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• The timing of diel stem growth of mature forest trees is still largely unknown, as empirical data with high temporal resolution have not been available so far. Consequently, the effects of day–night conditions on tree growth remained uncertain. • Here we present the first comprehensive field study of hourly-resolved radial stem growth of seven temperate tree species, based on 57 million underlying data points over a period of up to 8 yr. • We show that trees grow mainly at night, with a peak after midnight, when the vapour pressure deficit (VPD) is among the lowest. A high VPD strictly limits radial stem growth and allows little growth during daylight hours, except in the early morning. Surprisingly, trees also grow in moderately dry soil when the VPD is low. Species-specific differences in diel growth dynamics show that species able to grow earlier during the night are associated with the highest number of hours with growth per year and the largest annual growth increment. • We conclude that species with the ability to overcome daily water deficits faster have greater growth potential. Furthermore, we conclude that growth is more sensitive than carbon uptake to dry air, as growth stops before stomata are known to close.

Water transport is an integral part of the process of growth by cell expansion and accounts for most of the increase in cell volume characterizing growth. Under water deficiency, growth is readily inhibited and growth of roots is favoured over that of leaves. The mechanisms underlying this differential response are examined in terms of Lockhart's equations and water transport. For roots, when water potential (Ψ) is suddenly reduced, osmotic adjustment occurs rapidly to allow partial turgor recovery and re‐establishment of Ψ gradient for water uptake, and the loosening ability of the cell wall increases as indicated by a rapid decline in yield‐threshold turgor. These adjustments permit roots to resume growth under low Ψ. In contrast, in leaves under reductions in Ψ of similar magnitude, osmotic adjustment occurs slowly and wall loosening ability either does not increase substantially or actually decreases, leading to marked growth inhibition. The growth region of both roots and leaves are hydraulically isolated from the vascular system. This isolation protects the root from low Ψ in the mature xylem and facilitates the continued growth into new moist soil volume. Simulations with a leaky cable model that includes a sink term for growth water uptake show that growth zone Ψ is barely affected by soil water removal through transpiration. On the other hand, hydraulic isolation dictates that Ψ of the leaf growth region would be low and subjected to further reduction by high evaporative demand. Thus, a combination of transport and changes in growth parameters is proposed as the mechanism co‐ordinating the growth of the two organs under conditions of soil moisture depletion. The model simulation also showed that roots behave as reversibly leaky cable in water uptake. Some field data on root water extraction and vertical profiles of Ψ in shoots are viewed as manifestations of these basic phenomena. Also discussed is the trade‐off between high xylem conductance and strong osmotic adjustment.

The guillotine thermocouple psychrometer allows auxin action on cell enlargement to be investigated in intact plants. Because the technique measures all the physical parameters affecting enlargement in the same plants, close comparisons can be made of the changes brought about by this growth regulator. In etiolated seedlings of soybean (Glycine max L. Merr.). auxin was supplied endogenously by the intact plant or was depleted by removing the apical portion of the stem. We observed that, when stem growth was rapid in the intact plant, the water potential of the growing region was lower than in the nongrowing region but, as growth slowed during auxin depletion, the water potential rose until it became essentially the same as in the nongrowing region. This indicated that gradients in water potential had been induced by the demand for water during rapid growth but had decreased as growth decreased in the auxin-depleted cells. The turgor appeared to rise slightly as growth slowed which is in the wrong direction to account for the growth change unless compensating changes occurred in wall properties and/or synthesis. As growth ceased in the auxin-depleted tissue, the threshold turgor rose until it became nearly the same as the cell turgor, which indicates that auxin affected this wall parameter. The osmotic potential increased slightly, probably because of a dilution of the cell contents by the residual growth occurring after the stein apex (and cotyledons) had been removed. The hydraulic conductance for water was unaffected by auxin status whether it was measured in the whole enlarging region or in individual cortical cells from the region. It was concluded that auxin acts mainly on the metabolism of the cell walls manifested by the change in growth rate and threshold turgor. The other changes were passive responses to the changed growth rate.

