40 SEED TESTING INTERNATIONAL www.seedtest.org RULES DEVELOPMENT • of the seeds according to the ISTA Rules (ISTA, 2024). This was done as a check in case of any anomalous αw results and are not reported. Survey A survey was also prepared to better understand how widely αw is already being used in seed laboratories, including ISTA accredited laboratories, genebank and research laboratories. The survey questionnaire covered different areas such as type of laboratory and their current method of moisture content testing; if they would be interested in measuring αw; if they are already measuring αw; and if they would like to see αw measurement included as a method in the ISTA Rules. Links to the questionnaire were broadcast on the ISTA social media channels. Results Water Activity Measurements Two brands of water activity instruments, Rotronic (www.processsensing.com) and Novasina (www.novasina.ch), were used by the six laboratories (Fig. 1). Two laboratories (B and F) used Novasina instruments with built- in temperature control; their measurements were made exactly at the prescribed temperature of 20 °C. The mean recorded temperature was slightly higher for the other laboratories (highest mean temperature reported by laboratory D for the carrot measurements), but this source of variation did not have a systematic effect across the different moisture levels and crops, and hence is not considered significant in this data set. For cabbage, the maximum absolute difference between the value reported by a laboratory and the mean was 0.026; for carrot, it was 0.039; and for onion, it was 0.067. There were some systematic errors. For example, laboratories B and F generally reported water activity results that were lower than the mean; laboratory D tended to report higher water activity at the two lowest moisture levels, and lower water activity at the two highest moisture levels. Nonetheless, overall the repeatability within a laboratory was generally very high with low standard deviations for any given moisture level, crop and laboratory. Larger standard deviations were, however, observed for laboratory F, which may reflect a less efficient loading process. Reproducibility across laboratories was also acceptable. The measurement times recorded by the laboratories ranged from <5 to 45 min. Survey There was a total of 31 responses to the survey, primarily (20 responses) from people working in a seed testing laboratory, seven of which were ISTA accredited for moisture content determination. Eleven of the 31 respondents said that they were already measuring water activity, six of these were from seed testing laboratories, three from seed (gene) banks and two from university or other research laboratories. These measurements were on seeds from diverse species: cereals, legumes, trees and shrubs, and native or wild species. Some of these user respondents commented that they find it easy and efficient to assess seed moisture status using this method, with a clear advantage that it is non-destructive, so seeds are not wasted. Some of those not already measuring water activity were concerned that it might be expensive to introduce, but the main limitation was that there is no demand for water activity measurement. Conclusions The results of this study suggest that, provided the water activity meter is within calibration and temperature is controlled, water activity measurements are highly repeatable (within a laboratory and on the same instrument). Water activity measurements are also reproducible across laboratories using different instruments. Seed water activity is already being applied in some seed laboratories. Many seed (gene) banks measure water activity to confirm that seeds have dried enough to be packed for storage. Some seed companies are also using water activity internally during seed processing and research. However, since ISTA has not recommended its use, companies face challenges in adopting this method more broadly, particularly at the seed production stages. The results of this study represent a significant step towards establishing water activity measurement as an official ISTA method. Additional validation rounds could consider other types of seeds and different moisture levels. Also, the requirement for cutting larger seeds or seeds with physical dormancy could be included. Further work has already explored the measurement of αw for coated seeds (Vijayan et al., 2025), in which it was concluded that, in the case of film-coated seeds, measurements of αw provide an accurate indication of the αw of the seeds but the same cannot be said for pelleted seeds. Acknowledgements We gratefully acknowledge Jean-Louis Laffont and Kirk Remund (ISTA Statistics Committee) for analysing the data. References 1. Hay, F.R., Rezaei, S. and Buitink, J. (2022). Seed moisture isotherms, sorption models, and longevity. Frontiers in Plant Science, 13, 891913. 2. Hay, F.R., Rezaei, S., Wolkis, D. and McGill, C. (2023). Determination and control of seed moisture. Seed Science and Technology, 51, 267–285. 3. ISTA (2024). International Rules for Seed Testing. International Seed Testing Association, Wallisellen, Switzerland. 4. Vijayan, S.S., Rezaei, S. and Hay, F.R. (2025). Water activity-moisture content relations of primed, film-coated and/or pelleted vegetable seeds. Seed Science and Technology, 53, 197–210. Figure 1. Water activity (αw ) of seeds of three vegetable crops equilibrated to four different moisture levels and tested by six different laboratories (A–F). Within each species, the laboratories are ordered according to the recorded measurement temperature. Values are the mean ± s.d. of measurements made for three replicate samples. Horizontal lines represent the means for the crop at each respective moisture level.
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