Building (tall) Plants

1. Building (tall) Plants some problems, issues and solutions Thanks to Fabien Forget and Akeel Hajat (Bot 302 -06) for some ideas..

2. Living in water Living in water must be easy. No gravity effect, little torsional stress, plenty of water, maintain operational temperature, access to nutrients. right? wrong!

3. Overcome though the development of large, interconnected lacunae systems Gas exchange Gas exchange in hydrophytes becomes difficult with lowering temperature. As temperature drops, leaf gas partial pressure drops, so less gas exchange takes place, and less O2 becomes available for cellular and metabolic respiration.

4. How to live in water and get an air supply? Waterlilies have a higher H2O vapour pressure inside their leaves than outside, yet N2 and O2 gradients are lower, thus diffusion gradients for these two gasses exist. . Heating the leaves increases pressure on the inside (which is cooled through transpiration). Pressurised air flows from the younger leaves to the older through the lacunar system, causing air flow by a mass flow mechanism. Pressurised gas is also forced into the rhizomes. Using this mechanism it is calculated that a single floating waterlily leaf may have up to 22 litres of air enter the leaf, per day. Much of this ends up in the rhizome. According to Dacey (in Salisbury and Ross Plant Physiology) this is really an example of nature’s ‘little biological steam engine’.

5. Tall Land Plants Getting taller brings with it several cogent problems 1. resisting gravity 2. resisting torsional effects Yet, there is one other feature that is of greater significance: 3. moving water and moving sap

6. any other ideas? How tall is tall? 50 m? 75 m? 100 m?

7. How tall is tall? 113.1 m (371 ft) Sequoia sempivirens, coastal redwood forest, California nope…. 132.6 m (435 ft); Eucalyptus amygdalina, Watts River Victoria Australia (cut down in 1855)

8. moving water Water has to be able to ascend > 120 meters how ? – using a suction pump, barometric pressure is the limit (10.3 m; 17.5 mm hg, or 2.3 kPa; 0.0023 MPa at 20oC) To move water to the top of the Sequoia would require 10.9 atm; or 1.1 MPa required, given a smooth bore capillary -- but a more realistic figure would be > 2.2 MPa (> 2.5 kj kg-1)

9. 14.78 h= r capillary diameter and effectiveness

10. capillarity and flow In other words, the narrower the capillary (or cell) the higher the column of water will move, simply by capilarity. The narrower the diameter of the capillary the greater the frictional coefficient, and the slower the rate of transport is h According to the Hagen - Poiseuille equation, the flow rate would be proportional to the 4th power of the radius of the capillary r

11. Height comes at a cost • Vertical growth brings about mechanical stresses: - Hydraulic - gravitational - wind • Therefore mechanical stresses start to play a bigger role.

12. Wind stress and support • The larger the canopy the higher the drag • Significant physical structures required to support weight and resist gravity • Trees need to be resistant to tensional forces caused by wind • The taller the tree, the larger the bending moment experienced at its base (increasing wind speed with height) • When a tree bends – both wind and gravity are acting on it

13. Hydraulic stress • Gravity is a major factor • water and soluble nutrients from the soil need to be transported to the top of the tree for: - Metabolic processes & - providing turgor for growth. • Transport occurs along a gradient of negative pressure - Transpiration

14. consider In reality, all factors are not equal, as the equation applies to capillaries only (cannot account for pitting, sculpturing, or vesturing of pits or end walls).

15. tiny bubbles… Given that the water column must be under tension, then evolution of a fail safe xylem system was an essential component of the evolution of tall trees. The highest resistance to water flow is usually found in petioles and leaves, hence wilting occurs here first, possibly death, if the tension is not eased. Internal sculpturing of walls and of perforation plates might have an effect on cavitation-bubble size, keeping them smaller. Developing specialized cells, called tyloses that grow into vapour-locked tracheary elements, helps to form an effective seal.

16. -.5 -1.0 -1.5 -2.0 RH 6 8 10 12 14 16 18 the driving force.. atmosphere leaf cells P = 0.5 MPa ψ = - 2.7 MPa Osmosis keeps cells from collapsing T = 20 oC RH = 80% ψ = - 30.1 MPa decreasing negative potential gradient

17. Physiological responses to tree height Picture from ref.6

18. How does a tree this high manage to deal with stresses? >110m! Sequoia sempervirens 1

19. the problems • High negative pressure in xylem: • structural support to prevent collapse • Cavitation in xylem (embolisms) • Reduce stomatal aperture in prevention of cavitation affects gas exchange and photosynthesis affected • Gravity and friction

20. Mechanical engineering solutions Hydraulic stress solutions • Perforated walls of xylem • Stomatal conductance decrease to reduce risk of cavitation • Development of specialized cells : tylosoids • Narrower capillaries (trade off) • Smaller thick walled cells at top to cope with high negative pressures – must reduce photosynthetic capacity substantially

21. Mechanical Engineering cont… • Tortional and gravitational stress solutions • Lignification, increasing structural strength • Investment in higher density wood for taller more long lived trees • Changes in stem taper and canopy shape reduce stress intensities to manageable levels • Certain portions of mature trees are ‘designed’ to fail under high winds speeds

22. Changing morphology with height – stress manifestation Dean T. J., Roberts S. D., Gilmore D. W., Maguire D. A., Long J. N., O’Hara K. L., Seymour R. S., 2002, An evaluation of the uniform stress hypothesis based on stem geometry in selected North American conifers Trees, 16:559–568 Sequoia sempervirens

23. Summing up.... So, differential vapour pressure supplies sufficient driving force to ensure that tall tree can and do transpire, thus moving a large water mass, and accompanying this, required nutrients. All (sic) that is required was the evolution of efficient transport systems – the xylem in particular, required modification to withstand negative pressures. Prevention of collapse is essential, cavitations have to be minimized, tyloses must be able to form. BUT clear that large-diameter vessels (springwood) are NOT usually going to be effective water transporters, as the water potential availability in soils declines. NARROW vessels may well be the answer under droughting conditions.