Total energy consumption during the lifecycle of a female root-knot nematode (
Meloidogyne spp.) is 1 calorie.
The total biomass of a female root-knot nematode is 200 µg, including the egg mass (Melakerberhan and Ferris). For say 100,000 nematodes in a root system, the total nematode biomass is 20 g! Allowing for 50% production efficiency, total material extracted from the plant would be 40 g. So, the demand effect on the plant may be minimal unless plant is very stressed and resources are limited.
An adult Heterodera schachtii consumes 11 nL/day of cell content (Muller et al, 1981). So, it would take 1,000,000 such females to remove 11 ml of cell content in a day.
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Whole-plant effects - Disturbance of the biochemical network. Wallace (1987) points to the complexity of the biochemical pathways:
Photosynthesis divided into two basic phases - a light phase when light energy is converted into chemical energy, and a synthetic phase in which carbohydrates are formed in a series of reactions accelerated by light. Photosynthesis involves a chain of metabolic events cross-linked to other physiological processes, so disruption of one may have effects throughout system.
For example,
Bird suggested that photosynthesis is reduced in tomato by
Meloidogyne javanica by inhibiting production of cytokinins and gibberellins in roots, and/or by increased stomatal resistance due to water stress.
Fatemy et al. indicate that the response of potato to
Globodera rostochiensis is due to stomatal closure through water stress; the result is reduced photosynthesis.
However, generally the mechanisms by which root-infecting pathogens, including nematodes, affect physiological processes have been insufficiently studied.
d. Plant as an Integrator - Metabolic pool concept - plant as an integrator - concepts of demand and damage. Melakeberhan and Ferris characterized five effects of root-knot nematode infection in grape while exploring the impact in an energy partitioning and flow model:
- Reduction (disruption) of water uptake. Seinhorst, however, asserted that there is little evidence of reduction of water uptake in response to nematodes. He measured daily water uptake on a water usage basis - rate of water loss from pots minus increase in dry wt minus evaporation from surface. The rate was a linear function of total dry wt. So, he argued that rate of use per g tissue is constant. However, root damage could result in lower water uptake, and final dry weight could be a function of the rate of water uptake.
- Reduction in rate of photosynthesis.
- Reduced leaf expansion and total photosynthesis.
- Alteration of partitioning of photosynthate- change in root/shoot ratio.
- Increased leakage - direct effect and affect on other pathogens energy supply (Garrett - inoculum potential as a function of the abundance of infective units and the energy resources available to them).