Drought recovery gas-exchange of Larrea Tridentata

Principal questions include:
How fast do key physiological functions (CO2 assimilation, stomatal
conductance, leaf water status, leaf area , PS II function)
recover upon soil rewetting after drought, and in what order?

Details include:
Does stomatal conductance follow the Ball-Berry model,
gs = m A hs/Cs + b, with A = CO2 assimilation rate, hs, Cs =
relative humidity and CO2 mixing ratio at the leaf surface, and m, b=
empirical constants (but almost universally near 10 and 0 for well-
watered plants of diverse species and biomes)

Do the stomatal parameters vary systematically with leaf and soil
water status?

Does recovery depend systematically upon neighbor biomass, net
soil water input per area (dependent upon topography of water
distribution), and plant size?

Conversely, how do the functions change on entry into drought?

Initial analyses of these and allied data indicate:
Leaf water potential has anomalous diurnal trend in drought, peaking
in mid-morning. Also, psi(leaf) only recovers to about -2 MPa even
with abundant soil water.

Leaves lose apparent photosynthetic capacity (Vcmax) daily in drought;
much of this is loss of PS II function as measured by chlorophll;
fluorescence...but PS II function is restored nearly completely every
day. Recovery may be closely tied to high dark-respiration rates
seen even in intense drought. We hypothesize that continuous repair
enables the shrubs to capitalize upon transient water supplies after
erratic and small summer rainfalls.

Stomatal conductance, gs, drops with drought and diurnally within drought
episodes. However, leaf-internal CO2 concentration, Ci, is nearly
unchanged and does not decrease to improve water-use efficiency.

The signal for diurnal decreases in gs is not any simple measure of
water status, particularly not bulk leaf water potential or soil water
potential, unlike signals demonstrated in other plants to date.

Mild recovery from drought (modest rainfall - ca. 10 mm total) induces
recovery of photosynthetic capacity but no new leaf area in some
individual plants, new leaf growth in others. in strong recoveries,
both photosynthetic capacity and leaf area recover in all plants.