The speaking plant approach (SPA) is regarded as a desirable concept for environmental control in highly sophisticated greenhouses. An important feature of SPA is sensor-based plant diagnostic techniques, which are used to monitor plant physiological status. Although the concept of SPA was originally proposed approximately 30 years ago, the practical application of SPA techniques in commercial greenhouses has only recently become feasible, due to the innovation of easy-to-use devises and information and communication technologies in the present decade. In this review, two prospective plant diagnostic techniques that can be utilized for the SPA, and their applications in semi-commercial greenhouses, are introduced. The first technique is plant diagnosis based on a plant’s odor. Plants emit a broad range of volatile organic compounds and a plant’s emission profile changes depending on its physiological status. The second technique is chlorophyll fluorescence imaging. This technique measures the chlorophyll fluorescence induction phenomenon -a dynamic change in chlorophyll fluorescence intensity induced by illuminating a dark-adapted leaf with a stable intensity excitation light- and analyzes the shape of the induction curve. In addition, new challenges for an SPA-based mega-scale greenhouse are discussed.
