In vivo optical detection of pH in microscopic tissue samples of Arabidopsis thaliana

• Fiber-optic setup for in vivo detection of pH in microscopic samples was elaborated.
• Sensitivity ~ 0.04–0.15 pH and spatial resolution under 20 μm were demonstrated.
• Fast response within 10–20 s in the pH range 5.0 to pH 7.8 was reached.
• pH values in resistant tissues of Arabidopsis thaliana were monitored in vivo.
• Local pH gradients in microscopic resolution were detected.

Minimally invasive in vivo measurement of pH in microscopic biological samples of μm or μl size, e.g. plant cells, tissues and saps, may help to explain complex biological processes. Consequently, techniques to achieve such measurements are a focus of interest for botanists. This paper describes a technique for the in vivo measurement of pH in the range pH 5.0 to pH 7.8 in microscopic plant tissue samples of Arabidopsis thaliana based on a ratiometric fluorescence method using low-loss robust tapered fiber probes. For this purpose tapered fiber probes were prepared and coated with a detection layer containing ion-paired fluorescent pH-transducer 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (c-HPTS). A fluorescence ratiometric approach was employed based on excitation at 415 nm and 450 nm and on the comparison of the fluorescence response at 515 nm. The suitability of tapered fiber probes for local detection of pH between 5.0 and 7.8 was demonstrated. A pH sensitivity of 0.15 pH units was achieved within the pH ranges 5.0–5.9 and 7.1–7.8, and this was improved to 0.04 pH units within the pH range 5.9–7.1. Spatial resolution of the probes was better than 20 μm and a time response within 15–20 s was achieved. Despite the minute dimensions of the tapered fiber probes the setup developed was relatively robust and compact in construction and performed reliably. It has been successfully employed for the in vivo local determination of pH of mechanically resistant plant tissues of A. thaliana of microscopic scale. The detection of momentary pH gradients across the intact plant seems to be a good tool for the determination of changes in pH in response to experimental treatments affecting for example enzyme activities, availability of mineral nutrients, hormonal control of plant development and plant responses to environmental cues.