The following points highlight the seven main factors affected by parasites. The factors are: 1. Respiration 2. Photosynthesis 3. Nitrogen Metabolism 4. Translocation 5. Growth Substances 6. Responses to Environments 7. Host-Parasite Interaction.
Factor # 1. Respiration:
As a consequence of infection, the respiration increases many times in the host tissue. This increase may be due to parasite tissue as well. There is suggestion that parasites attack the membranes and thus break the barrier between the enzymes and thus increase in respiration occurs.
In some infections the respiration becomes uncoupled. In other words, the oxidation of carbon compounds is not associated with the normal production of ATP. As a result the temperature of the host tissue increases. Infection causes an upset in the synthesis and functioning of some enzymes. They may increase or decrease. There may also be a general shift from one pathway to another.
Factor # 2. Photosynthesis:
In the infected plants the level of photosynthesis increases either due to chlorophyll loss or due to its effect on the enzymology of photosynthesis. As a result of infection there may be an increase in the synthesis of growth substances following infection and this results in temporary stimulation of the process.
Factor # 3. Nitrogen Metabolism:
Parasites use large amount of nitrogen from the host tissues. Parasite synthesizes enormous amount of hydrolases which break the host protein and the nitrogenous compounds thus liberated, are used up in the synthesis of the parasite proteins. In the infected tissues RNA also increases.
Factor # 4. Translocation:
Parasites affect water transport considerably. If the infection is in the roots the effect is on water absorption. Sometimes the parasites may block the xylem or phloem and there may be a production of tyloses as well. As a response to disease, transpiration may also go up. Even translocation of solutes and their mobilization is also affected.
Factor # 5. Growth Substances:
There is an enormous increase in the synthesis of IAA which results in tumor formation, a change in the growth rate of the various organs of the plant, epinasty, malformation of the cell wall. Other growth regulators like ethylene, gibberellins and cytokinin also are imbalanced endogenously.
It is not fully understood whether the auxin is synthesized by the host or the parasite. In any case increase in the auxin, definitely helps in the softening of the parasite walls. The role of auxin in the translocation of the metabolites may also be borne in mind.
Factor # 6. Responses to Environments:
Disease infected plants are more susceptible to the drought, temperature extremes or even responses to plant’s water balance.
Injury:
In higher plants due to the attack of the parasite, the host tissues may be injured due to disorders in nutrition or metabolism. In some experiments where 14C sucrose was applied to the leaves of the host, it was transported to the parasite within 25 hours or so.
Further, the removal of the tip or the buds did not affect the transport of radioactive sugars. Sometimes a tree or a shrub fully engulfed by the parasite is also observed, as if it was strangulating the host. The growing stems and tips of the parasite e.g., Cuscuta were so much intertwined that they shade the host and fully eclipse the photosynthesis of the host and enforce death.
Factor # 7. Host-Parasite Interaction:
In recent years much work is being done to determine the specificity and selectibility of the parasite for a specific host. Canadian workers have brought out several levels of interaction between the host and the parasite (Fig. 16-2).
However, there is much to be learnt in this new system and that such an information would help in the control mechanisms.
