The Pennsylvania State University The Graduate School College of Agricultural Sciences LEAF THICKNESS AND ELECTRICAL CAPACITANCE AS MEASURES OF PLANT WATER STATUS A Dissertation in Agronomy by Sayed Amin Afzal © 2017 Sayed Amin Afzal Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy May 2017 The dissertation of Sayed Amin Afzal was reviewed and approved* by the following: Sjoerd Willem Duiker Associate Professor of Soil Management and Applied Soil Physics Dissertation Co-Adviser Co-Chair of Committee John E. Watson Professor of Soil Science/Soil Physics Dissertation Co-Adviser Co-Chair of Committee Paul Heinemann Professor of Agricultural and Biological Engineering Dawn Luthe Professor of Plant Stress Biology Erin L. Connolly Professor of Plant Science Head of the Department of Plant Science *Signatures are on file in the Graduate School. Abstract The goal of this study was to deliver a practical plant-based technique to monitor plant water status suited for automation. Three experiments were conducted in this study to determine feasibility of using leaf thickness (LT) and leaf electrical capacitance (CAP) to monitor plant water status. The objectives of these experiments were: 1) determine the relationship between leaf water content and LT across different crops and leaf locations on a plant; 2) examine the relationship of LT and CAP with growth medium volumetric water content (θ); and 3) determine the relationship of LT and CAP as a function of plant water stress levels determined by θ and visual wilting stages. In this study, LT and CAP were measured by a developed leaf sensor. This study helped in better understanding of the relationship between CAP and LT with plant water status to investigate whether these measurements are suitable for practical monitoring of plant water status. In the first experiment, the relationship between leaf relative water content (RWC) and relative leaf thickness (RLT) was determined on corn (Zea mays L.), sorghum (Sorghum bicolor (L.) Moench), soybean (Glycine max (L.) Merr.), and fava bean (Vicia faba L.). Leaf samples brought to full turgor were left to dehydrate in a lab. Piecewise linear modeling explained 86-97% of the variations of the relationship between RWC and RLT. The estimated piecewise parameters varied by species and leaf position on plant. The outcomes of this experiment showed that RLT has a strong determinable relationship with RWC, however, different crops may be required for different RLT thresholds as signals of water stress. The second experiment was conducted on tomato (Solanum lycopersicum L.) in a growth chamber with a constant temperature of 28°C and 12-hour on/off photoperiod for 11 days. Soil iii volumetric water content was measured by a soil moisture sensor. Soil water content was maintained at field capacity for the first three days and allowed to decline thereafter. The daily leaf thickness variations were minor with no significant day-to-day changes for soil moisture contents between the field capacity and wilting point. 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. CAP was consistently at a minimum value during the dark periods, but rapidly increased by illumination, implying that CAP was a reflection of photosynthetic activity. The daily CAP variations decreased when θ was below the wilting point, and eventually ceased during the final four days of the experiment. This result suggests that the effect of water stress on CAP would be through its negative impact on photosynthesis. The outcomes of this experiment show that LT and CAP can be used to monitor plant water status. In the last experiment, eight tomato plants were grown in a controlled greenhouse, four in pots filled with a potting mixture and the others in pots with a loamy mineral soil. One leaf sensor was clipped on a leaf of each plant. Ambient temperature, relative humidity, light intensity, θ for each pot, and LT and CAP for each leaf sensor were measured at five-minute intervals. The irrigation regime was designed based on observed visual wilting stages such that water stress level was increased over time. The stress levels ranged from a well-watered condition for the first and second irrigations to an extreme stress level for the sixth irrigation. The difference between night and day LT values increased with water stress. CAP was roughly at a constant minimum value during the nights and rapidly increased when the plants were exposed to light. The maximum daily CAP level decreased as water stress level increased. The daily night- and noon-time LT and noon-time CAP were used as daily critical values, and normalized iv by specific procedures to calculate their relative values. Piecewise linear regression gave strong relationships of these normalized daily critical values of LT and CAP with θ. The relationships between observed water stress θ, growth medium water potential (ψ), and the daily critical values of LT and CAP were assessed. The means of ψ were not significantly different across the water stress levels. Soil volumetric water content could identify the early water stress levels for the potting mixture. However, θ could not identify the water stress levels for the mineral soil. In contrast, relative noon-time LT clearly identified all the water stress levels for the potting mixture, and the early stress levels for the mineral soil. Relative night-time LT was insensitive to early stresses, while it identified the severe water stress levels. Relative noon-time CAP distinguished the mid-range stress levels. The results indicate that a transition from one stress level to the next level could be discerned by at least one of the relative values of LT or CAP. Therefore, in contrast to the soil moisture measurements, the combination of the relative values of LT and CAP could provide a complete coverage for identification of each water stress level. Finally, the results promise that LT and CAP measurements are suitable techniques for precision monitoring of plant water status. However, it is essential to consider that the results of this study were observed in limited conditions. Further studies are required to assess these techniques in various environmental conditions and on different crops. v Table of contents List of tables ................................................................................................................................... ix List of figures ................................................................................................................................ xii Acknowledgement ...................................................................................................................... xvii Chapter 1: Introduction ................................................................................................................... 1 Leaf thickness ...................................................................................................................... 5 Capacitive sensors ............................................................................................................... 8 Goal and objectives ........................................................................................................... 10 References ......................................................................................................................... 11 Chapter 2: The developed leaf sensor ........................................................................................... 19 Calibration of the leaf sensor measurement ...................................................................... 24 Effect of temperature on the output of the leaf sensor ...................................................... 24 Leaf thickness unit ................................................................................................. 24 Capacitance unit ..................................................................................................... 25 Quality tests of leaf thickness measurement unit .............................................................. 26 Precision ................................................................................................................. 26 Repeatability........................................................................................................... 27 Accuracy................................................................................................................. 28 Conclusion .............................................................................................................. 28 References ......................................................................................................................... 29 Chapter 3: Leaf thickness to predict plant water status ................................................................ 31 Abstract .............................................................................................................................. 32 Symbols and abbreviations ................................................................................................ 33 Introduction ....................................................................................................................... 33 Materials and methods ....................................................................................................... 38 Plant cultivation.....................................................................................................