Impedance Interpretation Guide

Principal Water Content Interpretation

Generally, higher impedance magnitudes correspond to tissues with lower water content and more negative water potential, while lower impedance magnitudes correspond to tissues with higher water content and less negative water potential. Both water loss and water supply affect tissue water content and water potential.

Anatomical Differences and Diurnal Patterns

This data was collected on a mature herbaceous plant grown within an indoor facility over a 4-day period.

The effect of plant anatomy on impedance signals may be seen in the relative positioning of the impedance traces. We expect that the closer a probe is placed to a primary site of transpiration (e.g. the leaf), the higher the signal. From highest impedance to lowest impedance, we have: the leaf, the small stem (petiole), and the large stem (branch near the base). Soil is even wetter and has lower impedance, but one should be cautious about the interpretation when the material measured is very different. The range of wet to dry soil impedance may be offset from wet to dry plant material for the same water potential ranges.

Examples of diurnal patterns (and variation of diurnal patterns in anatomy) may be seen in the amplitude of the impedance signals. We would expect a larger amplitude in leaf tissue over stem tissue because leaves have lower hydraulic capacitance and are the principle site of evaporation. The amplitude of the diurnal patterns is greatest in the leaf and noticeable in the small stem, but hardly discernible in the large stem. (The plot is a little misleading because of the log y-scaling… see that the leaf probe’s amplitude is ~150 kΩ, and that of the small stem is ~25 kΩ).

Note:
Within the facility, temperature was varied little between night/day, but lights and fans were shut off during the night. The little undulations in the impedance traces correspond with the HVAC cycles (every 12 minutes) within the grow-room.

Drying in the Field

This data was taken over months on a mature grapevine in a New Mexico vineyard. The probe was placed on a 10mm second-year fruiting stem.

Prior to irrigation we see an increasing impedance signal, associated with an initial plant drought stress. This signal, after irrigation begins on 4/28, drops dramatically. Consistent irrigation keeps impedance low throughout the remainder of the growing season.

Following Budbreak on 4/21, the rate of increase in impedance accelerates, presumably because of an increase in transpiration. This is also represented in the increase in amplitude of the diurnal patterns.

Note:
The amplitude difference in diurnal patterns between the grapes and the herbaceous plants from the previous data are a result of extreme environmental differences. In the outdoor vineyard, night/day temperature variation was as high as 20 C, while indoors temperature variations never exceeded 5 C.

Root and Vegetative Growth

This data was taken on mature herbaceous plants grown within an indoor facility over a 2-week period. Four probes were placed on the petiole of four plants recently repotted into larger pots.

The period 12/24-12/29 we observed downward trends in impedance in all four plants. This downtrend is believed to be associated with expansion of the root system, allowing the plant to uptake more water than it is losing.

The period of 12/29-1/6 we observed upward trends in impedance in all four plants. This increase is believed to be associated with an increasing shoot to root ratio, resulting in the plant losing more water than it is gaining.