I want to compare leaf water potential of cassava leaves grown under normal condition and mid-season water deficit stress. I am looking for suggestions or precautions to be taken at the time of recording leaf water potential in field, which may be the selection of leaf (lower/higher canopy), sampling size etc.
Select leaves fully exposed to the sun with no insect or mildew damage. Sample size depends on the confidence level and interval required and population size. If they are both 5% and the population is say 1000, then you need to sample 300-400 leaves :)
The inherent variation among plants needs also to be considered and this depends to a large extent on the variation in soil water content. Proper randomisation is of huge importance, especially if measuring outdoors as water potential will track changes in wind and irradiation. If you use paired measurements in time (one from each treatment) then you will minimise bias. Leaves are not replicates, just samples. Be aware of what are the true replicates in your experimental design (pots, plots, etc) and take mean or median of all leaves/plant measured within a single experimental unit. With a careful sampling strategy and well controlled conditions you will need fewer that 300-400 leaves per treatment. With a sampling strategy that does not control for bias, you will not get reliable estimates with any number of leaves.
Other important points: timing of measurements, traditionally, either pre-dawn or midday. In either case, the measuring period should not extend for more than one or two hours unless you include time in your data analysis. If outdoors, try to measure under clear sky conditions, and avoid days with changing cloud cover in the case of midday measurements. Handling of leaves: be very fast, the shortest the time between leaf excision and enclosing it into the chamber, the better. Try to keep this time to less than one minute, be even faster at midday if stomata are open.
Youngest fully expanded leaves are frequently used. If measuring at midday, make sure to avoid shaded leaves. Use a stereo microscope or magnifier and make sure you can easily see when the cut surface gets wet. Increase the pressure slowly enough so that you can record the exact balancing pressure value. Discard any leaf that is broken, especially in the petiole or veins as this will allow N2 to enter the xylem directly.
Rightly explained by previous researchers.Third fully opened leaf from top could be used.
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