Light-flecks cause non-uniform stomatal opening - studies with special emphasis on Fagus sylvatica L.

The appearance of stomatal patchiness in response tn rapid (seconds) change s in light has been studied in European beech, Fagus sylvatica L., and, by comparison, in a further 17 different woody species from the understorey of a European beech forest, using a simple water infiltration method. Water i nfiltrated areoles indicate open stomata. Since infiltration changes optica l characteristics of a leaf section it can be analysed by photography, comp uter-aided image analysis and by weighing. For F. sylvatica clear differenc es were found between infiltration of cotyledons (no patchy pattern) and an y other leaf type. Despite identical cultivation, leaves of the same type a nd age from different individual plants responded differently to applicatio n of 30 s of light after darkness. In contrast, the patchiness patterns wer e very similar for leaves of the same type originating from the same plant. Infiltration patterns after a light-fleck, observed on different leaves as a series of momentary clusters, probably indicate waves of opening stomata moving across the leaf blade. During and after a 30 s light-fleck infiltra tion increased and it continued to increase in the dark up to 10 min, indic ating increasing stomatal opening over that period. In general, shade leave s became more infiltrated (by weight) than half-shade or sun leaves, due to larger intercellular air spaces. All species, without exception, showed pa tchy infiltration and, thus, non-uniform stomatal opening. Measuring leaf g as exchange (as "quasi-steady states" using a fast responding system) durin g photosynthetic induction resulted in very similar CO2 responses of net ph otosynthesis (A/c(i)) as in the true steady state, proving that, in shade a nd half-shade leaves, the presence of stomatal patchiness does not necessar ily affect the calculation of intercellular CO2 concentrations. Causes and consequences of stomatal patchiness are discussed.