BIOL 1902 Lecture Notes - Lecture 9: Stoma, Photosynthesis, General Idea
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Continuing on with plants dealing with subzero temperatures:
●For some plants, the upper part freezes off and dies while the main part of the
plant is in the soil and survives.
○however, the main part still needs to be cold hardy.
■excess water withdrawn and evaporated from leaves and twigs
■water is drawn out of the cells, which increases solute
●protective sugars are also absorbed into the cell,
increasing solute concentration even more.
○as a result, the freezing point gets lower.
●the cell membrane becomes more flexible by the addition
of unsaturated fatty acids
■antifreeze proteins to suppress ice formation and dehydration
○ice can form inside, between the gaps of cells and can go through the
cell wall. however, the cell membrane is more flexible so they resist ice
penetration and just deflect inward.
●plants become cold hardy through acclimation, which has several steps.
○the first stage of acclimation is triggered by a change in the
photoperiod (the ratio from daylight to darkness).
■plants react to the change with their phytochromes.
■phytochromes are the light sensitive photopigments that get the
plant into action to prepare for cold temperature.
●also causes the cells to go dormant.
●makes them more responsive to low temperatures.
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○the second stage is triggered by cold (not sub-zero) temperatures (10-0
■in the second stage, the plants go in high gear and the cell
membranes start getting more flexible and the plant goes into a
state of dormancy (?).
In the far north, some trees are cold hardy to -80 degrees celcius.
For coniferous trees/plants, (and other trees that have needles) retaining needles
creates new problems:
●damage by solar radiation
○solution: they create new pigments: xanthophyll pigments that protects
them from radiation
○they enable chlorophyll to use sun’s energy to create heat, not
○Skunk Cabbage has an unusual adaptation for the cold:
■it turns up the heat
●desiccation (drying out) - (on calm sunny days, not windy days)
○the solution is within their size. size is important: the farther north you
go, the smaller trees are.
■small surface area is important, leaf surface area is small
●leafs close their stomata (pores) so water can’t escape
●needles have thick cuticles to lock in water.
○plants that retain their leaves have hairs on the underside of the leaves.
■hairs trap moisture.
■hairs also break up the wind that can dry out the plant.
○some of the plants that are evergreens (rock polypody for examples)
reduce leaf surface area by curling up to reduce desiccation.
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