Early Identification of Plant Drought Stress Responses: Changes in Leaf Reflectance and Plant Growth Promoting Rhizobacteria Selection-The Case Study of Tomato Plants

Drought is a worldwide problem, especially in arid and semi-arid regions. Detection of drought stress at the initial stages, before visible signs, to adequately manage irrigation and crop fertilization to avoid crop yield loss, is a desire of most farmers. Here, we evaluated the response of tomato plants to water scarcity, through changes in leaf reflectance due to modification in leaf pigments’ content, which translates into differences in spectral reflectance indices (SRI) values. Our methodology is innovative, as we were able to easily calculate and identify several SRIs for the early detection of drought stress “invisible” responses. We used a handheld spectro-radiometer to obtain SRI values from leaves of tomato plants growing under two different water regimes for 37 days. In an ensemble of 25 SRIs, we identified 12 that showed a consistent trend of significant differences between treatments along the experiment and within these, NDVI, SR, ZMI, Ctr2, GM1, and GM2 were already significantly different between treatments at day 7 after the start of the experiment and Ctr1 at day 9; although, no signs of damage were visible. Therefore, our results pinpoint these SRIs as promising proxies for the early detection of “invisible” responses to drought onset. We also analyzed the relationship between the monitored SRIs and plant morphological parameters measured during the experiment, highlighting a relationship between GM1 and plant height and leaf number. Finally, we observed a high abundance of putative beneficial bacteria among isolates collected from the tomato water-limited rhizo-environment at the terminus of the experiment, suggesting the active recruitment or selection of Plant Growth Promoting Rhizobacteria by tomato roots as a response to drought. Our work may be adapted into an easy protocol, of rapid execution, to be used in small-scale fields for early drought stress detection.