High resolution imaging of photosynthetic activities of tissues, cells andchloroplasts in leaves

Through imaging of chlorophyll fluorescence, it is possible to produce para meterized fluorescence images that estimate the operating quantum efficienc y of photosystem II (PSII) photochemistry and which can be used to reveal h eterogeneous patterns of photosynthetic performance within leaves. The oper ating quantum efficiency of PSII photochemistry is dependent upon the effec tive absorption cross-section of the light-harvesting system of PSII and th e photochemical capacity of PSII, The effective absorption cross-section is decreased by the process of down-regulation, which is widely thought to op erate within the pigment matrices of PSII and which results in non-photoche mical quenching of chlorophyll fluorescence. The photochemical capacity is non-linearly related to the proportion of PSII centres in the 'open' state and results in photochemical quenching of chlorophyll fluorescence. Example s of heterogeneity of the operating quantum efficiency of PSII photochemist ry during the induction of photosynthesis in maize leaves and in the chloro plast populations of stomatal guard cells of a leaf of Tradescantia albifor a are presented, together with analyses of the factors determining this het erogeneity. A comparison of the operating quantum efficiency of PSII photoc hemistry within guard cells and adjacent mesophyll cells of Commelina commu nis is also made, before and after stomatal closure through a change in amb ient humidity.