J. Sci. Fd Agric. 1978,29, 832-834

Agriculture Group Symposium Soil /Plant / Water Relations The following are summaries of papers presented at the joint meeting of the Agriculture Group and the Environmental Physiology Group of the Society of Experimental Biology at 14 Belgrave Square, London, on 18 April 1978.

Bound Water and the Adaptation of Tomato Plants to Water Stress

Basil Acock

Glasshouse Crops .Research Institute, Worthing Road, Littlehampton, West Sussex BN16 3PU

A reduction in the water potential of the rooting medium produces a reduction in the water poten- tial of plants growing in that medium. Loss of water is partly responsible for the adjustment but workers in the 1960s concluded, from measurements of solute concentration in sap, that plants adjusted to water stress primarily by accumulating solutes, especially salts. However, several studies in this decade, mainly using labelled salts, have shown that ion uptake is inhibited when plants are stressed.

One possible explanation of these conflicting results is that plants concentrate their sap by remov- ing some of the water from the pool available for dissolving solutes. Certainly an examination of the osmotic potentials (water potentials of frozen, and thawed tissue) of samples of plant tissue at various water contents suggests that in most plants 5 % or more of the water in the tissue is unavailable for dissolving solutes. The proportion of water that is thus ‘bound’ in tomato plants can increase markedly when they are stressed. When allowance is made for this ‘bound‘ water, the tomato plants exhibit only modest gains and losses of solutes.

Water Movement through Plant Roots

J. J. Landsberg

Long Ashton Research Station, Long Ashton, Bristol BSI8 9AF

Prediction of the water status of plants, and hence the effects of water deficits on physiological behaviour, growth and final yield, requires knowledge of the resistances in the pathway of water movement through plants. It appears that root resistance, which depends on the geometry and hy- draulic properties of the root system, is the major factor limiting water uptake from the soil by plants.

Two models of root resistance are considered. The first is analytical and leads to the conclusion that lateral spacing and the resistance of individual laterals are the dominant factors determining the flux per unit area into a root. The model predicts that when the surface layers of the soil are drier than the lower layers, root resistance tends to increase with flow rate. The reverse occurs when the surface is wetter than the lower layers.

The second model is a direct electrical analogue and is more useful for analysis of experimental results when water uptake from different depths can be estimated from measurements. I t has been used to calculate radial and total resistances of wheat root systems. Radial resistances were inversely related to flux into the roots and total root system resistance was inversely related to flow through the system. Values of the resistance to water movement through the roots in different layers were calculated from the set of equations describing uptake from each layer in terms of flow rates, poten- tial gradients and resistances; these values, inserted in a solution for #o, the water potential at the

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832 Agriculture Group Symposium

stern base, yielded total root resistances ( R T ) and estimates of the effective soil moisture potential for the whole profile.

Reductions in root resistance with flow rate and changes in root system resistance with time were discussed.

Simulation of Water Distribution in a Cropped Soil

H. R. Rowse

National Vegetable Research Station, Wellesbourne, Warwick CV35 9EF

Cultivation of the subsoil can improve crop yields by increasing rooting depth and water extraction but benefits vary between years and sites. A simulation model of crop water extraction is being used to help interpret and extend the results from deep cultivation experiments.

For simulation the soil is divided into horizontal layers and time into intervals of 1 h. Changes in the water content of each layer are calculated at each time interval from the rates of water flow be- tween layers (using Darcy’s Law) and the rate of uptake by the roots. Root uptake is found from the difference between soil and xylem potentials divided by the sum of the soil resistance (calculated from Gardnerl) and the root resistance (calculated from the measured root distributions and single root resistances). Plant potential is assumed to be constant throughout the root xylem and is initially estimated as that potential which gives the total extraction from all layers equal to the potential transpiration. Should this be below hp bars, the value at which stomata1 closure occurs, then the potential is assumed to be hp bars.

Simulated patterns of water extraction for a field-grown crop of broad beans were similar to those measured. Additional simulations were carried out with weather data from other years and with root systems that were either deeper than, similar to, or shallower than those measured in the field.

In all but very wet years, increasing root depth relative to that measured in the field increased the water extraction by the crop and would therefore probably increase dry matter production,z although the increases may be small in very dry years. A subsequent deep cultivation experiment in a year of moderate water stress on a site adjacent to the original experiment showed yield increases associated with deeper rooting.

References 1 . 2.

Gardner, W. R. Dynamic aspects of water availability to plants. Soil Sci. 1960, 89, 63. De Wit, C . T. Transpiration and crop yields. Versl. Landbouwk Onderz. 1958, No. 64.4.

Measurement of Soil Water Changes and Properties in a Cropped, Freely Draining Soil

M. E. Parkes and Patricia Waters

Department of Agricultural Engineering, University of Newcastle upon Tyne, and Field Drainage Experimental Unit, Cambridge

Information was obtained on the spatial variation of soil moisture changes at three sites and six depths within an area of 20 m2. Changes were measured to a depth of 1 m for five time intervals over a period from March to August. The largest variations were associated with surface layers and with large moisture changes.

One hundred and eighty undisturbed soil cores were taken to characterise three monolith lysi- meters, removed from the same site. These cores were used to determine soil moisture characteristics at intervals of 0.2 m from 0.2 to 1.2 m depth. The results compared favourably with those from in situ estimates based on paired neutron probe and tensiometric measurements for the lysimeters.

Soil/plant/water relationships 833

Measurements of unsaturated hydraulic conductivity were made on each lysimeter using the ‘instantaneous profile’ technique. Soil moisture characteristic data was used to predict the unsatur- ated hydraulic conductivity for the five or six layers representing soil contained in each lysimeter. Matching factors were used corresponding to zero soil moisture tension. The two determinations were compared in relation to the errors associated with the field determination of unsaturated hy- draulic conductivity. Poor agreement was obtained for depths below 0.2 to 0.4 m where there was appreciable air entrapment.

