Leaf Dark‐Adapter for Measuring Chlorophyll Fluorescence in Situ during Daytime

Chlorophyll a fluorescence is used as an indicator of stress damage to growing plants. Plants or plant samples first must be transported to a laboratory and dark adapted. If stress (e.g., drought) has not been severe enough to permanently damage the photosynthetic apparatus, plants may recover during dark adaption and stress may not be detectable. In situ readings are more reliable. Dark adaptation takes place after the onset of total darkenss. The objective of this work was to develop a device that would enable the dark adaptation of plant leaves in situ during hours when stress could occur. A thin, light weight sheet of metal was developed to clip onto corn ( Zea mays L.) leaves and allow dark adaptation during daylight. A sliding shutter was provided to expose a section of the leaf to light emitted from the fluorometer probe. Using the device, field measurements of daytime chlorophyll a fluorescence were obtained on attached leaves. Measurements were also taken at night on the same plants after dark‐adaptation. In situ day measurements were significantly lower than those taken at night, indicating that stress during daytime influenced the fluorescence response and that plants tended to recover before nighttime readings were taken. Daytime in situ measurements provide an opportunity to detect non‐permanent damage to the photosynthetic apparatus that may show transient drought effects and lowel photosynthetic activity.