The influence of light intensity and leaf age on the photosynthetic capacity of leaves within a tomato canopy

In dense crop stands, the decrease in leaf photosynthetic capacity (Amax) is paralleled by a decrease in photosynthetically active photon flux density (PPFD) and an increase in leaf age. In greenhouse horticulture, assimilation lighting is traditionally applied from above the canopy. Recently a new lighting technique has been developed in which assimilation lighting is applied within the canopy: intracanopy lighting. This development raises the question whether the decrease in the Amax of lower, thus older and shaded, leaves in a crop is solely due to the lower PPFD, or also partly due to ageing of these leaves. We investigated whether leaf ageing influenced changes in the Amax of tomato leaves during their usual life-span during cultivation in commercial crop systems (i.e., up to 70 d). To uncouple leaf age from the PPFD, tomato plants were grown horizontally, so that the PPFD was similar for all leaves. To investigate the effect of PPFD during leaf development (PPFDLD), Amax-leaf age profiles were determined for the leaves of plants grown under conditions with distinctly different natural patterns of PPFD (i.e.,Winter, early Spring, and late Spring). In addition, in half of the plants used per experiment, all fully-developed leaves were shaded to 25% of the normal PPFD in the greenhouse using a neutral density filter. Photosynthetic capacity and chlorophyll contents were higher in late Spring than in Winter, but were hardly affected by leaf age. In early Spring, the Amax and chlorophyll contents were higher in younger leaves than in older leaves. To a large extent, this was due to the differences in PPFDLD, and hardly due to leaf ageing. Shading fully-developed, mature leaves dramatically decreased their Amax and chlorophyll contents within a few days. We conclude that, during the normal 70 d life-span of tomato leaves in commercial cultivation, the decrease in PPFD within the canopy, and not leaf-ageing, is the most important factor causing changes in Amax with canopy depth.