Photoacoustic spectra of needles as an indicator of the activity of the photosynthetic apparatus of healthy and damaged conifers

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Abstract
Photoacoustic spectra were measured at chopping frequencies of the excitation light at 22 and 515 Hz in order to detect differences between needles from healthy and damaged conifers (Abies albaMill, andPicea abiesKarst.) with respect to pigment distribution and possibly also to photosynthetic activity. Fully green needles of healthy trees exhibit photoacoustic spectra at 22 Hz with a maximum in the red absorption region of the chlorophyll. This maximum is lost with increasing damage to needles and chlorophyll breakdown. The photoacoustic spectra at 22 Hz of the damaged needles therefore are characterized by a higher signal in the blue‐light region as compared to that in the red‐light region. This can be quantified by forming the ratio of the photoacoustic signals at 675 and 475 nm (ratio PA 675/475). The needles of the damaged trees possess a lower photosynthetic activity, as seen from the CO2fixation rate and the variable fluorescence (Rfd‐values). It is assumed that the changes in the PA‐spectra of the needles from damaged trees are the result of this decline of the photosynthetic apparatus and its function. In contrast to the PA‐spectrum at 22 Hz the PA‐spectra at 515 Hz, where the PA‐signals primarily emanate from the epidermal layer, exhibit a different shape with a maximum near 550 nm. In the needles from the damaged trees the PA‐signals are higher, particularly in this maximum range of the spectrum, than those of fully green needles from healthy trees.