The influence of soil water potential and soil temperature on the seedling emergence of wheat and barley

The time for 50 Vo emergence of wheat and barley increases linearly with decreasing matric potential. This increase actually begins at matric pressures above pF 2.7. The rise in temperature makes emergence faster within the range of minimum temperature (3.l°C for wheat and I.9°C for barley) and the temperature where growth begins to slow down (about 31°C for wheat and 27°C for barley). The optimum range for 50 Vo emergence was obtained at a matric pressure range of pF 1.3—2.7 or —5.0 0.20 m (water column) at a temperature of 10°C, which quite well corresponds to the situation in Finland during the emergence period in spring. A clear increase can be observed in the required heat sum for wheat and barley when the soil water potential reaches a critical point which was pF 2.8 or —6.3 m for wheat and pF 2.7 or —5.0 m for barley. The total emergence as a function of matric potential for wheat and barley was determined over a period of 30 days at 10°C. In the wet side, pF 1,0 can be considered a limit, the total emergence decreasing with lower values. In the dry side, a corresponding decrease can be noticed in total emergence at pF above 3.0. The water uptake by seeds speeded up with increasing temperature from 10to 25°C. Radicles of wheat and barley began to appear when the water uptake by the seed was approximately 50—60 % of the initial weight of the seed. The initial water uptake caused by the moistening of the pericarp due to capillarity was about 3 Vo for wheat and 5 Vo for barley at a soil water potential of pF 1.2. Index words: seedling emergence, soil water potential, soil temperature


Introduction
The two major factors affecting seedling emergence are the moisture and the tempera- ture of the seed bed.In Finland, temperature is the minimum factor for seedling emergence of cereals.Inversely, in arid and semi-arid regions the factor restricting emergence is generally the low moisture content of soil.
Because of the short growing season in Finland, early sowing in spring and fast emer- gence are very important aspects in obtaining a good yield.
The term soil matric potential is preferable to soil moisture when water uptake by plants is discussed, because in different soils the the water uptake by plants is dependent on the amount of energy needed to detach water for the use of plants.In different soils the plants have to exert a similar suctional power toward water despite the fact that the water content of clay for example is many times higher com- pared to that of sand.The present paper is dealing with the effect of soil matric potential and soil temperature on the seedling emergence of wheat and bar- ley.

