Chlorophyll Fluorescence Imaging for Health Condition Monitoring of Tomato Plants in Greenhouse

Chlorophyll fluorescence is red light emitted from chlorophyll a pigments in green leaf. Hence, accurate measurement of chlorophyll fluorescence allows us to assess photosynthetic functions, which can be used as indicator of a plant's health conditions. By illuminating a dark-adapted green leaf with an excitation light at a stable intensity, a dynamic change in chlorophyll fluorescence intensity can be observed. This phenomenon is called "chlorophyll fluorescence induction phenomenon" and the time course of the chlorophyll fluorescence intensity during this phenomenon is called "induction curve". The shape of the induction curve varies depending on the plant's health conditions. In this study, we developed a chlorophyll fluorescence imaging system to monitor health conditions of tomato plants in greenhouse by measuring the induction curves. First, we applied this system to detect physiological changes caused by high light treatment at the whole plant level to check its performance. Then, we extended this system to the plant canopy level. A 60x60 cm LED panel provided measuring light to illuminate the leaves around the shoot apex of the tomato plant. The induced chlorophyll fluorescence was measured by a CCD camera fixed in the middle of the LED panel. The intensity of the measuring light at the plant surface was about PPFD 100 mu mol m(-2) s(-1). All of the chlorophyll fluorescence imaging was performed at night. We measured the induction curves of 20 tomato plants grown in a north-south cultivation lane. Most of the induction curves of the tomato plants showed common shapes. However, two plants showed unusual shapes of induction curves. Seven days after the induction curve measurement, a visible symptom of physiological dysfunction was observed in one of the two plants. This study showed that chlorophyll fluorescence imaging is a promising technique for monitoring of health conditions of tomato plants in a greenhouse.