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The rates of net photosynthesis per unit ground area by a closed canopy of tomato plants were measured over a range of natural light flux densities. The canopy, of leaf area index 8.6, was divided into three horizontal layers of equal depth. On successive days the canopy was progressively defoliated in layers from the ground upwards, allowing the photosynthetic contribution from individual leaf layers to be determined. The uppermost layer, 23% of the total leaf area, assimilated 66% of the net CO2 fixed by the canopy and accounted for a similar percentage of the total leaf respiration. Net photosynthesis versus light response curves for individual leaves from different positions within the canopy were also obtained. Leaf conductances to CO2 transfer and the dark respiration rates of leaves from the uppermost leaf layer were approximately ten times those from the lowest layer. The canopy data were analysed using a simple model which assumed that the canopy was composed of leaves with identical photosynthetic and respiratory characteristics. The model fitted the data and allowed the characteristics of an ‘idealized’ leaf to be estimated. The estimated values of the leaf light utilization efficiency, α,and the leaf conductance CO2 transfer, τ, were similar to values directly determined for individual leaves in the uppermost leaf layer and the estimated rate of leaf dark respiration, Rd, corresponded to measured rates for leaves much lower in the canopy. The simple model may be used to examine gross effects of crop environment on the leaf photosynthetic characteristic of an ‘idealized’ leaf, but cannot be used to predict accurately canopy net photosynthesis from the photosynthetic and respiratory characteristics of any single real leaf. A more detailed model, developed to allow explicitly for the observed variation in τ and Rd within the canopy is appropriate for this purpose.
Acock et al. (Sun,) studied this question.