Development of microphotoacoustic techniques and studies of photosynthetic properties of carbon plants

The theory of photoacoustic spectroscopy was examined as currently applied in photosynthesis research. A transpiration component of the photoacoustic signal within leaves was detected. Discrepancies between energy storage measured at low and high frequencies were found using standard photoacoustic measurements. Based upon these findings, a new modified photoacoustic theory is proposed.; A micro-scanning photoacoustic spectrometer was developed in order to examine the photosynthetic properties of individual cell layers in leaves. By projecting microscopic linear laser beams (40–50 μgm wide) paradermally on cross sections of leaves, the new instrument can measure: (1) chlorophyll distribution; (2) the light absorption profile; (3) gradients of oxygen evolution; (4) the quantum efficiency for oxygen evolution. Some of these photosynthetic parameters (i.e. gradients of oxygen evolution) can be assayed only by this new technique.; Photoinhibition within intact leaves was studied by measuring changes in the O₂ evolution profile across sun and shade leaves of spinach after high-light treatments. Sun leaves were more resistant to high light than shade leaves. Photosynthetic tissues within the leaf showed differential sensitivity to excess light. In sun leaves, abaxial tissues were less resistant than adaxial tissues, while in shade leaves, there was not much difference between the two tissue types. The inhibition of photosynthesis caused by excess excitation energy occurred only when light intensity was very high. The extent of photoinhibition within the mesophyll tissues directly followed the light gradient within the leaf.; Comparison of the profile of O₂ evolution with that Of CO ₂ fixation and changes after photoinhibitory treatment raises several questions that cannot be answered by current models for whole-leaf photosynthesis: (1) there is an apparent mismatch between the CO₂ fixation profile and the light gradient across the leaf; (2) there is a mismatch between peaks in CO₂ fixation and O₂ evolution; (3) there is disagreement between responses of CO₂ fixation and O ₂ evolution to photoinhibition.; To resolve these discrepancies, I propose a conceptual model for energy transfer in leaves of C₃ plants: a shuttle, consisting of 3-PGA and 3-PGald/DHAP have the potential to transport chemical energy trapped by the light reactions between tissues. In this manner, excess light energy can be captured in tissues, and exported to tissues where light is limiting to photosynthesis. Such a mechanism would facilitate maximum rates of carbon fixation within the leaf and would allow spatial uncoupling between light absorption and carbon fixation. Experimental results obtained thus far are consistent with the operation of a triose phosphate shuttle, but more experimentation is needed to prove its existence.