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  • Foto del escritorALESSANDRA VARALLI

3…2…1…let’s start with the stable isotope analyses

Four weeks have passed since my last day in the office…and even more since my last lab work, how I miss those days!

I have started to work in the Raindrops project team last December, and my contribution consists in exploring C4 plants management, thanks to the combination of experimentally controlled data with ethnographic evidence for the assessment of crop water availability. Particularly, my purpose is to investigate how C4 plant were cultivated in the past to contribute to provide a long-term perspective on human adaptation and the establishment of resilient cultivation systems in drylands through stable isotopes analyses of sorghum, pearl millet and finger millet seeds cultivated under controlled water treatments.

Indeed, I’m specialised in stable isotope analyses that I usually perform in human, animal and botanical remains to reconstruct diet, mobility and subsistence strategies in the past. The application of carbon and nitrogen stable isotope analyses are widely used nowadays to investigate agricultural systems in pre-protohistoric times, but while humans and animals are quite well known, there are still few studies on archeobotanical remains….and even less on C4 plants…and that’s why I’m here!

Based on the investigations on C3 plants, like wheat and barley, we know that isotopic studies in charred seeds provide significant information to reconstruct agrarian practices, palaeoenvironmental conditions, and to detect foodstuff origins. Particularly, the analysis of stable isotopes in plants is a useful tool to infer natural and anthropogenic effects on the growing conditions of plants, because stable isotopes in plant tissues, like seeds and chaff, reflect the environmental conditions in which these tissues developed. Physiological and agronomic studies on C3 plants were carried out in the 80s, and following to these, recent investigations could detect a tight relationship between the carbon isotope composition (δ13C) of cereal crops and plant water status: thus, quantitative models to estimate past water inputs have been developed for these kinds of plants.

But…what about C4 plants?

Before studying archaeological remains, it is important to understand the reaction of C4 plants to different water status in modern samples because, at the moment, all the isotopic information we have about C4 are very few and controversial. So the first step of my study is to answer the following questions:

Can we detect any isotopic difference among the various water regimes? Do the different C4 plant species react differently? And what about African and Asian landraces…do they show any difference if similarly watered?

Just before Christmas time I spent one week at the University of Lecce working at the Laboratory for Mass-Spectrometry and Isotopic Analyses of the Department of Cultural Heritage, to analyse the first two sets of seeds. The first set is from ethnographical fieldwork. More particularly, they were sampled from plants of 12 villages in Pakistan by Carla and Stefano last year. Most of the plants where the seeds originate were grown in rain-fed fields and some others in irrigated ones. The second set of seeds were selected from the plants grown (with love!) by Francesca during her first controlled experiments in ICRISAT, in India (if you are curious to know how she cultivated them, have a look at her article on the blog, dated to March 2019). These last samples have been cultivated according to two extreme water regimes, that means that half were well watered and the other half were highly water stressed.

Because of the small size of the seeds and to be sure to have enough material to analyse in the mass spectrometer, each sample includes 7 grains: they have been finely powdered and I prepared 3 tin capsules (3 replicates) for each sample to check the variability within a sample.

I recently got the results and they seem very interesting…! I look forward to analyse more of them: all the other samples are ready to leave with me for the next round of isotopic analyses, so much work to do, I can’t wait!

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