Material and methods
The grain size of the sand used in the ex- periments as seed bed was 0.1 -0.6 mm.The water retention curve of the sand is presented in Figure 1.
Four methods were used to determine the points of the water retention curve for the soil used as seed bed.The matric potential values -l5O 10 m (water column) were ob- tained osmotically using a semi-permeable cel- lulose acetate membrane, pore size 5 nm, immersed in polyethylene glycol solution (PEG,     molecular weight about 20.000).Table 1 shows the correspondence of the PEG solu- tion and the matric potential.The points -1 m and -5 min the water retention curve were determined by a method developed in the course of the experiments.The sand was put into a small closed plastic vessel and moist- ened to saturation.Thereafter, pressure was applied to the sand to remove any extra water through the holes under the vessel.The points mentioned above were checked using a water retention curve determination device.The fourth method was used to determine the points -0.25 0 m in the water retention curve.The arrangement was based on the capillary rise of sand.The vessel containing sand was laid in another vessel containing water.This allowed the soil to take up water, and matric potential values of -0.25 0 m were determined according to the different heights of sand columns.This method did not allow to determine matric pressure values below -0.25 m, because the capillary rise of the sand was no higher.
The seedling emergence experiments were made with spring wheat (Tähti) and barley (Pokko).The investigations were carried out using three different methods.The first meth- od was used to regulate the moisture of the sand osmotically in the cellulose acetate membrane which allowed capillary rise of water in the funnel where the seeds were immersed into the depth of about 1.5 cm.Each funnel contained 20 seeds.By the second method the effect of low soil moisture on emergence was investigated using the first method described above for determination of the water retention curve.The matric poten- tialranged from -IOO to -lO m and 10 seeds were immersed into the depth of about 1.5 cm.
The third method which was the same as the one described in connection with the water retention curve determination by the fourth method was used to determine the effect of high soil moisture on emergence.Observations were recorded daily.Germination was defined as a seedling of 0.5 cm emerging from the soil surface.After the experiments the moisture of the seed bed was measured.
In addition to the above, water uptake by seeds was studied by immersing 5 seeds of wheat and barley in the soil at a depth of about 1.5 cm for different periods of time at pF 1.2.The moisture was maintained at the desired level by means of the fourth method as was done in the determination of the water retention curve.The seeds were thereafter dried and weighed to determine the amount of water imbibed.The seeds were reimmersed in the seed bed and the experiment continued.Also the number and length of radicles as well as the length of shoot were measured.
The experiments were made at the tempera- ture of 10, 15, 20 and 25°C.
The minimum temperatures for germination were determined at a soil water content of 10-30 % (vol.%) or -0.20 0.10 m.The reciprocal of time for 50 % emergence was calculated and a regression line was drawn (Feddes 1972).The minimum temperatures obtained were 3.l°C for wheat and I.9°C for barley.The corresponding heat sums (slopes of the lines) were 70 degree days for wheat and 73 degree days for barley.
Figure 3 shows the dependence of soil water potential on the 50 % emergence rate for wheat and barley at a temperature of 10°C which produced the most reliable results.Feddes (1971) observed slowing down in the emergence rate at pF values above 2.7 which can also be considered a limit to that of wheat and barley.In the wet side, however, Feddes reported of the limit for slowing down in the emergence rate of different vegetables to be about pF 2.0 or -1 m, when this was about pF 1, 3 or -0.20 m in the present study.Ac- cording to Figure 3, the optimum emergence rate is achieved at about pF 1.3-2.7 or -5.0 0.20 m.It is, however, possible that although the water potential of -0.20 m still produces fast emergence, it may be too high or too wet for a normal development of seedlings.Feddes observed the pF range 1.0-1.5 to produce too poor seedlings despite otherwise normal germination.These experiments were, however, made on radish and the results differed also in other respects from those of the cereals mentioned above, specifically in the wet side.
In Southern Finland the average tempera- ture in the latter half of May when the emer- gence of spring cereals is taking place is about 10°C.Thus Figure 3 gives an idea of the de- pendence of the emergence rate of cereals on the soil water tension in Southern Finland when sowing is carried out normally in about mid-May.During the emergence period, lack of water does not usually restrict emergence.Thus the optimum area in Figure 3 can be considered normal in regard to onset of emer- gence.It is, however, possible that the soil sur- face where the emergence takes place can in clay, silt and peat soils become too dry as a result of evaporation, emergence slowing down because of lack of water.
The effect of matric pressure on the heat sum required for 50 % emergence is presented in Figure 4.The heat sum was calculated using the following equation: Figure 4 shows a clear increase in the re- quired heat sum for wheat and barley when the matric pressure reaches a critical point which was pF 2.8 or -6.3 m for wheat and pF 2.7 or -5.0 m for barley which well cor- respond to the critical value pF 2.7 obtained by Feddes (1971) for four vegetable crops.It is not possible on the basis of the experiments to determine the lowest and highest matric pressures where no more emergence oc- curs.But within a certain period of time the emergence percentage for total emergence can be determined.Figure 5 shows the dependence of total emergence during 30 days on the matric potential of wheat and barley at a tem- perature of 10°C.In both cases, one point clearly differing from the other observations has been excluded.For barley the curve has been drawn directly through the observations, whereas for wheat the curve is partly drawn approximately.The results do not differ much from those obtained by Aura (1975) with sugar beet, although the experiments are not entirely comparable.
Figure 6 shows the imbibition by wheat and barley at the temperatures of 10, 20 and 25°C at a soil water potential of about pF 1.2.Com- pared to the imbibition by corn (Seneca Gol- den) at 24°C, for instance, the curve is fairly similar to that for wheat at 25°C with the difference that the rise of the imbibition curve of corn is to start with slightly steeper (Blacklow 1972).The imbibition results for wheat and barley do not show as clearly the point where the exponential phase becomes linear as presented by Blacklow for corn (UH 108) at 16°C and 24°C.There are con- siderable differences between plants and varie- ties.The matric potential of the seed bed greatly affects also the imbibition by different seeds (Hadas 1970).The course of the imbi- bition by wheat in the present study was sim- ilar to that described by Chino (1972) for wheat during 30 hours.Chino reported of an imbibition slightly faster in average, but this can be ascribed to an initially greater water uptake.The initial water uptake, defined ac- cording to Becker (1960) as the water uptake caused by the moistening of the pericarp be- cause of capillarity occurring during 1 min, was in the present study about 3 % for wheat and 5 % for barley.Blacklow reported an initial water uptake of 6 % by corn.
In conclusion, in Finnish conditions it is essential for efficient seedling emergence of wheat and barley in spring that the soil ma- tric potential is within the optimum range, i.e.
about pF 1.3 2.7 or -5.0 0.20 m.It is, however, possible that pF 1,3 is still slightly too wet for normal seedling to occur despite otherwise fast germination.Already a small rise in soil temperature greatly affects the rate of emergence in conditions like those pre- vailing in Finland.With efficient drainage it is possible to affect the soil temperature in the emergence period, thereby shortening for instance the ripening of wheat and barley by10 days.Which is of great importance in Finland because of the short growing season.Also by means of irrigation seedling emergence could be promoted in Finland during some years mainly in silt, clay and peat soils because of the possible drying of top soil.

Fig
Fig. I. Water retention curve for the sand used as seed bed in the experiments.

Fig. 3 .
Fig. 3. Effect of soil water potential on the rate of 50 % emergence of wheat and barley at 10°C.

Fig. 5 .
Fig. 5.Total emergence for wheat and barley occurring during 30 days as the function of matric poten- tial at 10°C.

Table
. The