1. Basic parameters influencing plant - water relations
The soil is a reservoir for the precipitation and it is assumed that there is no water coming from the ground water (that source of water which flows to the field below-ground). It depends on the organic matter, the soil structure (relation of sand to clay to silt) and the aggregation of the soil how much water the soil can store. The storage capacity is measured in “water holding capacity of the soil”. In normal “excellent aggregated soils” and sandy soils most of the water is “plant available”.
Loss of water from soil is by:
Surface run-off: This happens when the precipitation is offered in large amounts in small time, and when the soil is not able to absorb such amounts of water in that small time. Well aggregated soils and sandy soils absorb water very well and better than hydrophobic (water repellent) very organic soils and heavy compacted clay soils.
Drainage: If the soil is very sandy and water percolates quickly through the vadose zone, it does not become plant-available, since it cannot be stored in the soil for a longer period.
Evaporation: Evaporation is the most loss of water from the soil without being taken up by the plant.The amount of evaporation is often very high. It is that water which “transpires” from the naked soil, and it is clear that the hotter it is the more evaporates. Water is moved up in small porous canals in the soil to the surface where it evaporates and is lost (capillary forces are involved). The bad thing is that, in addition, with the water salts (which are solved in the water) are moved up to the surface – there the salt remains while the water evaporates.
Transpiration: This is the water which is transpired by the plant when it grows. It is likely to be the greatest but also productive part of water loss. With a very few exceptions plants MUST transpire to produce biomass – this is a natural law that photosynthesis and assimilation is combined with transpiration. Plants have specific tiny organs on the leaves which regulate the transpiration: these are the stomata – in most cases more present on that side of the leave which is not exposed to the sun. Through these stomata the water is lost (by transpiration) and by physical law the negative pressure of the air is so high that this transpiration process moves up (sucks) the water from the soil to the roots – through the roots into the stem and up into the leaves and transpires through the stomata – this suction is extremely high. The plant regulates transpiration by opening and closing these stomata. Stomata are closed during night and during very hot times of the day to prevent that too much water is lost – so to prevent drying out. During this time of “closure” the plant does not produce biomass. So, under hot climates, plants produce in the morning and in the afternoon during some hours, and in particular in the afternoon it is difficult for the plant to “start again” with the production process.
The water use efficiency. It is important to understand that transpiration is necessary for the biomass production of the plant, and that farmers thus try to optimize the efficiency of the water needed for the production of their goods: this is the water use efficiency: that is the biomass produced per litre of water. This can range depending on the plant species and the growing conditions and the climate in a range between a few hundred (e.g. 150 l for a kg barley) to a few thousand (e.g. 3000 l for a kg rice) litres of water per kg dry weight of biomass. So: it is all about water saving per kg biomass production, and with arbuscular Mycorrhiza we can improve this water use efficiency by several mechanisms.
2. Mechanisms to influence the water use efficiency
There are mechanisms to reduce the water losses from the soil and to optimize the biomass production. Some of the potential strategies are presented hereafter.
Influence of Mycorrhiza on soils
Improved soil aggregation through AM hyphae and Glomalin (see also report on Glomalin) – this is a medium to long term effect. Water infiltration is increased, surface run-off decreased, and water holding capacity increased.
Reduction of evaporation from soil surface by quicker establishment of a vegetation cover – this reduces evaporation by shading at the beginning of the crop establishment when most water is lost through evaporation.
Salt stress means in great part also “water stress” due to the osmotic pressure making water uptake for roots more difficult from soils – here Mycorrhiza may also help to a certain extent.
Influence of Mycorrhiza on the transpiration process
Stomata opening and closing is heavily influenced by optimum mineral nutrition of the plant, in particular by potassium (K) nutrition – stomata react quick - by closing stomata when there is a danger of “dehydration”, and they open quicker again for photosynthesis when the critical time is over – it is known from literature that plants with Mycorrhiza have a more balanced nutrition and that they react quicker to such critical periods of high temperatures during the day. By this they assimilate over a longer time of the day and thus produce more.
Water must move quick to all plant parts again after a period of water stress (even from a short period during the hot time of the day) so that it can produce again: it is known from literature that the hydraulic conductivity – this is water movement in the plant – is improved and the resistance of the plant tissue to water movement is decreased when the plant is mycorrhizal.
Water uptake by roots: During the hot time of the day the water loss is so high that the root shrinks because water is even extracted from the root. A gap is produced between the plant and the soil which has the water in its porous cavities – so the water flow is interrupted. In such moments the root external hyphae of the Mycorrhiza are important because they are the bridge between the shrunken root and the soil water – the bridge allows that condensing water still “flows to the root. The contact to the soil water is maintained.
During periods of water stress the availability of the plant nutrients decreases very quickly – in the soil the nutrients must be dissolved by the soil water and if there is less water the nutrients are only available in certain patches or regions of the soil. Under soil water stress the root external hyphae can still actively take up nutrients and transport them to the plant. Fine root hairs which are normally taking up nutrients are dying under water stress conditions. So, nutrient uptake by Mycorrhiza during water stress conditions is important benefit of Mycorrhiza.
Root architecture and root distribution in the soil of mycorrhizal and non-mycorrhizal plants differ. Mycorrhizal plants can have deeper roots and often also finer roots so that water can be better extracted from more area of the soil.
3. Benefits of Mycorrhiza for plant water relations in practical terms
a) You increase the water use efficiency.
b) As an example you need to produce the same biomass
10% to 40% less water
for Sorghum (Sorghum bicolor), and / or
30% to 60% less water for fenugreek (Trigonella foenum-graecum)
when you compared inoculated plants with non-inoculated plants and
had them grown over 5-6 weeks.
This
may give a first indication:
You can produce about 30% - 40% more biomass, with the same amount
of water.
c) The other point is that, if you compare the biomass production of inoculated and non-inoculated plants, then you need less time to produce a plant of a defined height or biomass when mycorrhizal. This means that you need to irrigate over less time. So, you may save water when you have the plants ready after four months instead of six months.