add study protocol and readme

studies/Ribes_nigrum_drought_exp2/README.md

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+# Drought stress experiment in young plants
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+A second greenhouse experiment was performed to investigate the influence of drought stress on the gene expression and primary metabolism of young plants of *R. nigrum* cv Rosenthals Langtraubige. Stressed plants (A-D) that were not watered for 9 days were compared to control plants (E-H) with continuous irrigation. Leaves were sampled for gene expression and metabolite analysis.
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+<img src="./protocols/Supplemental_Figure4.svg" width="600">
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+Figure from [Ziegler et al. 2024](https://doi.org/10.1093/hr/uhae313)
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