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Enabling better global research outcomes in soil, plant & environmental monitoring.

Water Use in a Plant – Parasite Interaction

Plant parasites have long been studied for the amount of carbon, nitrogen and other nutrients they acquire from their hosts. Water is a critical resource for all organisms yet the amount of water parasites attain from their host has not been widely studied. The primary reason for this shortcoming is simply a lack of appropriate equipment. In a nutshell, it is a very hard thing to measure accurately!

 

Plant parasites have long been studied for the amount of carbon, nitrogen and other nutrients they acquire from their hosts. Water is a critical resource for all organisms yet the amount of water parasites attain from their host has not been widely studied. The primary reason for this shortcoming is simply a lack of appropriate equipment. In a nutshell, it is a very hard thing to measure accurately!

A classic plant-parasite study system is the gall. These woody protrusions or warty masses, which can be found on stems, leaves, flowers or roots, are caused by numerous organisms. Insect induced galls are perhaps the most commonly studied, but galls are also caused by fungi, nematodes and bacteria. Galls appear as discrete units on plants and provide habitat and resources for the organism. Some insects can spend their entire life-cycle within a gall, and other galls contain populations and communities of organisms. Galls offer an excellent study system for socio-biology, as well as the physiology of plant-parasite interactions.

As galls are easily identified, and appear as a discrete unit, the possibility was raised that the amount of water, or sap, movement to the gall could be measured. A basic methodology would be to measure sap movement towards the gall for some time, and then remove the gall to see if this sap movement declined or entirely ceased.

If there were water resources being acquired by a gall, it would no doubt be small. A precise measurement technique, which can measure very low rates of water movement, was needed.

Scientists at ICT International decided to approach this problem by way of the SFM1 Sap Flow Meter. This instrument typically measures sap flow in woody stems, such as the tallest trees in the world (the Californian coastal redwoods), Amazonian rainforest trees, Japanese cedar, or papaya. However, it can also measure woody stems with a diameter of at least 10mm. Importantly, the SFM1 Sap Flow Meter employs the Heat Ratio Method (HRM) to measure sap movement in plants.

The HRM was specifically designed by researchers at the University of Western Australia to measure low and reverse rates of sap movement in plants. No other sap flow technique is capable of the job; therefore the SFM1 Sap Flow Meter was the only option available.

Scientists at ICT International then had to find an appropriate gall system to study. Fortunately, galls are very common, and natural, in the national parks and adjoining farmland near the ICT International office in Armidale, northern New South Wales, Australia. The Hickory Wattle (Acacia implexa) is commonly infested with the fungal rust gall, Uromycladium tepperianum (Figure 1). The rust gall is initially formed by the fungus but is subsequently colonised by a community of invertebrates including mites, mealy bugs and predatory beetles.

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Figure 1. (A) An infestation of fungal rust galls on the Australian native tree, Hickory Wattle (Acacia implexa).

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(B) Galls form woody protrusions, or warts, on plants such as this fungal rust gall.

The galls are large, and easily identified, and therefore made an excellent choice to test whether the SFM1 Sap Flow Meter could measure water movement.

Five independent branches on a host Hickory Wattle were selected for an installation of a SFM1 Sap Flow Meter (Figure 2). These branches had no photosynthetic organs on them, there were only galls, thereby minimising confounding influences. These branches also had a minimum diameter of at least 10mm.

The SFM1 Sap Flow Meters were installed towards the end of summer and 28 days of measurement were made. On the 29th day of

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Figure 2. An example of a SFM1 Sap Flow Meter installed on a branch in this study.

measurement, the galls were removed from the branches. The galls were only found at the end of branches, so the entire branch was not removed, only the portion where the galls were located. If there was sap movement, then the removal of the galls would see sap flow return to, and stay at, zero.

A SFM1 Sap Flow Meter was also installed on the host tree to measure total tree water use. Total gall biomass was measured and the amount of water the galls were parasitising from the tree could be determined. A nearby weather station measured meteorological variables, such as vapour pressure deficit (VPD), in order to ascertain any possible driving mechanism of sap flow.

The results of the study exceeded expectations. It was unequivocally found that galls acquired water from their host. This is demonstrated in Figure 3 which shows the last seven days of measurement. There is sap movement towards the gall, mimicking daily cycles of the host plant. Once the galls are removed, sap movement returns to, and stays at, zero. The amount of flow was very small when compared with total tree water use – note the different values on the axes of Figure 3. It was also found that galls were parasitising 4.63% of total tree water use. This may seem insignificant but as infestations increase on a tree, or during drought periods, this amount of parasitism could be critical to host plant health.

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Figure 3. An example sap flow dataset from galls and their host tree. The gall data is the mean of five branches. Galls were removed from branches on the second last day of measurement. Sap flow return to, and stayed at, zero, thereby confirming galls were causing the daily pattern of sap movement in this study.

A multivariate, statistical analysis showed that the most important driver of water movement in the gall was vapour pressure deficit (VPD).

This is a measure of how dry the atmosphere is. Therefore, as the atmosphere dries, more water is lost from the gall.

There was a strong water potential gradient from the host to the gall, driving water flow. In fact, this is the mechanism other plant parasites, such as mistletoe, have shown to acquire water from the host. The next step that ICT International scientists will make will be to install PSY1 Stem Psychrometeron the tree, branch and near the gall in order to quantify the water potential gradient.

This study showed that the SFM1 Sap Flow Meter can be used to measure water use in a plant-parasite interaction. Many hypotheses can now be tested, as well as monitoring of management strategies in plant-parasite control.

References

Forster MA, Quantifying water use in a plant-fungal interaction, Fungal Ecology (2012), http://dx.doi.org/10.1016/j. funeco.2012.06.005