Some effects of light on the mineral nutrition of plants

Despite the many advances that have been made during the past two decades in the understanding of the mechanism of the absorption of ions by plants, relative little is still known about their translocation through plants. In several recent publications it has been suggested that the transfer of nutrients across the symplast of the root is a 'passive' process dependent entirely upon transpiration. Two reasons hove been advanced in support of this viewpoint. Firstly the close correlation which exists under certain circumstances between the rate of transpiration and the rate at which ions are transferred to the plant shoot. Secondly the evidence from many sources that the outer surface of the cytoplasm does not present a high resistance barrier to the entry of ions - that is to say the concept of apparent free space.
As against this concept of passive movement there has existed for the past two decades a considerable body of evidence which has been regarded as establishing the dependence of nutrient accumulation on metabolism. The widely known work of Hoagland and Broyer is particularly relevant. These workers showed that whereas variations in the rate of transpiration exerted a large effect on the movement of ions to the shoots of plants of initial high nutrient status, only a small effect was observed when the initial nutrient status of the plants was low.
The recent suggestion of Hylmo and Epstein that ions are transferred passively across roots rests on Hylmo's observation that transpiration and the translocation of ions to the shoots of young pea seedlings varied concomitantly. Since the seedlings were of high salt status his results accord with those of Hoagland and Broyer. The tacit conclusion that the relationship observed by Hylmo between transpiration and translocation is of general occurrence however appears unwarranted without further consideration of the effect of transpiration on uptake by plants of different salt status and by plants supplied with different concentrations of salts.
Accordingly an investigation of this subject was undertaken. Transpiration was varied by feeding and the duration of the experiments was sufficiently short for the effects of shading other than that of varying the rate of transpiration to be negligible. Phosphate and rubidium, labelled with 32P and 86Rb, were the ions studied. Barley was the principal experimental plant, but in some experiments sunflower plants were used. The work was carried out in water culture. The plants were raised in a greenhouse, and when they had attained a suitable size and had been subjected to any necessary pretreatment to adjust their nutrient status, they were sealed in the experimental vessels which contained the appropriate radioactive nutrient solution. They were then transferred to the controlled environment chambers where they could be exposed in the dark or in varied light intensities, under constant conditions of temperature and humidity. Transpiration losses were determined by weighing, and the absorption of nutrients was determined by the assay of roots and shoots separately.
The extent to which the rate of transpiration affected the transfer of phosphorus from the root to the shoot varied both with the concentration of phosphorus supplied to the plant and the initial phosphate status of the plants. When low levels of phosphorus (0.1 p.p.m. P) were employed and the initial phosphate status of the plants was low, the shoot content was either independent of transpiration or was only slightly affected by it. When however the concentration of phosphorus in the external medium, or the phosphate status of the plants was increased, a dose correlation between shoot content and transpiration occurred. With rubidium also transpiration affected the shoot content more when rubidium supplied was supplied at a high concentration (85 p.p.m. Rb), but some effect of transpiration remained at the lowest concentration employed (0.28 p.p.m. Rb). Qualitatively therefore there is a similarity between the effects of transpiration on the absorption of both phosphorus and rubidium.
These findings, which are in close agreement with those of Broyer and Hoagland, are most simply interpreted as indicating that ions are not transferred from the external medium across the root and thence to the shoot by the transpiration stream, but that one step in the overall process, which can on occasions be rate limiting, is dependent on transpiration. This situation occurs when the external concentration of ions is high, and especially when the plants are of high nutrient status.
Evidence of many types shows that the transpiration dependent step is not the transfer of ions from the external medium across the symplast to the xylem: the correlation between oxygen uptake of plants and salt uptake is well established: respiratory inhibitors hove been shown to reduce both the absorption and the translocation of ions to the shoots, in a manner similar to the findings in tissue slices. The present investigation provided further evidence on this subject.
Since phosphate and rubidium in the transpiration stream are present predominately as free ions, it is legitimate to calculate the mean concentration of ions in the transpiration stream, from the quantity of the ions in question which reaches the shoot and the quantity of water transpired. When transpiration is low and the concentration of salt supplied to the plant is small, the mean concentration of ions in the transpiration stream may exceed that In the external medium by factors greater than 100. Moreover when the external concentration is high the T.S.C.F. may be significantly less then unity. The high concentration gradient against which ions are moved is conclusive evidence of the expenditure of metabolic energy in the process of translocation to the shoot. Furthermore the existence of significant parallel pathways for passive diffusion is precluded since their existence would preclude the possibility of the large concentration gradients being maintained between the internal and external concentration.
It is considered that the interpretation suggested by Hoagland and Broyer is supported by the present evidence, namely that transpiration can affect the rate of upward movement to the shoot of ions which have already been released into the xylem. This step may become rate limiting in the overall process of translocation when the external concentration of salt is high and/or the plants are low salt status.
The high internal concentration of ions suggests the existence of a barrier against the free diffusion of ion into the xylem. Its effect may be compared to that of the tonoplast of a single cell. Anatomical considerations suggest that the endodermis may be the site of this barrier. This in no way disputes the possibility that considerable zones of the root cortex are not separated from the external medium by a high resistance barrier.
Some observations were made on the effects of shading individual leaves on their accumulation of phosphate and rubidium. Sunflower plants were used for this work. During the short experimental periods employed, shading of an individual leaf reduced the content of that leaf. As it would be expected that shading would reduce the rate of transpiration from a leaf, the effects observed accord with the postulate that the transpiration stream passively transports ions in the xylem.
Having observed the large effect that transpiration exerted on the absorption and translocation of phosphorus and rubidium, consideration was given to the effects of transpiration on salt uptake by plants that had been grown continuously under three levels of light intensity, ranging from full daylight to 14% full daylight. The experiments, which were of a preliminary nature, were carried out in the open. Immediately prior to the 24 hour treatment period in which the absorption of phosphorus and rubidium was measured, the plants from each light intensity were divided into three groups and were then treated under the three levels of light.
In contrast to the experiments carried out in the controlled environment chambers, variations in the transpiration rate had but a small effect on salt uptake. It is postulated that this situation was due to the fact that in the plants which had been grown and treated under the same level of light intensity and with the same concentration of nutrients, that a 'balance' had been attained between the metabolic processes responsible for the active transport of ions across the root, and the utilisation in the shoot, with the result that variations in the rate of transpiration could exert but a small effect on the upward movement of ions.
A markedly different situation occurred in the experiments carried out in the controlled environment chambers since all plants were subjected to a marked environmental change during the treatment period. It appears that all reported observations of the effect of transpiration on salt uptake have been obtained under artificial conditions. It is apparent that the striking effects of transpiration on upward movement of salts only occurs under such conditions. Come modification of the distribution of nutrients within the individual leaves may nonetheless result from natural shading.
Thus the present data suggest that the effect of constant shading on the nutrient balance of plants is to be explained in terms of the effect of variations in light intensity on metabolic processes.