Introductory Crop Physiology (1+1)

Path of Translocation (or) Tissue Concerned with Translocation- Evidences

Path of Translocation (or) Tissue Concerned with Translocation

As early as 1837, Hartig discovered the importance of phloem in the conduction of food materials. Since then, there are number of evidences to prove that the phloem tissues are well suited for the conduction and transport of food materials.

The following are some of the evidences to prove the above concept
1. Phloem- Tissue concerned with Translocation

• The cells of ground tissues like parenchyma, collenchyma etc. is comparatively poor in organic substances.
• The existing food materials are also in insoluble form. This clearly indicates that these tissues are not well suited for conduction because they never form a continuous system between supply end and consumption end. They have been well suited for storage only.
• Xylem is also not well suited because the cells are dead and they have a rapidly moving water current upwards, from root to stem and leaves.
• So food materials cannot move downwards through this tissue.
• From this it is clear that the remaining phloem tissue is the actual path for the downward flow of organic food materials.
2. Structure and distribution of phloem

• In higher plants, the sieve tubes constitute the chief component of phloem tissue.
• The sieve tubes are elongated, placed end to end and their end walls are provided with sieve plates.
• The sieve plates are perforated and through the pores protoplasmic connections are established between the sieve tubes above and below.
• In cucurbits large amount of food materials have to be conducted through a narrow stem.
• Hence, they are provided with bicollateral vascular bundles i.e. with phloem on either side of the xylem.
• Thus the structure of phloem and its distribution in plants clearly indicate that phloem tissue is the only most suitable tissue for conduction of food material.


TOP

3. Chemical Analysis of Phloem Tissue

• The phloem cells contain comparatively large amount of carbohydrates and they are present in soluble state. This clearly indicates that these food materials are not' for storage but for translocation only.
• Mason and Maskel (1928) by chemical analysis of phloem tissues showed that there was a diurnal (day and night) variation in the sugar content of the phloem tissue. High concentration of sugar was observed in the phloem tissues during noon time when the photosynthetic rate and food production are maximum.
4. Evidence from Ringing Experiment

• Malpighi (1679) demonstrated that the removal of a ring of bark (with phloem tissue) from the stem of the trees, blocked the movement of sap down the trees and this caused a swelling of the tissues above the ring.
  
• This type of swelling is due to the accumulation of food materials. Since the continuity of phloem tissue is cut off due to ringing, food materials cannot be transported below the ring.
  
• The downward moving food materials accumulate above the ring and cause swelling.
• This also clearly indicates that phloem tissue is concerned with conduction of food materials. .
5. Effect of Blocking the Phloem Tissues

If the path of phloem tissue is blocked by means of liquid paraffin, the conduction of food materials becomes blocked. Similar conditions are noticed if the phloem tissue is injured or blocked due to diseases.

6. Evidence from the Use of Radioactive Isotopes

• Biddulph and Markle (1944) studied the translocation of radioactive isotopes of phosphorus (P32) after it had been introduced into the leaves of cotton plants. They separated the xylem and phloem tissue at a particular place by introducing a wax paper in order to prevent lateral conduction. After a short period, they found the presence of greater amount of P32 in the phloem tissue. These results clearly indicate that the downward movement of P32 labeled organic compounds occurred in the phloem.
• Rabideau and Burr (1945) supplied the bean plant with CO2 marked with radioactive C14. They traced the path of C14. Carbon-di-oxide, with C14 was used by the plants to pre¬pare the organic food materials. Thus the path of organic food mate¬rials with C14 was traced. By this method they proved that the phloem was concerned with the translocation of organic solutes.
• Biddulph (1956) by the use of radioactive tracers and autoradiography has shown that sieve cells of conifers and the sieve tubes of angiosperms is the path of movement of organic food materials.
7. Evidence from Aphid Stylet Studies

• Aphids are parasite insects, parasitic on the food materials translocated in plants.
• When the aphids insert their stylet to feed on, they are anaesthetized and their stylet is cut off .
• The exudates from such cut stylets directly are related to the food materials being translocated in the phloem.
• Microscopic examination of sections of the stem, on which the aphid was allowed to feed, always revealed that it was the sieve tubes or a group of sieve elements which were punctured and which supplied the exudates.
• All the above given evidences proved beyond doubt that phloem is the only tissue that conducts organic food materials.
TOP