Bio 1B Section 120 May 11, 2005

Topics to study for the Plants section of the final exam

Water relations  How does water flow up plant? Chain of water molecules. Driven by transpiration.  Water potential: tendency of water to move from one area to another. Denoted by (psi).  Water moves from an area of higher (less negative) to lower (more negative) .

P: Pressure/tension, caused by mechanical forces: Can be 0, <0, or >0.

: Osmotic potential, caused by solutes. Always ≤ 0.

P +  (can be 0, <0, or >0)

System P   Open container of pure water 0 + 0 = 0 Open container of salty water 0 + -1 = -1 Pressurized container of pure water +1 + 0 = +1 Xylem (water sucked upward) -5 + -5 = -10 Turgid plant cells +2 + -5 = -3 Cells at incipient plasmolysis 0 + -5 = -5

 We can measure the total water potential of a plant with a Scholander bomb (pressure bomb).  Plants control transpiration (water loss) by opening and closing their stomata.  Incipient plasmolysis is when the cell is just about to “collapse” from dehydration, i.e. the

walls are no longer pushing back against the cell contents and therefore P is 0.

Hormones  Auxin (=IAA) o How was it discovered? Plants (coleoptiles) bending toward light. o Most important effects: apical dominance; root formation. o Also ripening of fruit and abscission of leaves. o Different parts of plant are sensitive to different concentrations.

 Giberellins o Discovered in spindly rice seedlings with fungus infection: “foolish seedling disease.” o Causes stem elongation by increasing cell length. o Also breaks dormancy and has several other effects.

 Ethylene

1 o Fruit ripening, flower and leaf senescence. o Only plant hormone which is a gas. o Has commercial importance to make fruit ripen on demand.

 Cytokinins o Cause cell division. o Can cause differentiation of organs (e.g. leaves), if auxin also present in right amount.. Can cause tumors if too much.

 Abscisic acid (=ABA) o Inhibits growth o Plays a role in germination and breaking winter dormancy. o Not actually involved in abscission, that was a mistake.

Controls on flowering  We don’t know what “florigen” is, but grafting experiments suggest that it is a physical substance that flows in the plant.  Plant flowering controlled by night length: o Long-night plants will only flower if there is a long enough dark period. o Short-night plants will only flower if there is a short enough dark period.  Short-day = long-night; Long-day = short night  Sometimes, cold period is necessary to make plants flower heavily (e.g. magnolias)  Cold (stratification) also sometimes crucial for seeds sprouting.

Mineral nutrition  A nutrient is an element that is essential for the survival of an organism.  The following substances are crucial for the survival of plants but are not usually considered nutrients because they are required in such large amounts: C, O, H2O (also light).  Macronutrients are required in fairly large quantities: N, P, K, S, Ca, Mg  Micronutrients are required in smaller quantities: Fe, Mn, Zn, Cu, Mb, Ni, Cl, B  Some nutrients, such as N, are mobile (meaning that the plant can move them to where they are needed. If a plant has healthy young leaves and sick old leaves, it is probably suffering from a deficiency of a mobile nutrient.  Other nutrients, such as Fe, are immobile. Once they become incorporated into the plant’s body, they cannot be moved. If a plant has healthy old leaves and sick young leaves, it is probably suffering from a deficiency of an immobile nutrient.

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