The Effects of Severe Drought on Spring Barley

B. J. Legg and W. Day

Rothamsted Experimental Station, Physics Department, Harpenden, Herts AL.5 2JQ

A drought experiment was done on spring barley in 1976. The crop was protected from rain by auto- mated shelters that covered the crop while rain was falling and were moved away when it ceased. There were 12 irrigation treatments, ranging from no water applied after emergence to weekly irriga- tion returning the soil to near field capacity. The yields of total dry matter, including roots, at har- vest were 8.4 and 16.9 t ha-1 for the extreme treatments.

The results were analysed to indicate how much of the decrease in total dry matter could be attri- buted to different physical factors. The factors considered were: decrease in foliage area and dura- tion; change of leaf angle altering the amount of radiation intercepted by unit leaf area; stomatal closure decreasing the rate of photosynthesis; and change of internal resistance to COz transfer affecting the rate of photosynthesis especially at light saturation. It was found that changes in leaf angle and internal resistance had very little effect; increased stomatal resistance of leaves had a small effect; but the decreased foliage area of the stressed plants was the major factor causing a decrease in yield.

Influence of Agronomic Practices on Water Use by Apple Trees

D. Atkinson and J. M. Farre

East Malling Research Station, Maidstone, Ken! ME19 685

Much of the apple acreage in western Europe is now comprised of continuous hedgerow orchards where the tree is grown with a row area kept free of vegetation by the use of herbicides and a grassed interrow area. Light interception by this type of orchard is only 15-20 % of that available in the first cropping year building up to 50-60% at maturity 3 to 5 years later. This results in substantial light interception by the grassed interrow which competes with the tree for available water.

At normal tree densities (< lo00 trees ha-1) soil water deficits are higher under grassed interrow than under herbicided row areas. Differences which are highest in the surface 50 cm are reduced in very high density plantings. With overall herbicide management, deficits in both row and interrow areas are reduced and become more uniform over the soil area.

Where a tree is grown in a herbicided row, in contrast to either overall herbicide or grass, it is presented with two dissimilar soil environments one with and the other without interspecific root competition. Studies of the distribution of roots between these two zones has shown that in trees under 10 years old all or most apple root growth and nutrient uptake occurs in the herbicided row. Use of the grassed area is increased in high density plantings. Under overall herbicide treatment root growth and activity is distributed over the entire area.

Without irrigation and under medium to high evaporative demand stomatal resistance is higher in young and mature trees with grass competition. Grass competition reduces average crop and most of its components and increases variability in cropping between years.

The amount and pattern of water use in orchards is related to root distribution and modified by variation in tree density and by the presence of grass in part or all of the orchard.

834 Agriculture Group Symposium

Water Use and Root Distribution of Grass Species

E. A. Garwood and J. Sinclair

Grassland Research Institute, Hurley, Maidenhead, Berks SL6 5LR

The amount and distribution of rainfall during the growing season is a major source of variation in the yield of grass. At Hurley the average loss in production is some 20% of the potential yield.

In an experiment to examine the growth and water use of five grasses (perennial ryegrass, cocksfoot, timothy, tall fescue and Poa trivialis) frequent measurements of the soil water profiles were made. A subsidiary experiment showed negligible upward movement of water to the plant roots, but slow drainage occurred over several weeks.

During periods of rapid drying water was removed from successively deeper zones in the soil until the rate of removal was similar over a range of depths, implying that longitudinal resistance in the roots was small compared with other resistances in the system.

There were large differences between the species in the pattern of water removal with depth, root distribution and DM % yield. Tall fescue removed considerable water to at least 100 cm, perennial ryegrass and cocksfoot to 80 cm, timothy to 60-70 cm and P. trivialis little below 40 cm depth. These differences were in accord with the distribution of roots at depth. For example, at 60 cm there were 5 cm root cm-3 soil under tall fescue and only 1.9 cm (3111-3 under timothy. During a period of drought (maximum potential soil water deficit of 270 mm) DM yields ranged from 3.3 t ha-l (tall fescue) to 1.1 t ha-1 (timothy), reflecting differences in the amount of soil water extracted by the species. P. trivialis produced only 0.1 t DM ha-l and did not survive the drought.

Observations made under even more severe conditions showed large variation in efficiency of use of soil water between cultivars of tall fescue and perennial ryegrass. It is concluded that selection of grasses for depth of rooting and efficiency of water uptake may prove profitable.

The Effects of Subsoiling and Incorporating P and K in the Subsoil on Crop Yield

J. McEwen

Rothamsted Experimental Station, Harpenden, Herts AL5 2JQ

In the summer of 1973 a small-plot experiment was started on the sandy loam at Woburn to test the effects of subsoiling by hand and of incorporating a large dressing of P and K (1930 kg ha-1 P205+ 460 kg ha-l KzO) into the subsoil. The rates were calculated to make the subsoil as rich in available P and K as the topsoil. Two additional treatments were included-the same amount of P and K worked into the topsoil and no soil disturbance or P and K.

The treatments have not been re-applied since and their effects have been assessed on a rotation of potatoes, wheat, sugar beet and barley. All crops were present each year and received regular dress- ings of N, P, K, Mg and lime in amounts used in normal farm practice on this land.

Subsoiling alone increased the 4-year mean yields of wheat by 21 x, of barley by 24% and of sugar from sugar beet by 11 %. Potatoes were unaffected.

Incorporating P and K into the subsoil increased the yields of potatoes by 16% and further in- creased the yields, in addition to the effect of subsoiling, of barley by 2076 and of sugar by 4%. Yields of wheat were not further affected.