   So no crossing of membranes, no energy required

-          Other plants sugar is transported against concentration gradient – active transport (requires energy) (Fig. 33.24b)

-          a proton pump uses ATP to pump protons out of phloem companion cell

o   creates a proton gradient