Rapid adjustment in epidermal UV sunscreen: Comparison of optical measurement techniques and response to changing solar UV radiation conditions.

The accumulation of soluble and cell-wall bound UV-absorbing compounds (i.e., flavonoids) in the epidermis and the mesophyll of leaves is a response of plants to UV exposure. These compounds are known to function in UV screening, but they are also of potential value for food quality. One way to non-destructively monitor UV screening in leaves is by optical methods, from which UVA-PAM and Dualex instruments stand out. The degree and rapidity to which plants can modulate UV screening in response to fluctuating solar UV conditions is poorly understood. In this study, okra plants were exposed to two solar radiation treatments (near-ambient UV [+UV] and attenuated UV [-UV]) and the epidermal UV transmittance (TUV ; UVA-PAM) and flavonoid index (Dualex) were measured in the youngest and second youngest mature leaves over three consecutive days and within an individual day. The day-to-day (measured near solar noon) and diurnal (over the course of a day) measurements of leaf optical properties indicated that TUV decreased and flavonoid index increased in the adaxial epidermis ~50% until 15:00 CDT then returned close to morning values later in the day. Correlations between UV-B radiation and TUV and flavonoid index revealed highest values 30 min to 1 h prior to the measurements. These findings indicate that plants can respond quickly to fluctuating solar UV conditions and underlines the importance of the harvest-time point for health-promoting compounds in fruit and vegetables. Our findings also indicate that the UVA-PAM and the Dualex instruments are both suitable instruments to monitor rapid changes in UV screening in plants.