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 photosyn- thesis were of limited use, so we resorted to Chl fluorescence imaging (CFI) and spot measurements to evaluate photosyn- thetic electron transport rates (ETRs) and non-photochem- ical quenching (NPQ). Imaging revealed a remarkable transient heterogeneity of NPQ during photosynthetic in- duction 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 fouth-order veins. Both responded to external CO2 concentration ([CO2]), occlusion of stomata with VaselineTM, leaf dehydra- tion and relative humidity (RH). We interpreted these re- sponses in terms of independent behavior of stomata in adjacent areoles that was largely expressed through CO2-limited photosynthesis. Heterogeneity was most pro- nounced 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 VaselineTM, after dehydration or at low RH, also re- stricted ETR to -75% of control values. Low NPQ in chloro- plast-containing cells adjacent to major veins but remote from stomata suggested internal sources of high [CO2] in these tissues.