Optimising LAI in bent rose shoots

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 CO2 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