Proc. III IS Rose Research Eds. N. Zieslin & H. Agbaria Acta Hort. 547, ISHS 2001 319 OPTIMISING LAI IN BENT ROSE SHOOTS H. Pien, E. Bobelyn and R. Lemeur Laboratory of Plant Ecology, Ghent University Coupure Links 653, 9000 Ghent Belgium M.C. Van Labeke Research Centre for Ornamental Plants Schaessestraat 18, 9070 Destelbergen, Belgium Keywords : Rosa hybrida cv. Frisco, leaf photosynthesis, upscaling to canopy level, arching, leaf area index Abstract The arching technique for cut rose cultivation knew a great success due to a higher yield and a better quality of the flowering shoots. The success is generally attributed to a higher carbohydrate content supplied to the flowering stem by the bent photosynthesising leaves. However, the photosynthesis rate of shaded (and older) leaves diminishes. As a consequence the undermost bent stems might not contribute to the carbon gain, especially under low natural light intensities. In January, the photosynthetic photon flux density (PPFD) was measured above, in the middle of, and under the leaf mass of bent shoots of Rosa hybrida cv. Frisco. The measurements showed an exponential light extinction pattern. The leaf area index (LAI) of the bent shoots, simultaneously registered, varied from 1.5-2 in the middle leaf layer to 4-5 under the bent shoots. Photosynthesis rate was measured at different light intensities for leaves of the upper and the lower leaf level of the bent shoots. The saturated photosynthesis rate in the lower bent layer was reduced to ca. 15 % of the CO 2 exchange measured for upper leaves of the bent shoots. The dark respiration rate diminished and the light compensation point increased in leaves of the lower bent layer. In light limited circumstances, the intensity in the lower layer can drop below the light compensation point. Hence, the carbon balance of the lower leaf layer can become negative. Mathematical upscaling of the leaf photosynthesis rate to bent canopy level revealed that the contribution of the lower bent layer to the total carbon gain of the bent shoots was negative. Hence, it might be better to restrict the LAI. Based on our results, an LAI ranging between 1 and 3.5 is advised for the bent rose shoots in order to optimise the amount of photosynthesising leaves on bent shoots. 1. Introduction The arching technique is an advanced method for cut rose cultivation that was developed in the late 80’s in Japan. The technique knows a great success (Okhawa & Suematsu, 1999). Bending down the rose shoots instead of pruning results in rose stems of a better marketing quality (Okhawa & Suematsu, 1999, Särkkä & Rita, 1999), less blind shoots and a higher yield (Särkkä & Rita, 1999). The success is generally attributed to a higher carbohydrate content supplied to the flowering stem by the bent photosynthesising leaves (Särkkä & Rita, 1999). Indeed, an increase in assimilate supply, induced by a greater number of mature leaves on the primary shoots, results in thicker and larger axillary shoots (Marcelis-Van Acker, 1994). Apart from the increased assimilate supply an altered hormone balance might influence the flower development (Kool, 1996), as was shown in Japanese pear (Ito et al., 1999). However, it is also known that the photosynthesis rate of shaded leaves changes. In shade-grown leaves, light saturated photosynthesis rate occurs at much lower photosynthetic photon flux densities (PPFD) than in leaves grown in full sunlight (Björkman, 1981, Jones, 1992, Lambers et al., 1998). At the same time, respiratory losses