PHYTOGRAM™

Agricultural Electronics Corporation Home Page

The PHYTOGRAM™ is an electrochemical monitoring capability designed to assess inside-the-plant water status and level of metabolic activity of the plants, seedlings, or trees and their response to natural and artificial environmental influences. The PHYTOGRAM™ consists of a sensor and equipment to apply electrochemical testing procedures to the sensor circuit.

The sensor is a thin rod (about three times the size of a human hair and made of precious metal) permanently resident inside the plant, seedling, or tree. It is implanted in the extracellular region of the tissue in a non destructive manner and as such provides an indication of the normal transfer of constituents into and out of cells adjacent to its surface. The sensor functions as a "chemical window" inside the plant through which one can observe changes in extracellular constituents due to cellular activity.

The same sensor is used in three procedures (chemical assays): (1) measurement of inside-the-plant hydration (extracellular water content); (2) measurement of changes in proton or hydrogen peroxide concentration, and (3) measurement of sucrose transport between petiole and fruit.

Measurement of inside-the-plant Water Content (Patent Pending)

On a diurnal time scale water moves out of the cells and into the extracellular region in the morning and back into the cells in the late afternoon and evening. This leads to a diurnal cycle in the extracellular water content. The magnitude of the diurnal cycle gives a measure of the cross sectional area of the extracellular region that is filled with water during the daytime period. Over a multi-day period, the minimum value of the diurnal cycle will vary in proportion to the water content of the plant as a whole. As the plant dries out, both the magnitude of the diurnal cycle and the minimum value decrease.

Water in the extracellular region wets the surface of the sensor. Ionized oxygen within this water forms one charge layer of a parallel plate electrical capacitor. The other charge layer is present within the metal surface of the sensor. Ihe electronics in the POD measures the capacitance of this parallel plate capacitor which is in turn proportional to the area of the wetted surface. The total sensor surface within the plant is measured with a caliper. The ratio of the wetted surface within the plant expressed as a value of electrical capacitance to the total surface within the plant expressed as a length of the cylindrical sensor within the plant gives a measure of water content in units of nanofarads/millimeter. This ratio changes with changes in sensor wetted surface area. The ratio can then he used as a measure of plant water content which is independent of the time and conditions of the measurement. This permits comparison of the ratio from day to day, year to year and site to site. The day to day variation, termed a water content profile, can be used as a criterion of plant water content. The magnitude and timing of irrigation events can he set to maintain a desired water content profile.

Measurement of proton or hydrogen peroxide concentration

Protons are employed in many cellular membrane cotransport processes. Changes in proton concentration in the extracellular region can be used as a measure of the timing and magnitude of these processes. The PHYTOGRAM™ sensor measures changes in the proton concentration by means of changes in the electrical potential between the sensor surface and the extracellular fluid. The difference between the maximum and minimum sensor electrical potential values yields a single numerical daily index of metabolic activity.

Under the special condition wherein a pathogen is present in the extracellular fluid, the cells increase the concentration of hydrogen peroxide in the extracellular fluid. When this happens, the potential between the sensor surface and the fluid is set by the relative proton and hydrogen peroxide concentration. Proton concentration sets the potential value (the proton mode) over a range of potential up to 700 millivolts relative to the standard hydrogen electrode. Above this level, the concentration of hydrogen peroxide sets the sensor electrical potential (the peroxide mode).

This gives rise to a special application of the PHYTOGRAM™ in plant pathology and plant immunology. Hydrogen peroxide is a major constituent of the active oxygen defense mechanism. The PHYTOGRAM™ can be used to measure the magnitude, timing and spread of this mechanism through the plant.

Measurement of Sucrose Transport(Patent Pending)

Sucrose moves across cell membranes as a charged entity consisting of the sucrose molecule and a hydrogen ion (proton). The hypothesis underlying this measurement is that sucrose moves within the extracellular fluid also as a charged molecule. As such, sucrose movement becomes an electrical current and the plant as a whole can be considered an electrical circuit. Movement within the extracelltilar region (apoplast) and across cell membranes via proton/sucrose symports are electrically facilitated. Movement within the phloem is osmotic in accord with the Munch Hypothesis. This leads to an electro/osmotic model sucrose movement from leaf to fruit.

Quantification of sucrose movement within this model is based on the difference potential of sensors in the petiole and fruit. Data taken in year 2004 at twelve winegrape sites in which simultaneous petiole and berry extracellular potential were measured indicates the difference potential varies strongly with fruit loading on the shoot. Cumulative difference potential over the period from pre-veraison to harvest correlates with sucrose per berty.

General characteristics and applications of the PHYTOGRAM™ are shown below.