Yield stress threshold (Y) and volumetric extensibility (θ) are the rheological properties that appear to control root growth. In this study they were measured in wheat roots by means of parallel measurement of the growth rate (r) of intact wheat roots and of the turgor pressures (P) of individual cells within the expansion zone. Growth and turgor pressure were manipulated by immersion in graded osmoticum (mannitol) solutions. Turgor was measured with a pressure probe and growth rate by visual observation. The influence of various growth conditions on Y and θ was investigated; (a) At 27 °C.In 0.5 mol m−3 CaCl2 r, P, Y and θ were 20.7±4.6 μm min−1, 0.77±0.05 MPa, 0.07±0.03 MPa and 26±1.9 μm min−1 MPa−1 (expressed as increase in length), respectively. Following 24 h growth in 10 mol m−3 KC1 these parameters became 12.3±3.5 μm min−1, 0.72±0.04 MPa, 0.13±0.01 MPa and 21±0.7 μm min−1 MPa−1. After 24 h osmotic adjustment in 150 mol m−3 mannitol/0.5 mol m−3 CaCl2 r= 19.6±4.2 μm min−1, P = 0.68±0.05 MPa and Y and θ were 0.07±0.04 MPa and 30±0.2 μm min−1 MPa−01, respectively. After 24 h growth in 350 mol m−3 mannitol/0.5 mol m−3 CaCl2 r= 13.3±4.1 μm min−1, P= 0.58±0.07 MPa, Y=0.12±0.01 MPa and ø 32±0.2 tim min−1 MPa−1. During osmotic adjustment in 200 mol m−3 mannitol/0.5 mol m−3 CaCl2, with or without KCl, the recovery of growth rate corresponded to turgor pressure recovery (t1/2 approximately 3 h). (b) At 15 °C. Lowered temperature dramatically influenced the growth parameters which became r= 8.3±2.8 um min−1, P=0.78 MPa, r=<0.2 MPa and θ=15±0.1 μm min−1 MPa−1. Therefore, Y and θ are influenced by 10 mol m−3 K+ ions and low temperature. In each case the effective pressure for growth (P-Y) was large indicating that small fluctuations of soil water potential will not stop root elongation.

Yield stress threshold (Y) and volumetric extensibility (phi) are the rheological properties that appear to control root growth. In this study they were measured in wheat roots by means of parallel measurement of the growth rate (r) of intact wheat roots and of the turgor pressures (P) of individual cells within the expansion zone. Growth and turgor pressure were manipulated by immersion in graded osmoticum (mannitol) solutions. Turgor was measured with a pressure probe and growth rate by visual observation. The influence of various growth conditions on Y and phi was investigated; (a) At 27 degrees C. In 0.5 mol m-3 CaCl2 r, P, Y and phi were 20.7 +/- micrometer min-1, 0.77 +/- 0.05 MPa, 0.07 +/- 0.03 MPa and 26 +/- 1.9 micrometer min-1 MPa-1 (expressed as increase in length), respectively. Following 24 h growth in 10 mol m-3 KCl these parameters became 12.3 +/- 3.5 micrometer min-1, 0.72 +/- 0.04 MPa, 0.13 +/- 0.01 MPa and 21 +/- 0.7 micrometer min-1 MPa-1. After 24 h osmotic adjustment in 150 mol m-3 mannitol/0.5 mol m-3 CaCl2 r = 19.6 +/- 4.2 micrometer min-1, P = 0.68 +/- 0.05 MPa and Y and phi were 0.07 +/- 0.04 MPa and 30 +/- 0.2 micrometer min-1 MPa-1, respectively. After 24 h growth in 350 mol m-3 mannitol/0.5 mol m-3 CaCl2 r = 13.3 +/- 4.1 micrometer min-1, P = 0.58 +/- 0.07 MPa. Y = 0.12 +/- 0.01 MPa and phi 32 +/- 0.2 micrometer min-1 MPa-1. During osmotic adjustment in 200 mol m-3 mannitol/0.5 mol m-3 CaCl2, with or without KCl, the recovery of growth rate corresponded to turgor pressure recovery (t1/2 approximately 3 h). (b) At 15 degrees C. Lowered temperature dramatically influenced the growth parameters which became r = 8.3 +/- 2.8 micrometer min-1, P = 0.78 MPa. Y = < 0.2 MPa and phi = 15 +/- 0.1 micrometer min-1 MPa-1. Therefore, Y and phi are influenced by 10 mol m-3 K+ ions and low temperature. In each case the effective pressure for growth (P-Y) was large indicating that small fluctuations of soil water potential will not stop root elongation.

