Incomplete stomatal closure during the night is observed in a diverse range of C3 and C4 species ([Fig. 1][1] ; Supplemental Table S1) and can lead to substantial nighttime transpirational water loss. Although water loss is an inevitable consequence of stomatal opening for photosynthetic carbon gain

Nighttime Stomatal Conductance and Transpiration in C3 and C4 Plants

Plants raised from shoot-tip cultures of apple (Malus x domestica Borkh.) were grown in divided pots so that approximately half the root system could be exposed to soil drying whilst the remainder was well watered. Water was withheld from half the root system in this way for 24 d. During this time, the daily increment in leaf area declined to 65% of that of control plants, which had all their roots in well watered soil. Both leaf expansion and, more particularly, leaf initiation were inhibited. Water loss per plant also fell to 70% of the rate exhibited by control plants. No significant differences were detected between the leaf water status of the partially dried plants and that of the well watered ones. After this drying period, the half-dried plants were treated during the night of the 24th day as follows; one group had its roots in dry soil rewatered, another had them excised, and a third continued half dry. Both the rewatered group and the group with roots excised showed significant recoveries in leaf growth rates compared with the group kept half dry. The inhibition of leaf growth by drying soil is discussed with regard to root-to-shoot signalling. The alleviation of this inhibition by the excision of roots in dry soil, is taken as evidence of a positive inhibitor, produced by drying roots, influencing shoot growth.

A Positive Root-sourced Signal as an Indicator of Soil Drying in Apple, Malus x domestica Borkh.

Mature leaves of Phaseolus vulgaris L (red kidney bean), Xanthium strumarium L (cocklebur), and Gossypium hirsutum L (cotton) were used to study accumulation of abscisic acid (ABA) during water stress The water status of individual, detached leaves was monitored while the leaves slowly wilted, and samples were cut from the leaves as they lost water The leaf sections were incubated at their respecitive water contents to allow ABA to build up or not At least 8 h were required for a new steady-state level of ABA to be established The samples from any one leaf covered a range of known water potentials (ψ), osmotic pressures (π), and turgor pressures (p) The π and p values were calculated from “pressure-volume curves”, using a pressure bomb to measure the water potentials Decreasing water potential had little effect on ABA levels in leaves at high turgor Sensitivity of the production of ABA to changes in ψ progressively increased as turgor approached zero At p=1 bar, ABA content averaged 4 times the level found in fully turgid samples Below p=1 bar, ABA content increased sharply to as much as 40 times the level found in unstressed samples ABA levels rose steeply at different water potentials for different leaves, according to the ψ at which turgor became zero These differences were caused by the different osmotic pressures of the leaves that were used; ψ must cqual -π for turgor to be zero Leaves vary in π, not only among species, but also between plants of one and the same species depending on the growing conditions A difference of 6 bars (calculated at ψ=0) was found between the osmotic pressures of leaves from two groups of G hirsutum plants; one group had previously experienced periodic water stress, and the other group had never been stressed When individual leaves were subsequently wilted, the leaves from stress-conditioned plants required a lower water potential in order to accumulate ABA than did leaves from previously unstressed plants On the basis of these results we suggest that turgor is the critical parameter of plant water relations which controls ABA production in water-stressed leaves

Correlation between loss of turgor and accumulation of abscisic acid in detached leaves.

Predawn leaf water potential (Ψ w) and xylem pressure potential (Ψ p ) are expected to be in equilibrium with the soil water potential (soil Ψ w ) around roots of well-watered plants. We surveyed 21 plant species (de- sert, chaparral, and coastal salt marsh species, as well as two temperate tree and two crop species) for departures from this expectation and for two potential mechanisms explaining the departures. We measured soil Ψ w , leaf Ψ w , and xylem Ψ p in the glasshouse under well-watered conditions that eliminated soil moisture heterogeneity and ensured good soil-root hydraulic continuity. Most species failed to equilibrate fully with soil Ψ w , depend- ing on whether leaf Ψ w or xylem Ψ p was used as the measure of predawn plant water potential. The contribu- tion of nighttime transpiration to predawn disequilibrium was assessed by comparing plants with bagged canopies (enclosed overnight in plastic bags to eliminate transpi- ration) to plants with unbagged canopies. Nighttime tran- spiration significantly reduced predawn xylem Ψ p for 16 of 21 species and the magnitude of this contribution to predawn disequilibrium was large (0.50-0.87 MPa) in four woody species: Atriplex confertifolia, Batis maritima, Larrea tridentata, and Sarcobatus vermicula- tus. The contribution of nighttime transpiration to pre- dawn disequilibrium was not more prevalent in mesic compared with xeric or desert phreatophytic compared with non-phreatophytic species. Even with bagging that eliminated nighttime transpiration, plants did not neces- sarily equilibrate with soil Ψ w . Plant xylem Ψ p or leaf Ψ w were significantly more negative than soil Ψ w for 15 of 15 species where soil Ψ w was measured. Predawn dis- equilibrium based on leaf Ψ w was of large magnitude (0.50-2.34 MPa) for seven of those 15 species, predomi- nately halophytes and Larrea tridentata. A portion of the discrepancy between leaf and soil Ψ w is consistent with the putative mechanism of high concentrations of leaf apoplastic solutes as previously modeled for a halophyte, but an additional portion remains unexplained. Predawn leaf Ψ w and xylem Ψ p may not reflect soil Ψ w , particu- larly for woody plants and halophytes, even under well- watered conditions.

Predawn plant water potential does not necessarily equilibrate with soil water potential under well-watered conditions

The regulation of cell-division activity in the vascular cambium and of secondary xylem and phloem development is reviewed for temperate-zone tree species in relation to auxins, gibberellins, abscisic acid, cytokinins, and ethylene. Representatives of the first four of these PGR classes (IAA, GA1, GA4, GA7, GA9, GA20, ABA, Z, ZR, DCA) have been identified conclusively by mass spectrometry in the cambial region in some Pinaceae, but not in any hardwood species. Endogenous ethylene has yet to be definitively characterized in this region in any species. Evidence concerning the source and metabolism of cambial PGRs is scanty and inconclusive for both conifers and hardwoods.

The role of plant growth regulators in forest tree cambial growth

During progressive stages of drought and subsequent recovery through rewatering, foliage of 2-year-old seedlings of Douglas-fir was sampled for water potential, stomatal resistance, and abscisic acid content (ABA). Simultaneous measurements of soil matric potential were also made. There was a rapid rise in stomatal resistance in the range of 1500–2000 ng ABA g−1 dry weight of foliage corresponding with a plant water potential of –10 to –12 bars. Soil matric potential seemed more closely associated with ABA increase than did plant water potential. After rewatering, seedlings returned to normal ABA levels within 6 days.

The Association between Soil and Xylem Water Potential, Leaf Resistance, and Abscisic Acid Content in Droughted Seedlings of Douglas‐fir (Pseudotsuga menziesii)

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The Association between Soil and Xylem Water Potential, Leaf Resistance, and Abscisic Acid Content in Droughted Seedlings of Douglas‐fir (Pseudotsuga menziesii)