The use of photothermal radiometry in assessing leaf photosynthesis: I. General properties and correlation of energy storage to P₇₀₀ redox state

Energy storage measurements by modulated photothermal radiometry (PTR) were carried out on intact leaves to assess the value of the PTR method for photosynthesis research. In particular, correlations to the redox state of P₇₀₀ under various conditions were examined. PTR monitors modulated light conversion to heat by sensing the resulting modulated infra-red radiation emitted from the leaf. It is, therefore, a complementary method to photoacoustics for estimating energy storage and its time variation, particularly under controlled leaf atmosphere. With modulated light-1 (λ>690 nm) the energy storage approached zero and P₇₀₀ was maximally oxidized. When background light of shorter wavelength (λ<690 nm-light-2) was added, energy storage momentarily increased (a manifestation of Emerson enhancement) while P₇₀₀ was reduced. The values of both parameters varied as a function of the background light intensity, keeping a mutual linear relationship. Following the initial change, there was a slow reversal transient of P₇₀₀ oxidation with a parallel decrease in energy storage. Temporal correlation to P₇₀₀ redox state after dark adaptation was observed also for the energy storage measured in modulated light 2 when combined with background actinic light of medium intensity (about 50 W m²). Under these circumstances P₇₀₀ was almost totally oxidized initially and then gradually reduced while energy storage was initially low and then increased parallel to P₇₀₀ reduction. A comparison between the maximum energy storage in modulated light 1, enhanced by background light 2, to the energy storage with short wavelength light (where light tends to be more evenly distributed) indicates a comparable contribution to energy storage from each active photosystem. The above experiments indicate that energy storage contribution from PS I is directly related to the extent of openness of its reaction-centers. While some aspects of the data call for more experimentation, these experiments already establish PTR as a valuable method to monitor photosynthetic energy storage activity in vivo, particularly when used simultaneously with other non-invasive methods.