Effect of plant growth regulator on electrical impedance spectroscopy during ripening process in kiwifruits

Abstract: Plant growth regulators were used frequently to improve the yield of kiwifruits in recent years. But the use of plant growth regulators often caused low internal quality and deformity of fruits, and the expanded kiwifruits easily decay after softening. It is necessary to explore the effect of plant growth regulators on kiwifruit tissue during the ripening process. Up to now, the traditional chemical methods and the other modern techniques such as NIR, FTIR and NMR have been adopted to analyze the influence of plant growth regulators on fruit tissues. But these approaches are difficult to achieve fast and cheap measurement. Electrical impedance spectroscopy (EIS) is a fast developing method in analyzing materials' characterization. When currents with different frequencies flow through the biological tissue, cytomembrane, the extracellular and intracellular fluid would show different electrical properties. Therefore, EIS has the potential to be used for detecting the change of kiwifruit cell microstructure during the ripening process. To develop a quick, convenient and economic method for exploring the effect of plant growth regulators on kiwifruit fruit, EIS technique was used to analyze the change of biological tissue at cellular level during the ripening process of the expanded and control kiwifruits by using an equivalent electrical circuit. Hayden model was chosen to analyze kiwifruit ripening process, which is composed of extracellular resistance, intracellular resistance and constant phase element representing the cytomembrane. Complex nonlinear least square (CNLS) method was used for fitting the EIS data and determining the parameters of Hayden model. The results indicated that the impedance magnitude of the expanded and control fruits decreased with increasing frequency, and impedance magnitude decreased with kiwifruit ripening only at low frequencies. It was obvious during the ripening process that the impedance of control fruits decreased more quickly than that of the expanded fruits. The phase angles of two types of kiwifruits increased sharply and then decreased quickly with the increase of frequency, and reached the maximum values at 12 kHz. Cole-cole plots of kiwifruit tissues presented the semicircles with different radiuses, which represent the characteristics of typical biological tissue. Cytomembrane impedances of expanded kiwifruits changed minimally during the ripening process of 15 days, but after 7 days, cytomembrane impedances of the control fruits dropped sharply. Extracellular fluid resistance of two types of kiwifruits initially decreased, then increased and finally decreased with ripening process. Extracellular fluid resistance of expanded fruits was larger than that of control fruits, and extracellular fluid resistance of control fruits dropped more quickly than that of expanded fruits after 7 days. Variation of intracellular fluid resistance of two types of kiwifruits was not obvious. Because plant growth regulator could alleviate the damage of cell membrane, EIS properties of two types of kiwifruits were different. So electrical impedance spectroscopy is useful for the identification of expended kiwifruits from untreated kiwifruits by impedance properties. Consequently use of EIS to analyze the ripening process provides a technological foundation for the detection of expanded fruits.