Leaf thickness and electrical capacitance as measures of plant water status

Plant-based sensors can potentially be an answer to mitigate the shortcomings of the common plant water status monitoring methods. In this study, leaf thickness and leaf electrical capacitance (CAP) were investigated as indicators of water stress. The experiment was conducted on a tomato plant (Solanum lycopersicum) in a growth chamber with a constant temperature of 28°C and 12 h on/off photoperiod for 11 days. The growth medium used was a peat potting mixture, with its volumetric water content (θ) measured by a soil moisture sensor. The soil water content was maintained at field capacity for the first three days and allowed to dehydrate thereafter, over a period of eight days. The daily leaf thickness variations were minor, with no significant day-to-day changes between soil moisture contents from field capacity to wilting point. However, leaf thickness changes were more noticeable at soil moisture contents below the wilting point until leaf thickness stabilized during the final two days of the experiment when the moisture content reached 5%. The CAP stayed roughly constant at a minimum value during dark periods and increased rapidly during light periods, implying that CAP was a reflection of photosynthetic activity. The daily CAP variations decreased when soil moisture was below the wilting point and completely ceased below the soil volumetric water content of 11%, suggesting that the effect of water stress on CAP was observed through its impact on photosynthesis. The results suggest that leaf thickness and CAP can be used for estimating plant water status.