September/October – 2006
Author: Dr Allen Cooper
Growth of the spring cabbage plants was vigorous and very healthy.
Dr ALLEN COOPER demonstrates that water taken up by plant roots is not lost solely from the leaves, but that a large amount is lost from the roots themselves. In this article, the father of NFT documents four scientific firsts, including correlations between day-length and nutrient uptake, between nutrient uptake and the direction of change in day-length, and the photoperiodic relationship with the interaction of day-length and the direction of change in day-length.
At the age of 83 I do not have many years left in which to complete the development of a cheap, simple form of NFT. I am therefore publishing, while there is still time, the information that I obtain as soon as I obtain it in the hope that if I do not have the time left to complete the work, someone may have become sufficiently interested to finish the job. It may be an imprudent course of action but I know that I shall not live to regret it.
It is well established that plants do not grow sufficiently well in a static nutrient solution to produce a crop. Even bubbling oxygen into static nutrient solution is not sufficient to ensure the long-term maintenance of plants or good growth. A minimal requirement in water culture is the re-circulation of a sufficiently large total volume of nutrient solution. The simple form of NFT that I am trying to develop requires no electricity to achieve the circulation. The plants themselves provide the motive power.
There is much about the functioning of plants that has never been questioned. There are widely held beliefs that are accepted as being true without ever having been tested. These believed “truths” are sometimes not explicitly stated but are tacitly implied. The very general subject of plant metabolism provides an example. A visit to a cowshed shows that animal metabolism is imperfect. The organs of the cows for excreting the waste products of their metabolism are clearly apparent and the products of excretion are even more apparent. However, it is tacitly implied that plant metabolism is perfect, presumably because the organs of excretion are not apparent and the products of excretion are even less apparent. Another example of the mythology in plant physiology is the belief that roots absorb water and leaves lose it. Like many beliefs with conviction there is an element of truth in this belief but it is not the whole truth.
A critical examination of the spatial growth pattern of Helianthus annulus throws some light on both of the above beliefs. H. annulus is a perennial plant that, as its name implies, grows with the passing of the years in the form of a ring. If a young plant of H. annulus is placed in the bare soil in the centre of the ring it will die. However, if the soil is removed from the centre of the ring and all root material is carefully sieved out of the soil and the soil is thoroughly washed before being replaced in the centre of the ring, then a young plant will grow well in the centre of the ring. A large scale version of this “poisoning” of the soil is provided by the peach tree replant problem in California where it is well known that young peach trees have been grubbed.
Fundamentally, plants are not very different from animals. In the nineteenth century in London when the products of human excretion were separated from one of the main products for ingestion, namely the water supply, mortality decreased dramatically and population growth escalated. This analogy suggests the untested possibility that in both H. annulus and in peach trees the organs of ingestion and the main organs of excretion are the same organs, namely roots.
It is obviously impossible for the volume of soil in which a plant is growing not to have moisture gradients within the volume of the soil. It is reasonable to make the untested suggestion that the roots at the wet end of a moisture gradient absorbs water that contains nutrient while the roots at the dry end of the gradient excrete liquid containing the waste products of metabolism, in other words plant urine. There is no need for an anus because, unlike animals, plants do not ingest solid food. The leaves are ideally situated to handle gaseous ingestion and the excretion of gaseous waste products of metabolism. Thus, in both leaves and roots the same organ may be capable of both ingestion and excretion. The closest analogy in animals is the mouth that is capable of both swallowing and vomiting.
The above suggestions rest on the ability of “wet” roots to absorb liquid and the ability of “dry” roots to exude liquid. To test this hypothesis I set up a horizontal length of roofing gutter closed at both ends, and beside it and touching it a very slightly sloping adjacent gutter open at one end. The open end was sited above a bucket. Spring cabbage plants were propagated in pots filled with soil until there was sufficient root growth to enable the mass of roots and soil to be slit vertically with a knife from the base upwards to provide a plant with a root system divided into two parts. The plants were placed astride the touching gutters so that one half of the root system was in the horizontal supply gutter that was filled with nutrient solution and the other half was in dry drainage gutter.
The spring cabbage plants were grown outdoors in the simple equipment from September to April inclusive. The volume of nutrient solution in the supply gutter decreased progressively until it was replenished, and a considerable volume of liquid was collected daily in the bucket under the open end of the drainage gutter. There was no doubt that the “wet” roots were absorbing liquid and the “dry” roots were exuding liquid. In this way the plants provided the motive power that achieved the circulation of the nutrient solution without a need for electrical power. Re-circulation could have been achieved by manually emptying the contents of the bucket into the supply gutter. Millennia ago the Egyptians perfected a simple technique for lifting irrigation water that is still in use.
The photograph (please ignore the stone trough; it was too heavy for me to move) shows that the growth of the spring cabbage plants was vigorous and very healthy, suggesting that the plants had benefited from being provided with a dining room separated from their lavatory. Like the Victorian Londoners and H. annulus, they seemed to thrive when not forced to ingest waste products of metabolism. However, this is an untested concept of plant mythology that was fashionable only until the nineteenth century when the work of the German chemists led to the founding of the artificial fertilizer industry and created the resulting increases in crop yields. It is a concept that has remained unfashionable ever since and has been little examined.
Correlation between day-length and nutrient uptake
Another item of plant mythology is the belief that some aspects of plant growth and development are controlled by day-length per se. An enormous amount of work has been done on the influence of day-length on flowering despite the fact that flowering provides a very limited assessment of the influence of day-length on the physiology of plant growth and development because flowering is a fairly discrete response and its progress is difficult to follow without destructive sampling. What is required for study is, firstly, a response to day-length that is continuous and, secondly, a non-destructive method of measuring the response continuously or, at least, daily.
