Growth control and physiology

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The growth control, i.e. premature distension of the internodes, is a very important aspect for horticultural nurseries and the cultivation of young plants. The phenomenon depends primarily on the density of cultivation, on the available light (intensity and duration of illumination are positively correlated with phototropism phenomena) and temperature (in particular the difference between day and night temperatures). However, the nutritional formula of the substrate can also help regulate the phenomenon. Contrary to popular belief, high levels of Phosphorus (> 40 mg/l), often used with the intention of promoting rooting, can be a predisposing factor that should not be underestimated. But, above all, is Nitrogen, in its soluble forms (nitric and ammoniacal forms in the circulating solution), that exerts the most appreciable action. Reducing nitrogen supply to a minimum, or even eliminating it, as is sometimes observed in cultivation, is however a mistake. Much more useful is to take care of the ratios between nitric and ammoniacal forms of nitrogen, which must be markedly in favour of the nitric one. Plant roots actively take up nitrate from the circulating solution via numerous low- and high-affinity nitrate-proton cotransporters. The first step in nitrate assimilation is the cytosolic reduction to nitrite. The enzyme nitrate reductase catalyses this reaction:

NO3- + NAD(P)H + H+ + 2e- → NO2- + NAD(P)+ H2O

Nitrite (NO2-) is a highly reactive and potentially toxic ion. Plant cells therefore immediately transport it from the cytosol into the chloroplasts, leaves and plastids, and roots. In these organelles, the enzyme nitrite reductase reduces nitrite to ammonium according to the following reaction:

NO2- + 6 Fdrid + 8 H+ + 6e- → NH4+ + 6 Fdoss + 2 H2O

where Fd is the oxidised and reduced ferredoxin.

At this point, plant cells avoid the toxicity of ammonium by rapidly converting it into amino acids.

The main pathway for this conversion involves the action of glutamine synthetase. Glutamine synthetase combines ammonium with glutamate to form glutamine:

Glutamate + NH4+ + ATP → glutamine + ADP + Pi.

The uptake of nitrate from the circulating solution is affected by the metabolic activity of the aerial part, as a large availability of carbohydrates (energy-ATP) is required to reduce the absorbed nitric ion into ammoniacal form.

NO3- uptake is linked to leaf surface area and photosynthetic efficiency of the leaves.

When plants have mainly the nitrate form at their disposal, they must first carry out an expensive reduction process that involves a not inconsiderable amount of energy.

A substrate fertilisation with predominantly nitrate nitrogen will force the plant to make a greater metabolic effort, than one with an abundance of ammoniacal nitrogen. Furthermore, as this energy metabolism is linked to photosynthesis, bland photosynthetic rhythms, typical of the short autumn and winter days (the same ones that induce phototropism) will lead to a lower uptake and use of the nitrogen resource.

This will provide a substrate, which we could define as “responsive”, i.e. one that will feed the plant by stimulating its growth more when photosynthesis is taking place at a high rate.

During periods of low light, the lower photosynthetic rate, on the other hand, will be accompanied by lower nitrogen uptake and utilisation, helping to control the growth.

An aspect not to be neglected is also the availability of Calcium.

Calcium is a very important mesoelement because it is related to the building of cell walls.

It acts by stimulating tissue hardening, but is subject to frequent absorption and translocation problems. Calcium transport is most active towards tissues with intense respiratory activity. During autumn and winter, less light and lower temperatures reduce tissue transpiration resulting in a greater need for the element. For this reason, supplementation during cultivation is important, choosing preparations that can efficiently transport the element within the plant tissues.