LI-6400XT: Controlling the Environment

The CO2 Source Assembly is designed with robust materials for safe operation. 12g cartridges provide up to 8 hours of operation in the field. The mini-cartridges are accessible outside the instrument, making them easy to change.

The 6400-01 CO2 Injector System consists of an electronic controller, a CO2 Source Assembly that uses mini-cartridges for portable operation, and a CO2 tank fitting for greenhouse or laboratory operation.

All parts integrate directly into the standard console with no external batteries or control modules. The CO2 Injector System provides a constant CO2 input from 50 to 2000 µmol mol^-1. CO2 is controlled by delivering a precisely controlled pure CO2 stream into air that is CO2 -free. The rate of pure CO2 injection is varied according to demand from the leaf chamber, or from the input CO2 set point, whichever approach you specify.

The 6400-01 facilitates measurements at elevated CO2 concentrations and the easy generation of A-Ci curves. The CO2 injector is under complete software control, allowing you to manually set CO2 levels from the console, or use AutoPrograms to make measurements at a series of concentrations.

The LI-6400XT controls chamber humidity by automatically varying the flow rate to null-balance at the chamber humidity level you specify in software; the input flow rate can also be held constant. Flow rate is controlled by pump speed in the standard system. With the 6400-01 CO2 Injector System (optional), pump speed is constant and flow rate to the chamber is controlled by redirecting excess flow. This “shunt regulation” allows flow to be controlled smoothly and quickly across a broad range.

Whether the controller in the 6400-01 is used or not, the air supplied to the chamber may be dry or moist. Supplying the chamber with moist air allows higher flow rates to be used to balance low transpiration rates, which provides more stable control and more accurate measurements. Inaccuracies and time delays due to water sorption on the air lines between the console and the sensor head are eliminated by measuring the reference and sample water vapor concentrations in the sensor head.

Integrated Peltier coolers control temperature based on either leaf temperature or chamber block temperature (software selectable). Chamber block temperature can be set to any value within ± 6 °C of ambient temperature. Temperature control is a standard feature of the LI-6400; no external power supplies or accessories are required.

The 6400-02B LED Light Source, 6400-40 Leaf Chamber Fluorometer, and 6400-18 RGB Light Source are totally integrated with the hardware and software of the LI-6400XT System.

The use of LEDs with low power consumption makes them a practical light source because of their small size and ability to operate from the LI-6400XT battery. LEDs also minimize the influence of the light source on the leaf temperature and are easily computer controlled.

The 6400-02B LIght Source red LEDs are used to provide radiant output at 665 nm (nominal), while blue LEDs provide output at 470 nm. The output of the blue LEDs is crucial for studying stomatal kinetics.

The Light Sources are continuously variable over their entire measurement ranges, so you can specify any light level without needing to make adjustments or change filters. Light levels are automatically cycled through user-entered set points when using the “Light Curve” AutoProgram, making light curve generation automatic and unattended.

The Light Sources are easily installed, since they replace the upper half of the standard LI-6400XT leaf chamber (the 6400-18 can be hand-held or mounted to the 6400-17 Whole Plant Arabidopsis Chamber. Having the light source as part of the leaf chamber ensures that the geometry between the leaf sample and the light source will be the same for every measurement. Careful placement of the LEDs also ensures uniform light distribution at the leaf surface.

Accuracy during operation is assured by feedback control that adjusts the light source to maintain the target value.

When using the standard leaf chamber, PAR is measured in the chamber near the leaf plane using a miniature GaAsP sensor. The external ambient Photosynthetically Active Radiation (PAR) can be measured with an optional LI-COR quantum sensor located outside the chamber.