IMPORTANCE OF THE GRADIENT IN PHOTOSYNTHETICALLY ACTIVE RADIATION IN A VEGETATION STAND FOR LEAF NITROGEN ALLOCATION IN 2 MONOCOTYLEDONS

Carex acutiformis and Brachypodium pinnatum were grown with a uniform distribution of photosynthetic photon flux density (PFD) with height, and in a vertical PFD gradient similar to the PFD gradient in a leaf c anopy. Distribution of organic leaf N and light-saturated rates of pho tosynthesis were determined. These parameters were also determined on plants growing in a natural vegetation stand. The effect of a PFD grad ient was compared with the effect of a leaf canopy. In Brachypodium, p lants growing in a vegetation stand had increasing leaf N with plant h eight. However, distribution of leaf N was not influenced by the PFD g radient treatment. The gradient of leaf N in plants growing in a leaf canopy was not due to differences within the long, mostly erect, leave s but to differences between leaves. In Carex, however, the PFD gradie nt caused a clear increase of leaf N with height in individual leaves and thus also in plants. The leaf N gradient was similar to that of pl ants growing in a leaf canopy. Leaf N distribution was not affected by nutrient availability in Carex. In most cases, photosynthesis was pos itively related to leaf N. Hence, light-saturated rates of photosynthe sis increased towards the top of the plants growing in leaf canopies i n both species and, in Carex, also in the PFD gradient, thus contribut ing to increased N use efficiency for photosynthesis of the whole plan t. It is concluded that in Carex the PFD gradient is the main environm ental signal for leaf N allocation in response to shading in a leaf ca nopy, but one or more other signals must be involved in Brachypodium.