Measurement of the Potential Velocity
Measurement of the Potential Velocity of Water Transport in Stems of Strong Stressed Tomato Plants
In the literature (KURSSANOV, 1984) are very high discrepancies how fast the transport velocity of water and soluble substances like sugar and ions is. In cucumber and tomato sterns (DREWS and HOLZ, 1977) the so called sap flow velocity lies in the range of 50...IOO cm/h. The measurements were made with the heat impulse technique. The last technique has the great disadvantage that only the velocity of the peak is measured. The first warmer water molecules reach the measuring point distinctly faster. There is a mean and a potential velocity.
Some accidental observations by the measurement of the inner stem temperature with thermistors and the transport of C-14 into the root tips have shown that the potential velocity of water and sugar is wide higher then known until now.
By irrigation of the soil with cold water we measured in one meter distance from the soil a decrease of temperature after one minute. Again a temperature decrease was observed when we gave cold water drops on the tip of the shoot but the velocity was not so fast. Both observations show that this is an active water transport phenomenon in the phloem then it is bidirectional. Also the great velocity of C-14 transport into the root tips could be interpreted at ease when we postulate an active water moleculc transport in the phloem (AUERSW ALD, 1973).
The water content of the stem and its thickness are very tight correlated (TORII et al., 1988). The last are signal regulated (AUGUSTIN, not published). The treatment of roots or leaves with Bi58, a phosphor-organic pesticide, which blocks the acetylcholinesterase, makes also under strong light stress whereby the stem is normally ever shrinking that it is growing. It is a right growth then the thickness of the stem is higher than ever before.
All these observations show, that the active water transport plays an important role in plant life. Because the transport is
bidirectional it is very difficult to measure how fast it is. To measure the potential active transport velocity we used the measurement of the stem thickness dynamics. The stem thickness has a very good correlation with the hydrature and the transpiration rate of the whole plant (McBurney et al., 1984).
Materials and methods
The tomato plants stood in five litre plastic pots in soil, The climatic conditions in the growing room were daily ten hours light (20 klx) from HQI-lamps, day/night temperatures 23/18 centigrade. We began to measure as the plants were two meters long. An inductive measuring device was elastic fastened on the stem in one meter distance from the soil. The light strength was increased until 50 klx and a long time not irrigated. When the shrinking velocity of the stem was higher than 10 micrometer per minute began the direct observation of the shrinking, The velocity continually was timed, After 10,..20 timings the soil was irrigated with an overflow of water in a very short time, The timing of the velocity was continued until the growth began. The measure for the velocity of the water was the time between irrigation and the crossing point of the straight lines of shrinking and the braking of the shrinking (see figure). This method overestimates the true reaction time, but it is correct enough for our purpose.
Results and discussion
The velocity lies in the range between 85...324m/h. In all cases this is higher as every measuring in herbs known until now. Water thickens the stem. The thickness of the stem after the irrigation leads to a greater thickness as ever before, The great velocity in the stress phase could be explained by two facts:
1. There is no bi-directional transport
2. There is a switching through of all signals in all fibres simultaneously in one direction and not alternating like in the
normal phase, when is no water stress.
That the velocity of water transport is higher as measured until now is easy to calculate with the help of measurement of transpiration, stem diameter and percentage of area of xylem and phloem vessels (ca. 1...2%). The tomato stem has a diameter of 1 cm near the soil and transpiration rates in peak times is more than 120 ml/h. When we take into account that the whole cross section of the stem transports the water, the velocity would be 1.538 m/h, But we know, that the transport vessels have only a very tiny percentage of the cross section area, This calculated and measured velocity is so high that it is very difficult to maintain the opinion that the water transport is passive. It is in our opinion a capillary-peristaltic pump phenomenon.
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