Gas Exchange and Water Relations in Mediterranean Sclerophylls

1. Lecture 3 Gas exchange and Water relations of Mediterranean sclerophylls • Aims: to consolidate the applications of plant water relations methodologies in a comparative study of water use by mediterranean sclerophylls • Learning outcomes: to understand how measurement of water potential during the progressive dehydration of leaves and or shoots can be used to give an ecological insight into niche differentiation

2. 3.1 Mediterranean vegetation: some definitions • 3.2 Pressure bomb and plant water relations 3.2.1 The pressure-volume curve 3.2.2 The Höfler diagram • 3.3 Olive, carob and bay: a Mediterranean odyssey • 3.4 Gas exchange components • 3.5 Diurnal response of impact of Quercus suber Key Reference: Lo Gullo MA and Salleo S (1988) Difernt strategies of drought resistance in three Mediterranean sclerophllous trees growing in the same environmental conditions New Phytologist 108, 267- 276 Background texts: Lambers et al (1988) Plant Physiological ecology Springer Nobel PS (1999) Physicochemical and environmental plant physiology Academic Press DA 258

4. 3.1 Mediterranean vegetation: some definitions • Macchia, garrigue, Matorral – all descriptions of the stress- tolerant, late-succession, high-diversity, shrubby, scrubby forest typical of the Mediterranean climate • Highly disturbed, often burned, (sprouters and seeders) • Main canopy forming species: Evergreen sclerophylls (literally hard- or tough- leaved) (eg Quercus coccifera, ilex and suber; Arbutus unedo, Olea europea,(Olive) Pistacea lentiscus, Juniperus, Laurus nobilis, (bay) Ceratonia siliqua (carob), • semi-deciduous drought- tolerant shrubs (soft-leaved malacophylls)-Cistus spp., Rosmarius, Phlomis, Lavendula …

5. 3.2 Pressure bomb and plant water relations • When a leaf or stem is removed from the plant, the tension in the xylem causes the sap to withdraw from the cut surface • Provided that the osmotic concentration of solutes is low in xylem sap and the apoplast (usually less than 0.1 MPa), the slight overestimation of xylem sap pressure given by the pressure bomb is negligible

6. 3.2.1 The pressure-volume curve • Methodology: first catch your shrub, stem or leaf preferably somewhere in the Mediterranean basin • Place stem or leaf overnight in a beaker of water in a dark cupboard, covered with a plastic bag, to allow complete rehydration to full turgor • Method 1: invert stem in pressure bomb, pressurise to reach balance point, invert pre-weighed tube containing absorbent material over petiole, apply over-pressure of 0.2 to 0.5 MPa for 5-10 minutes; depressurise, weigh tube, re-measure new balance point; repeat procedure for 10 data points through point of turgor loss • Method 2 : weigh stem/leaf, measure initial balance point, remove, put branch on bench and leave to dehydrate for 10-20 mins (repeat for replicate 2 and 3); reweigh, re-measure balance point, repeat…… • Tabulate data as  and Relative water content and plot as 1/ against RWC…….

7. The curved part of the plot represents approach to loss of turgor; during this period, there is a a progressive decrease in turgor and as water is extruded, increase in osmotic potential/pressure; • Extrapolate linear portion of curve from turgor loss point to 1/ axis to derive 1/s or 1/ since below TLP,  = s (or -) • For a given volume of water extruded, calculate turgor pressure

8. Elasticity of cell walls- the shape of the curve is markedly dependent on the wall rigidity; if the wall is very rigid, the water potential components change very rapidly for a given water loss; more elastic cells allow turgor to be maintained as cells progressively dehydrate, because of the extra “give” in the cell wall • Bulk modulus of elasticity,  (MPa), amount by which a small change in volume (DV) brings about a small change in turgor (DP), such that DP =  . DV/V, and  = dP/dV . V • Modulus of elasticity () can therefore be derived from the slope of the linear portion of the P-V curve; a high value of  corresponds to low cell-wall elasticity in rigid cell, as a large change in P occurs for a small amount of water expressed; a lower value corresponds to an elastic cell 3.2.2 The Höfler diagram

