This post is part of the series Simulating Climates in Growth Chambers.
In continental North-West America, summer heat is correlated with drought. To make our climates more realistic, we added drought treatments. The simplest way to do this is subject plants in a ‘dry treatment’ to drought cycles, with soil moisture content dropping to 25% relative to total saturation1, before plants are re-supplied with water and fertilizer. In the ‘wet’ treatment, soil moisture content is maintained above 65% relative to saturation. Wet and dry treatments receive the same amount of fertilizer2. In order to apply drought treatments consistently, we use boxes of 40×36 cm where 100 plants share the same soil volume (15.8 l). This circumvents the problem of larger plants being more drought-stressed, as can happen when using individual plant cones. The boxes are made from Coroplast, a plastic corrugated cardboard, which can easily be cut with an Exacto knife. This allows us to size the boxes exactly the way we want them, and optimally use the space in the growth chamber.
Plant box opened on the side with the plants tagged for root washing
As with using individual containers, planting distance needs to be balanced against experiment duration, chosen climate and resulting expected plant size, unless mortality due to plant competition is desired. However, too low a plant density will result in very few and long drought cycles, smaller treatment effects, reduced opportunities to fertilize, and possibly malnutrition. If you need to separate the plants at the end of the experiment, check out this blog post to find out how we avoided the problem of the roots becoming too entangled.
Example of box weights for dry and wet treatments. The first column indicates the target maximum and minimum weights. All wet and all dry boxes are fertilized at the same time, hence individual box weights may differ slightly from the target
Come back tomorrow to find out how I applied day length regimes or go back and see some of the other posts in the series.
1 This is a point we previously established for the given soil mix to correspond to a soil water potential of -1 MPa, the point at which permanent damage starts to occur.
2 That is, at the end of each drought cycle and in comparative amounts (topping up with water if needed).