Orientation of the lateral roots of trees: II. Hydrotropic and gravitropic responses of lateral roots of Sitka spruce grown in air at different humidities.

Sitka spruce [Picea sitchensis (Bong.) Carr.] plants were grown under controlled conditions in specially designed boxes in which the plagiogravitropic lateral roots grew out from moist peat into an air space at one side. In one treatment the air was saturated with water vapour. In the other a linear horizontal gradient of water vapour pressure was maintained between the vertical peat surface and a porous membrane made from black cloth. Near the peat surface the vapour pressure gradient was equivalent to a soil water-potential gradient of about 0.48 MPa mm-1 . The angles of the roots before and after emergence from the peat were measured in different planes. In the saturated air treatment there was no significant change of angle after emergence but roots which emerged into the air with the humidity gradient grew more slowly and showed a marked change of direction. This deflection had two components; there was some tendency for the roots to be attracted to the moist peat surface (hydrotropism), and there was also a downward component (gravitropism). Mean deviation from the angle of emergence which would have resulted from a hydrotropic response alone was 58°, while deviation from the true gravitropic position was only 31°. Therefore under the conditions of the experiment the gravitropic response was considerably stronger than the hydrotropic one. The implications of these results are discussed for the growth of roots in soils, where water potential gradients generally do not seem to be sufficient to cause hydroptropic curvature. However, there was an enhanced gravitropic response of the tips of lateral roots subject to water stress and this may help control the orientation of roots near the soil surface.

Orientation of the lateral roots of trees: I. Upward growth of surface roots and deflection near the soil surface.

The main lateral roots of some tree species grow close beneath the soil surface, even on irregular or sloping ground. Experiments were carried out to explore the origin and behavior of such surface roots. Pinus contorta Douglas ex Loudon and Picea sitchensis (Bong.) Carr. plants were grown under growth room and glasshouse conditions. Lateral roots were excavated and vertical and horizontal co-ordinates measured. The liminal angles (0°= vertically downwards) of first-order lateral roots of seedlings of both species decreased linearly with depth of origin on the taproot, Some of the roots of highest origin had liminal angles > 90° (ie they grew upwards). However, these roots deflected downwards on approaching the soil surface. If additional soil was added soon after germination the upper lateral roots grew further in an upward direction and then deflected beneath the elevated surface. Two-year-old Picea sitckensis transplants were planted centrally in large containers of soil and an elevated ridge of soil was made 160 mm to one side of the plants. Vigorous upwardly growing roots developed from the damaged and bent root systems that had been produced in the forest nursery. The upwardly oriented roots deflected horizontally when their tips arrived below the flat peat surface, but when they extended beneath the ridge they turned upwards within it. As they approached the top of the ridge they again deflected downwards. It is concluded that the direction of growth of surface roots is the result of two opposing influences. Certain of the main lateral roots are plagiogravitropic and grow obliquely upwards but as these roots approach the soil surface they respond to some signal from the environment that causes downward deflection. The nature of the signal was not determined but it was shown that downward deflection occurred whether or not the soil was exposed to light. The biological importance of this form of surface root orientation is discussed. The growth behavior described enables the main lateral roots to explore the nutrient-rich surface layers of the soil while avoiding the injury to the root tip that would result from exposure.

Developmental changes in Sitka spruce as indices of physiological age: II. Rooting of cuttings and callusing of needle explants.

Adventitious rooting ability is considered a good indicator of juvenility in trees and has often been used to assess explants subjected to treatments to alter developmental stage. However, rooting assessments are not always practical and the callusing of needles in vitro was investigated as an alternative bioassay of physiological age. Shoots were sampled from the uppermost whorl of Sitka spruce trees aged 1, 3, 4, 6, 11, 16, 21, 35 and 39 years and from the lowest whorls of 3- and 4-year-old trees. Needles were sampled for measurement of length, width, length/width ratio, projected and total surface areas, dry weight and specific leaf area. For explants of uppermost whorl origin, there were decreases in the rooting ability of cuttings and the callusing response of needles with increasing age of tree. Associated with the loss in callusing ability was an increase in the production of a black exudate. The changes with age for rooiing, callusing and exudation could each be adequately described by the Gompertz growth function which showed their rates of change to be similar. The observed changes are closely associated with changes in needle area, weight and specific leaf area. Euclidean distances (using the mean values of selected characteristics for each age class) were computed as a measure of physiological age. The shoots and needles of basal shoots are more juvenile in character than those taken from the uppermost whorls of the same trees.

The dynamics of rooting in Triplochiton scleroxylon cuttings: their relation to leaf area, node position, dry weight accumulation, leaf water potential and carbohydrate composition.

Single-node, leafy stem cuttings of Triplochiton scleroxylon K. Schum. were collected from successive nodes down the uppermost shoot of 2-shoot stockplants. The leaves were trimmed to 10, 50 and 100 cm(2) before the cuttings were set under intermittent mist to root. Batches of cuttings were harvested after 0, 14, 28 and 42 days to assess leaf water potential, dry weight and carbohydrate content of their leaf and stem portions. Cuttings with leaf areas of 10, 50 and 100 cm(2) increased in total dry weight by 29, 61 and 90%, respectively, during the 6-week period. The increase in dry weight was accompanied by increases in reflux-extracted soluble carbohydrates (RSC), water-soluble carbohydrates (WSC) and starch. By contrast, increase in leaf area reduced leaf water potential of cuttings before root emergence. Fewer large-leaved cuttings rooted than smaller-leaved cuttings, suggesting that rooting ability is at least partially determined by the balance between photosynthesis and transpiration. Fewer roots per cutting were produced on cuttings with 10 cm(2) leaves than on cuttings with larger leaves. Node position affected increments in dry weight, carbohydrate content and leaf water potential, with differences between nodes on day 0 generally being lost or slightly reversed by day 14. Rooting ability was not related to initial (day 0) carbohydrate content, suggesting that rooting is dependent on carbohydrates formed after severance. During the rooting period, the proportions of total non-structural carbohydrate as WSC and starch were reversed, from mostly WSC on day 0 to mostly starch by day 42. These changes in WSC and starch occurred most rapidly in large-leaved cuttings.

Developmental changes in Sitka spruce as indices of physiological age I. Changes in needle morphology.

There is a paucity of data regarding developmental changes in trees. Measurements of length, width, length/width ratio, perimeter/width and width/height ratios of tranverse sections, projected and total surface areas, dry weight, specific leaf area and the weight of epicuticular waxes per unit leaf area were made on needles sampled from the uppermost whorl of Sitka spruce trees aged 3, 5,10,15, 20 and 38 years. All characteristics, except needle length and the weight of epicuticular waxes, showed asymptotic changes with age that could adequately be described by the Gompertz growth function, and offer potential as indices of physiological age. Asymptotes were reached at different ages (width and the length/width ratio having the slowest rates of change and the perimeter/width ratio, specific leaf area, and the width/height ratio having the fastest) suggesting a predictable sequence of changes. The observed changes in needle morphology are interpreted as a transition from shade to sun leaves.