Integrating transient heterogeneity of non-photochemical quenching in shade-grown heterobaric leaves of avocado (Persea americana L.): responses to CO2 concentration, stomatal occlusion, dehydration and relative humidity.
Long-lived shade leaves of avocado had extremely low rates of photosynthesis. Gas exchange measurements of photosynthesis were of limited use, so we resorted to Chl fluorescence imaging (CFI) and spot measurements to evaluate photosynthetic electron transport rates (ETRs) and non-photochemical quenching (NPQ). Imaging revealed a remarkable transient heterogeneity of NPQ during photosynthetic induction in these hypostomatous, heterobaric leaves, but was adequately integrated by spot measurements, despite long-lasting artifacts from repeated saturating flashes during assays. Major veins (mid-vein, first- and second-order veins) defined areas of more static large-scale heterogeneous NPQ, with more dynamic small-scale heterogeneity most strongly expressed in mesophyll cells between third- and fourth-order veins. Both responded to external CO2 concentration ([CO2]), occlusion of stomata with Vaseline™, leaf dehydration and relative humidity (RH). We interpreted these responses in terms of independent behavior of stomata in adjacent areoles that was largely expressed through CO2-limited photosynthesis. Heterogeneity was most pronounced and prolonged in the absence of net CO2 fixation in 100 p.p.m. [CO2] when respiratory and photorespiratory CO2 cycling constrained the inferred ETR to ~75% of values in 400 or 700 p.p.m. [CO2]. Likewise, sustained higher NPQ under Vaseline™, after dehydration or at low RH, also restricted ETR to ~75% of control values. Low NPQ in chloroplast-containing cells adjacent to major veins but remote from stomata suggested internal sources of high [CO2] in these tissues.
Laboratory of Physiological Green Systems, Department of Biomechanical Systems, Faculty of Agriculture, Ehime University, 3-5-7, Tarumi, Matsuyama, 790-8566 Japan.
Kotaro
K
Takayama
Diana
D
King
Sharon A
SA
Robinson
Barry
B
Osmond
D002245:Carbon Dioxide / Q000494:pharmacology*; D002734:Chlorophyll / Q000378:metabolism; D003681:Dehydration; D004579:Electron Transport; D006813:Humidity; D008027:Light; D027421:Persea / Q000033:anatomy & histology / Q000187:drug effects / Q000502:physiology*; D010577:Petrolatum / Q000378:metabolism; D010788:Photosynthesis / Q000502:physiology*; D018515:Plant Leaves / Q000033:anatomy & histology / Q000187:drug effects / Q000502:physiology; D054046:Plant Stomata / Q000033:anatomy & histology / Q000187:drug effects / Q000502:physiology*; D018526:Plant Transpiration / Q000502:physiology*; D014867:Water / Q000502:physiology; D024341:Xanthophylls / Q000378:metabolism
D024341:Xanthophylls; D014867:Water; D002734:Chlorophyll; D002245:Carbon Dioxide; D010577:Petrolatum
Avocado; Heterobaric anatomy; Patchy stomata; Photosynthetic induction; Vaseline™; Xanthophyll pigments