Education Zone

Plants require a large supply of nitrogen to make protiens both for their structure and biochemistry.

supplied via uptake of nitrate and ammonia from the soil. (naturally occuring or from fertilizers).

Another main source of nitrogen for some plants is via nitrogen fixation.

The plants can do this by forming symbiotic relationships with nodule forming bacteria that have the capacity to carry out nitrogen fixing reactions.

On a global scale natural fixing reactions provide three times the amount of nitrogen to the soil as conventional fertilizers.

The reaction involved in nitrogen fixing reduces N2 to NH3 and is catalysed by the enzyme nitrogenase.

The rhizobium species that carry out these reactions have specific relationships with a particular plant species.

The specific plants release substances which induce chemotaxis of the rhizobium towards the host plant.

when the concentration of these substances increase the a set of genes in the bacteria are induced called the nod genes. These allow the bacteria to absorb to the host root surface.

once attatched the bacteria releases substances which induce the host plants cells to divide to form a set of cells (nodule meristem)

The cells are then filled with the next generation of bacteriods and are involved via their connection with the nodule, via the infection thread, in the passage of photosynthesis to the nodule.

Early stages of infection are affected by calcium content and soil ph, low ph reducing levels of attatchment and calcium ions amelioating this effect.

Nitrate supply affects nodule morphology with high supply causing more clustered groupings known as supernodules.

Until this process is complete the plant does not recieve any nitrogen but still has to supply the nodule with photosynthates. (may take 21 days)

If another supply of nitrogen during this period is present for nodulated plants the yield is often increased. As it allows higher leaf LAI and hence a compensation in photosynthate levels to make up for those provided to the nodules.

Carbon costs for nodulated plants seem high at first when compared with other sources of nitrogen but become similar when assimilation and uptake costs are included. Carbon supply however is the most significant nutritional component in terms of its effect on assimilation rate.

apart from photosynthates and calcium supply, phosphorous supply is the other major factor affecting assimilation, high levels required for effective assimilation.

Other forms of nitrogen affect N2 assimilation in a manner which produces a hyperbolic response curve. High levels of mixed nitrogen can however cause a suspension of nitrogenase activity.

Nitrate ion must be assimilated into an appropriate form before it can be incorparated into protiens.

This is a two step process, both steps are catalysed by enzymes

Reduction of nitrate to nitrate - (catalyst, nitrate reductase)

nitrite to ammonia - (catalyst, nitrite reductase)

The reduction of nitrite to ammonia is particularly quick to avoid the buid up of nitrite in the plant, a toxic substance. Both catalsysts are inducible in the presence of nitrate. This is important as the enzymes only have a half-life of a few hours. A high CYT/ABA ration will also induce nitrate reductase.

As well as the enzymes the two reactions also require reducing agents, supplied by photosynthesis. In C3 plants this leads to a competition between nitrogen and carbon assimilation which both need the reducing agent. In C4 plants the nitrate assimilation occurs in different cells, (the mesophyll cells as opposed to the bundle sheath cells) to carbon assimilation, reducing the intense level of competition and produces efficient use of nitrogen.the intense CO2 regime found in the bundle sheath cells also dramatically reduces the proportion of nitrogen needed by RuPB production.

Nitrate is transported to the root to be assimilated, rather than occuring in the roots, due to it being a highly mobile ion.

A number of factors affect the proportion of assimilation that occurs either in the roots or shoots.

Species- genotype determines where in the plant nitrate reductase is capable of being produced.

The second factor is which cation partner the nitrate is associated with when entering the plant. If the majority enters with K+ most assimilation occurs in the shoots. If the most nitrate enters with Ca2+, assimilation occurs in the roots.

Nitrate assimilation is greatest during periods of cell division and exspansion, reaching a peak at the peak of growth. As nitrate isn't very mobile in the pholem it's import into the leaf needs to be timed correctly. Importing lots of nitrate into a leaf that is about to cease growth is a waste of resources.

In leaves the level is also effected by light, with little nitrate being assimilated during the night. Fewer nitrates are transported in the dark, due to low transpiration, this is reflected in the reduction in amounts of nitrate reductase present. The roots are not effected due to the more constant supply of nitrate.

• Effects nutrients have on the development of plants

• Effects nutrients have on yield

• Factors that effect the rate of mineral uptake

• Explanation of the mineral uptake into the cells of plants

• Uptake of mineral nutrients in the apoplasm

• Transport of nutrients throughout the plant

• Nitrogen nutrition