The yielding properties of the cell wall, irreversible wall extensibility (m) and yield threshold (Y), are determined for stage I sporangiophores of Phycomyces blakesleeanus from in-vivo creep experiments, and compared to the values of m and Y previously determined for stage IVb sporangiophores using the same pressure-probe method (Ortega et al., 1989, Biophys. J. 56, 465). In either stage the sporangiophore enlarges (grows) predominately in length, in a specific region termed the \"growing zone\", but the growth rates of stage I (5—20 μm·min-1) are smaller than those of stage IVb (30—70 μm·min-1). The results demonstrate that this difference in growth rate is the consequence of a smaller magnitude of m for stage I sporangiophores; the obtained values of P (turgor pressure), Y, and P—Y (effective turgor for irreversible wall extension) for stage I sporangiophores are slightly larger than those of stage IVb sporangiophores. Also, it is shown that the magnitude of m for the stage I sporangiophore is regulated by altering the length of the growing zone, Lg. A relationship between m and Lg is obtained which can account for the difference between values of m determined for stage I and stage IVb sporangiophores. Finally, it is shown that similar changes in the magnitude of m and φ (which have been used interchangeably in the literature as a measure of irreversible wall extensibility) may not always represent the same changes in the cell-wall properties.

The rate of cell enlargement depends on cell-wall extensibility (m) and on the amount of turgor pressure (P) which exceeds the wall yield threshold (Y). The difference (P—Y) is the growth-effective turgor (Pe). Values of P, Y and Pe have been measured in growing bean (Phaseolus vulgaris L.) leaves with an isopiestic psychrometer, using the stress-relaxation method to derive Y. When rapid leaf growth is initiated by light, P, Y and Pe all decrease. Thereafter, while the growth rate declines in maturing leaves, Y continues to decrease and Pe actually increases. These data confirm earlier results indicating that the changes in light-stimulated leaf growth rate are primarily controlled by changes in m, and not by changes in Pe. Seedlings incubated at 100% relative humidity have increased P, but this treatment does not increase growth rate. In some cases Y changes in parallel with P, so that Pe remains unchanged. These data point out the importance of determining Pe, rather than just P, when relating cell turgor to the growth rate.

Plants of Phaseolus vulgaris L. were grown at 22.5⚬C and transferred to nutrient solution held at that temperature or at 10⚬C; other plants were subjected to a pruning treatment that removed all lateral roots and part of the main root. Both root treatments resulted in a substantial reduction in growth of the primary leaves but did not affect leaf water potential or its components, π and P, except briefly for the root-cooled plants. Treatments did not increase the wall yield threshold, Y, of leaf tissue which was always lower than in control plants; consequently the effective turgor for growth, P - Y, was greater in treated plants. Both treatments reduced wall extensibility, measured by a modified Instron technique and from the relationship between growth of leaf pieces and their turgor, and it is suggested that this is responsible for the reduction in leaf growth.

Theory predicts that, for growing plant cells isolated from a supply of water, stress relaxation of the cell wall should decrease cell turgor pressure (P) until the yield threshold for cell expansion is reached. This prediction was tested by direct P measurements of pea (Pisum sativum L.) stem cortical cells before and after excision of the growing region and isolation of the growing tissue from an external water supply. Cell P was measured with the micro-pressure probe under conditions which eliminated transpiration. Psychrometric measurements of water potential confirmed the pressure-probe measurements. Following excision, P of the growing cells decreased in 1 h by an average of 1.8 bar to a mean plateau value of 2.8 bar, and remained constant thereafter. Treatment with 10-5 M indole-3-acetic acid or 10-5 M fusicoccin (known growth stimulants) accelerated the rate of P relaxation, whereas various treatments which inhibit growth slowed down or completely stopped P relaxation in apical segments. In contrast, P of basal (nongrowing) segments gradually increased because of absorption of solutes from the cell-wall free space of the tissue. Such solute absorption also occurred in apical segments, but wall relaxation held P at the yield threshold in those segments which were isolated from an external water supply. These results provide a new and rapid method for measuring the yield threshold and they show that P in intact growing pea stems exceeds the yield threshold by about 2 bar. Wall relaxation is shown here to affect the water potential and turgor pressure of excised growing segments. In addition, solute release and absorption upon excision may influence the water potential and turgor pressure of nongrowing excised plant tissues.