Within the manual Protocols for experimental plot sampling, handling and processing of cereals in field experiments by G.J. Rebetzke (Greg.Rebetzke@csiro.au), A. van Herwaarden, B. Biddulph, C. Moeller, R. Richards, A. Rattey and K. Chenu.
Crop canopy temperature (CT) is a complex, integrative trait that measures a genotypes ability to maintain transpiration and gas exchange. Thus, CT is correlated with plant water status. CT was first proposed as a measure of dehydration avoidance, and has subsequently been used in numerous studies, and is measured remotely by an infrared thermometer. Several suppliers (available on the internet) sell suitable instruments with various levels of repeatability, range, fixed angle view and of course price. In some cases, special lenses can also be obtained to alter the effective field of view.
There are numerous considerations before attempting to measure CT.
- Full canopy closure: It is essential that full canopy closure is reached before CT data is obtained (Fig. 17). Soil has a very different temperature than the crop, and even small amounts of dirt will bias the results.
- Time of day: CT around solar noon was initially advocated as a test of crop dehydration avoidance, especially in moderately stressed crops. Genetic differences will often be largest here (as much as 5∘C). Observations in early afternoon will also provide information on crop dehydration avoidance, although genetic differences are smaller (usually <2∘C). Early-mid morning observations provide an indication of the crops ability to rehydrate; genetic differences are smaller (usually <2∘C).
- Cloud and wind: CT will change quickly in response to wind and cloud. It is best to avoid days that are cloudy, especially where there is intermittent cloud intensity. Consistent, light breeze (<~5km/hour) is OK but not optimal, especially where trials are large. Variable wind events must be avoided.
- Crop growth stage and canopy height: Phenological stage (see -Development’ section) has a big impact on crop CT; plant stems (eg peduncle), inflorescences and spikes have different temperatures than the canopy. Thus genotypic variation for Zadok score should be avoided. Typically, CT is measured during the vegetative phase (before spike emergence) and again after anthesis (grain filling). Care should be taken during grain fill to stop observing CT when leaf senescence of the upper canopy begins, as loss of green leaf area is correlated to CT. Finally, note should be made of canopy height owing to potential for boundary layer effects on CT.
- Trial size and layout: As environmental stability is important for effective and efficient capture of biologically meaningful CT data, thought should be given to trial design. Smaller trials are better, and blocks should measured/distributed along time. It is helpful to run check genotypes, known to be cool or hot, regularly through the trial.
When the above issues are reached and appropriate for the objectives of the research, there are several things to consider whilst estimating crop CT.
- Start CT observations in a noted part of the trial and follow a defined path. If possible, repeat the measurements using another path and check the consistency of the results.
- Instrument field of view is dependent on the instrument chosen (by the fixed angle view). For example, with a narrower angle, the operator should be a couple of metres away from the target canopy to ensure sufficient representation of the crop. However, it is critical that all plots are estimated in the same manner to remain relativity. Canopies should be scanned for several seconds to integrate the entire plots.
- Be mindful of the position of the sun. The sun should be behind the operator, and care taken to ensure that the operator position does not shade the plot (Fig. 17).
- Be mindful of the number of data points the thermometer logs (usually a few hundred), as new data will over-ride old data. Some thermometer units require resetting.
- Observe obvious differences that may help explain large differences. For example, rolled and wilted compared to normal leaves could result in >5∘C differences between genotypes. Such scores will need to be undertaken during or as soon as possible after CT collection to ensure they are relevant.
Once data collection has been completed, statistical analysis is required. In addition to standard within environment analyses, there are some factors that need to be considered in analysis of CT datasets:
- Even minor, sometime inconspicuous, weather changes will influence CT. Thus, temporal effects within an observation event should be modelled.
- As multiple CT observations are usually collected on each plot within a single trial, analysis across these repeated observation events are potentially complicated by serial correlations.
Figure 17. The angle of the thermometer, the distance to the target area, stage of crop development, and amount of ground cover are all critical considerations when assessing canopy temperature in the field. Here a Mikron® hand-held thermometer was used with a thermometer angle to the canopy of c. 25�
Appendix 1. Text description of phenological scale, Zadoks decimal code (DC).
Appendix 2. Picture description of phenological scale, Zadoks decimal code (DC).
Zadoks JC, Chang TT, Konzak CF (1974) A decimal code for the growth stages of cereals. Weed Research 14(6),415-421. doi: 10.1111/j.1365-3180.1974.tb01084.x
Notes and troubleshooting tips
Download complete manual: Protocols for cereal field experiments_Nov2012.pdf
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