The work that has been done on the response of flowering to day-length has been done in constant day-lengths. In most of the world constant day-lengths do not occur but, despite this the myth has arisen that the same response to day-length obtained in constant day-lengths will apply in day-lengths that are increasing or decreasing. In other words, the response to day-length will be the same in an increasing day-length as in a decreasing day-length. The validity of the assumption has never been tested. The mythology would be in trouble if it were found that plants do not respond to day-length per se but to the interaction between day-length and the direction of change in day-length.
In order to ensure good plant growth I concerned myself with changes in the concentration of the nutrient solution and the maintaining of concentrations that were favourable to plant growth. Consequently, detailed data were obtained daily on the changes in concentration to decide whether to add water or nutrient solution to the supply gutter. The electrical conductivity of the liquid in the supply channel and of the liquid draining from the drainage channel was measured daily. These data are shown in Diagram 1 as the reduction in the cF of the liquid in the drainage channel relative to that of the liquid in the supply channel. Although the electrical conductivity of the liquid in the drainage channel would be increased by the excretion of any waste products of metabolism, and reduction in the cF of the liquid in the drainage channel relative to the cF of the liquid in the supply channel would provide a measure of nutrient uptake.
To eliminate any short-term fluctuations so that the long-term trend could be seen clearly, the data in Diagram 1 are for 10-day means plotted against the time of year. There was no significance in using 10-day means; it merely made the arithmetic easier. The data show that from late September to early November (i.e. from A to B) nutrient uptake decreased, whereas from early November to mid-December (i.e. from B to C) it increased. In mid-December a pronounced discontinuity occurred (i.e. between C and D) when the highest recorded uptake was followed by a very low value. From late December to mid-February (i.e. from D to E) nutrient uptake increased. The subsequent pattern of nutrient uptake (i.e. from E to F) was not well defined.
In looking for an explanation of this seasonal pattern of nutrient uptake, the discontinuity in mid-December is a good starting point. The greatest nutrient uptake occurred on the 8th of December, and the subsequent very low value occurred on the 18th of December. Therefore, an approximate estimate of the date of occurrence of the discontinuity is the 13th of December. What happened about this date?
The data in Diagram 2 suggests a possible answer to the question. In this diagram the day-length experienced by the plants in minutes from sunrise to sunset at the latitude of 50O 45′ N (the location of the experiment) has been plotted from early December to early January. Throughout early December the day-length progressively and regularly decreased, but on the 15th of December the regular daily decline in day-length ceased. The close correspondence between this date and the date of the discontinuity in nutrient uptake suggests the following hypothesis for disproof.
Correlation between nutrient uptake and the direction of change in day-length
There was a biochemical mechanism within plants that was detecting the direction of change in day-length. Immediately the decline in day-length ceased the relation between day-length and nutrient uptake changed. If this were correct then it would follow that plants were responding, not to day-length per se, but to the interaction between day-length and the direction of change in day-length. This possibility is shown in Diagram 3 where the reduction in the electrical conductivity of the liquid from the drainage channel relative to that of the supply channel is plotted against day-length in both decreasing day-lengths. The same lettering of A to F has been used as in Diagram 1. The data obtained in decreasing day-lengths are shown as open circles; the data obtained in increasing day-lengths are shown as solid circles.
It can be seen that in a declining day-length of not less than 564 minutes (the day-length of the 8th of November at the location of the experiment), nutrient uptake decreased as the day-length decreased (A to B). With further decrease in day-length, nutrient uptake increased as the day-length continued to decline (B to C).
In an increasing day-length, nutrient uptake increased (D to E) with increasing day-length until a day-length of 608 minutes was reached (the day-length of the 16th of February at the location). With further increase in day-length (E to F) the relation between nutrient uptake and day-length was not well defined.
It is of interest that the influence of day-length on nutrient uptake between E and F appeared to be less dominant than in other day-length conditions, presumably because other influences were capable of affecting the relationship when the day-length was greater than 608 minutes and was increasing.
Despite this variability of response there appeared to be an optimal day-length of 608 minutes for nutrient uptake by spring cabbage plants when the day length was increasing.
It would be interesting to see if the volume of water lost by the roots is also dependent on the interaction of day-length and the direction of change in day-length. If nutrient uptake is responding to such an interaction, it is reasonable to suggest that excretion would react correspondingly.
The correlation between nutrient uptake and day-length is obvious from the data. What is required is a controlled environment facility in which to determine whether the relation is casual. The use of a divided root system as described earlier would provide a simple, very sensitive method of exploring the responses of root physiology to the environment in a controlled environment research facility. It would provide both a response to day-length that was continuous and a non-destructive method of daily or even continuous sampling. Such an exploration would enable much of the potential of a simplified NFT system to be achieved. It might even make the difference between the long-term operational success and the failure of the simplified form of NFT by obtaining a greater understanding of the influence of the re-absorption of the waste products of metabolism on plant growth and development.
Preliminary indications are that re-absorption hastens the passage from root growth to shoot growth to leaf growth to fruit growth to death, and increases the amount of growth of each stage. The growth regulatory chemical involved might even be identified. The possible potential for the control of development of the part of the plant that is to be marketed is obvious. Applying artificial fertilizers merely increases the growth of the whole plant. Precise targeting would be a commercial advantage.
Surely there is a plant physiologist somewhere who is interested?
For further information contact:
Dr Allen Cooper,
Worthing BN14 9NW,