Comparative study of the content of starch and sugars of Tribulus terrestris, Lucerne, some Gramineae and Pentzia incana under different meteorological, edaphic and physiological conditions. Paper no. 2 - Carbohydrate nutrition

Abstract

The daily march of assimilates in different fodder plants, lucerne, the Karoo bush Pentzia incana, the Gramineae Algerian oats and Panicum minus and Tribulus terrestris, is discussed. The assimilates were determined under different meteorological and physiological conditions. All plants were grown on ordinary Karoo soil on the Veld Reserve, Fauresmith, and had, except for lucerne, rain as the only source of water. The lucerne plots were regularly irrigated and the plot had been fertilized 3 years previously with a heavy dressing of superphosphate. Lucerne was also used in pots. Lucerne, Tribulus and the Gramineae were investigated in fresh, temporarily wilted and permanently wilted conditions or in condition of progressive wilting. Pentzia which does not show signs of wilting was collected fresh or dry after long droughts. Under favourable meteorological conditions and working with a fresh plant the curves of the assimilates of no plant show anything unusual, the maximum of starch being either in the afternoon or early evening. It is under unfavourable meteorological conditions, resulting in drooping or wilted plants, that disturbances in the carbohydrate metabolism occur such as dissolution of starch, increase of sugar and unusual times of the maximum of starch, particularly in leaves. All the plants mentioned above contain starch, sucrose, fructose and glucose. But the ratio of the different carbohydrates varies with the genus. Gramineae at the best of times contain little starch, but rather more total sugars than lucerne. In the Gramineae therefore the daily variations of the sugar are more pronounced than those of the starch. The latter is most abundant in the guard cells of the stomata and may not be quite the same as ordinary starch produced daily during photosynthesis. Pentzia, too, never reaches amounts of starch as those present in fresh lucerne. Whilst it is generally accepted that the leaves are the organs for the formation of starch. it has been observed in lucerne and Tribulus, particularly in the latter. that the stems take over the photosynthesis or at least the starch formation in times of drooping and incipient drying; more starch is found in stems than in leaves at that time. Pentzia does not show such a phenomenon. Pentzia, particularly in drought loses all its starch, whilst lucerne, except at the beginning of winter, always contains an appreciable amount. In all the feeding experiments on the Veld Reserve, Fauresmith, no animal showed any disturbance; on the contrary, hamels thrived on all the plants under all meteorological conditions. When under similar meteorological conditions in the surroundings of Fauresmith disturbances such as bloating, dikoor and dikkop were observed, it seemed logical to think that another factor besides the animal played a role and that this factor was a soil factor. Actually it was suggested that this factor was a zinc deficiency in flush periods of growth in the sense of Hoagland (1944). Although this hypothesis could not be confirmed owing to war conditions, it seemed reasonable to accept that under the influence of different soils the plants might differ in some of their chemical constituents. Actually it was found that Tribulus and lucerne from farms where there was a dikkop outbreak or where bloating occurred contained a saponin during the height of summer. This saponin was absent in Tribulus and lucerne from the Fauresmith Veld Reserve during the summer of 1946. The glucoside was, however, absent in all plants either very early or very late in the season. Another unfavourable condition for lucerne besides drought, which decreases the amount of starch and increases the sugar content in leaves and stems, is frost. but this occurrence is presumably due to migration from the root and not only to assimilation. To make sure that this factor is conditioned by the soil and not by meteorological influence, soil of places renowned for dikkop outbreaks was brought to the Veld Reserve, Fauresmith, and beds were established with Tribulus (seeds from Fauresmith), getting only rain or distilled water. These plants on soil from Calvinia, Waagkop or Leeuwfontein contained the same saponin during the height of the season 1946, whilst in the surrounding Tribulus on Fauresm1th soli it was absent. Having Tribulus from different soils near the laboratory it was possible to study the daily march of the assimilates as influenced by the soil. In principle there was no difference in the daily curve, although on some soils the plants wilted quicker. The usual changes in starch and sugars occurred in this case as described above. Some chemical qualities of this saponin in question are described. It is possible that this glucoside is not the only constituent which varies in the plants from different soils. American authors have described a polyphenolic substance, a phytosterin, which is formed amongst other disturbances in the plants on zinc deficient soil. It is thought that the present saponin might be a related compound. In how far it is connected with dikkop, future animal experiments will have to show. If the saponin conditioned by the soil factor proves to be a link in the chain of Tribulosis, it must be understood that it is only one of the conditions necessary to produce dikkop, but a primary one. There are quite a few others. First there is the question whether in high summer the glucoside is present throughout the plants on the particular soils, whether its effect is only obvious when the veld is bare, or whether saponin is only present at certain times and whether it is always absent on Fauresmith soil. If sheep are grazing on veld with few other plants but Tribulus, on which they are forced to live, the occurrence of dikkop is much more likely. If plenty of other plants are available an occasional mouthful of Tribulus with the saponin may prove absolutely harmless. Hamels may graze without any ill-effect for a long time whilst hungry ewes may immediately succumb. Climatic conditions play as big a role, as they are the first to influence the veld, abundance of fodder being available with regular good rains, whilst plenty of "dubbeltjies" (Tribulus) is available with occasional small showers. Moreover, they also cause flush periods and incipient drying and wilting of the Tribulus itself. So, as mentioned before, it seems that the glucoside has a seasonal trend, being present in very small quantities at the start of the season, and absent at the end of the summer. Up to now other fluctuations within the summer are still unknown, but these will be investigated in the near future at the Veld Reserve, Fauresmith.