10. 3.3 Olive, carob and bay: a Mediterranean odyssey (Lo Gullo and Salleo 1988) • Olea oleaster, Ceratonia siliqua and Laurus nobilis are all classed as sclerophylls, with similar leaf Dry Weight: Surface Area ratio (1.1 –1.2 kg m-2) • “sclerophyll” = “leaves hard and coriaceous, breaking when folded” • Olea is “microphyllous”, the others have relatively large leaves • But some are more sclerophyllous than others, since leaf components may vary eg. cuticle thickness, tough conducting tissues (sclereids and fibres) and lignification of epidermal and parenchyma cells • All show different habitat preferences, leading to this analysis of water relations among the Olea- Ceratonion plant community in Sicily • There was a gradation in leaf water relations as determined from the P-V curve, shown for Olea and Laurus in September

11. Large seasonal change in temperature and soil water status as hot, dry summer develops in Mediterranean region

13. Olive can be dehydrated to a much greater degree before reaching the turgor loss point • For 5% decrease in water content, the decrease in  corresponded to –0.78 MPa for Olive, -1.06 for carob and –1.48 for bay • Olive sustained a much greater range of osmotic potentials, suggesting that the cytoplasm is more resistant to desiccation, and showed a greater capacity to adapt from May to September -1.95 to -2.5 MPa for Olive, but remained at –1.7 MPa for carob •  was much higher for bay than olive or carob- with the most elastic cells in Olive consistently for May and September

14. Carob had much higher leaf conductances in May and September, but neither carob or bay sustained a large decline in RWC,as compared to Olive

15. Xylem conduit diameter was narrowest in Olive, and greatest in Bay (but this may be carob –see p274 para 1) • But Olive has a greater conducting cross sectional area per unit of leaf surface area, and maintains the highest rate of water supply to leaves

16. 9.5 Ecological implications for root and shoot allocation • Degree of sclerophylly only thing in common! • Olea: drought tolerance extends to low conductances, large daily reductions in RWCand ; large seasonal osmotic adjustment, cells highest elasticity;  approaches Turgor loss point in May and Sept; narrow xylem constrains cavitation but large area re-supplies water; • Ceratonia: high conductances but RWC loss minimal; leaves approach relatively high TLP in Sept; little osmotic adjustment, but elasticity decreases by 20% seasonally; wide xylem conduits effect rapid recovery, (but increases risk of cavitation) • Laurus: relatively low conductances, but high RWC: whilst leaves retain water, small losses of water cause largest diurnal change in leaf  which recover rapidly; lowest cellular elasticity decreases seasonally; large xylem diameter aids replenishment, but risks cavitation • Ecologically, Olea would extend to drier habitats that Ceratonia, the latter requiring higher soil water contents; Laurus is normally found in more humid habitats. So all are sclerophylls, but some more so than others…. • Ceratonia siliquus- “water spender” • Olea oleaster- “drought tolerating”; • Laurus nobilis- “water saver”

17. 3.4 Gas exchange components Measure photosynthetic CO2 assimilation (A) as a function of external CO2 supply and plot as a function of internal, intracellular CO2 (which may be expressed as Ci or Pi) calculated from stomatal conductance

19. (1984) Planta 162, 193-203

21. Diurnal Gas exchange patterns in Cork oak in September: the mid-day depression of photosynthesis; Stomata close when “cost” of water exceeds “benefit” of carbon; So control of leaf water status over-rides requirement for CO2; Leaf water potential () actually recovers at mid-day! Leaves susceptable to photoinhibition at midday- NPQ will be high relative to ETR G, stomatal conductance DW, LAVPD Tr, Transpiration , Water potential NP, CO2 assimilation

22. What happens in the field: the mid-day depression of photosynthesis 8.6.2 How carboxylation and oxygenation compensate for CO2 limitation • Conduct serial, abbreviated A/ci analysis throughout the morning and afternoon • A progressive decline in carboxylation efficiency (CE: Rubisco activity) and increase in compensation point (G) a.m; recovery p.m. • Stomatal and mesophyll conductances alter in tandem to protect leaf water status, and any loss in carbon fixation is compensated by oxygenase activity, seen in the increasing CO2 compensation point.