Effects of quantity and daily distribution of irradiance on photosynthetic electron transport and CO2 fixation in tomato

Abstract

The purpose of this work was to analyse in situ the acclimation of the different components of the photosynthetic apparatus of tomato (Lycopersicum esculentum var Moneymaker) to the PAR environment. Tomato plants were grown under computer controlled banks of lamps that permitted the generation of a sinusoidal daily irradiance environment. Two daily integrals of irradiance (19 and 28 mol m-2) were combined with two photoperiods (8 and 16 hours) to give four different irradiance environments with maximum irradiances of 510, 760, 1020 and 1520 mmol m-2 s-1. The CO2 fixation, the quantum yields for electron transport by photosystems I and II, qQ, Fv?/Fm?and the rate constant for electron transport of plants grown under these differing irradiance environments were then measured using a sinusoidal irradiance regime similar to that to which they had been acclimated. Leaf absorbance, chlorophyll concentration and chlorophyll a/b ratios were also measured. Different components or processes of photosynthesis responded differently to changes in daily integral or photoperiod. Given the relationships between the different components of the photosynthetic apparatus, this resulted in a complex set of changes in the detailed functioning of the different leaves. The result, however, was that the CO2 fixation / irradiance relationship was similar for plants with the same daily integral of irradiance despite the doubling of the peak irradiance as the photoperiod was halved from 16h to 8h. Likewise, though the relationship between electron transport (as the product of photochemical efficiency and irradiance) and CO2 fixation was broadly similar regardless of the environment, there were differences between plants that depended on daylength and not the daily integral.