Studies on the pithiness of radish root. III.

Relationship between the process of pithy tissue formation and the changes of pectic substances in the cell wall

Abstract

This paper deals with the. changes of pectic contents in radish root during the process of pithy tissue formation.
The pectic substances decrease in content in dry weight basis during the process of thickening growth or of pithy tissue formation. So that, the fractionation of fractions was tried for the first time, by the usual way. In general, the middle lamella fraction was large and the protopectin fraction was medium and the water soluble fraction was very small in amount. It appears that the root contains little water soluble pectin. Each fraction decreased in amount on fresh weight basis during the process of pithy tissue formation.
Secondly, the fractionation of pectic substances in dry material was carried out by the extraction with one per cent ammonium oxalate at room temperature (F₁), with N/30 hydrochloric acid at 85°C (F₂), and again with one per cent ammonium oxalate at room temperature (F₃). It seems that the fractions F₁, F₂, and F₃ correspond to the easily soluble pectic acid or its salts, protopectin, and the highly insoluble pectic acid or its salts, respectively. If this is true, protopectin was high in the normal tissue, but low in content in the pithy tissue and the very insoluble pectate increased in percentage during the process of pithy tissue formation. It is possible that the pectic substances in pithy tissue was tansformed from protopectin into the highly insoluble pectic acid or its salts and this transformation was related to the cause of the sclerosis of the cell wall.
Use of the intense marceration agent, the mixture of N/30 hydrochloric acid and one per cent ammonium oxalate, decreased the fraction of pectic acid or its salts in amount. If the middle lamella pectin consists mostly of pectic acid, the amount of pectic acid should show an inverse relationship between the normal tissue and pithy one. For that reason the middle lamella contains the pectic substances such as the protopectin in addition to the pectic acid. It seems therefore that the fractions of water soluble pectin, protopectin and the middle lamella pectin in the plant do not correspond to the fractions obtained by marceration agents in the chemical analysis.
Furthermore, the amount of esterified pectins in flesh roots is determined by the histochemical test for pectin based on the reaction of hydroxamic acid with ferric ion. There is a linear relationship between the reflection density reading on the absorbance scale and the color intensity in the test sections. If the test sections are stored overnight in 0.5N hydrochloric acid in dry methanol, completely esterified sections are obtained. Total pectic subutances are estimated, using these sections. According to this method, it is clear that total pectic substances and the esterified pectin decreased markedly in content in a cross section area basis during the process of thickening growth or of pithiness and the esterified pectin was localized in the middle lamella and the primary cell wall of young parenchymatous cells, sclerenchymatous cells and meristematic tissues. And also these tissues had a high pectinesterase activity.
As the non-pithy tissue has a number of vascular bundle in a cross section area basis, the reflectance density of color in test sections is given high values. In other words, there is a closer relationship between the reflecatance density of color and the number of coducting tissue or meristematic one. If the flesh tissue in radish root has a high distribution density of meristematic island, it seems impossible to suppose that the pithiness occurs.