1.Leaf (or cell) expansion lower leaf area less Pn Effects of water deficit:

 Volume  time = m (P – Y) cell wall extensibility yield threshhold of cell wall Growth rate = Reduction in cell expansion: Both m and Y are determined by chemical-mechanical properties of the cell wall

m (P – Y) (6 – 4)growth (4 – 4)no growth when (P – Y) approaches zero no growth Even if P is restored, growth may still be inhibited. Why?

Both m and Y can be altered by metabolism, enzymatic activity leading to softening or stiffening of wall Drought m decreased Y increased (i.e., stiffer cell walls) … and the change is slow to be reversed This slows or stops growth …

Effect of water deficit on maize shoot development H 2 O content: 100% 16% 4% 2%  w (bars)

What do you same , leaf/stem growth inhibited to a greater degree than root growth What do you hypothesize?  V/  t = m(P – Y) roots in dry soil have: higher m lower Y

What’s your next hypothesis? Some (unknown) biochemical-enzymatic factor acts to cleave bonds in the cell wall, loosening wall structure and permitting growth. Sharp, Silk & Hsiao: found XET (xyloglucanendotransglycosylase) induced by ABA

divides again when cell regains ~ original volume no stress stress At any given point in time, cells are smaller, but DW growth is virtually unchanged. time volume growth slower

In non-stress conditions, rate of in growth is faster. Therefore: acquire & allocate C much faster i.e., re-invest “capital” or not Growth depends on growth that has already occurred (compound interest analogy).

Effects of water deficit: 2. Reduction in photosynthesis  Stomatal closure – impedes influx of CO 2  Effects on biochemical capacity for photosyn. electron transport rubisco activity

Effects of Water Deficit 3. Phloem translocation phloem transport photosynthesis sink activity governed by Stress effects? decrease photosynthesis  less exported decrease sink activity  leaf expansion hard hit

Reflects sink activity (impaired very early) % 14 C remaining in the source leaf after 24 h Net CO 2 assimilation  mol m -2 s C translocation velocity (cm h -1 ) Leaf  (- MPa) Reflects phloem loading and export Not strongly inhibited until leaf  falls lower than ~ -12 bars

escape desiccation desiccation postponement tolerance (maintain high  ) (tolerate low  ) H 2 0 savers H 2 O spenders osmotic adjustment protoplasmic resistance

4. Osmotic adjustment (a mechanism of tolerance of low  ) An increase in the solute concentration of cell sap resulting from more solute molecules per cell rather than from a lower cell volume. Effects of water deficit:

 a process by which  can be decreased without an accompanying decrease in turgor  net increase in solute content per cell  do not confuse with increase in solute [ ] that occurs as a result of dehydration and shrinkage Osmotic adjustment

leaf cell  = - 8 bars xylem  = - 6 bars soil  = - 1 bar  w =   +  p -8 = water flows in

cell  = - 15 bars (or water will flow out) xylem  = - 14 bars soil  = - 2 bar  w =   +  p -15 = consequence?   p decreases  (P – Y)  zero  growth stops

If you were a bio-mechanical engineer:  design this plant so that it cope with this situation.  what would you modify or enhance?