The Effect of Irradiating Adaxial or Abaxial Side on Photosynthesis of Rose Leaves

In many cropping systems most of the light irradiates the adaxial side of leaves. However, in cropping systems with intra canopy lighting a reasonable fraction of light may irradiate even the abaxial side of the leaves. The aim of this study was to investigate the effect of irradiating the abaxial leaf side compared to irradiating the adaxial side, in rose plants grown in glasshouse with the bending technique. The instantaneous effects on the optical properties and the light response of photosynthesis were analysed in intact leaves. Results demonstrated that the rate of net photosynthesis was higher when leaves were lighted from the adaxial side compared to the abaxial side. This was the consequence of both a higher light absorption and higher quantum yield (photosynthesis per unit absorbed light) in adaxial-lighted leaves. INTRODUCTION In many greenhouse crops, manipulations of the plant architecture are applied to increase the light interception and to optimize the photosynthesis efficiency of the different leaf layers (Buck-Sorlin et al., 2011). For instance, in cut rose crops, the shoot bending results in increased photosynthetically active surface per plant, by forming an extended horizontal canopy (Kim et al., 2004). Bent canopy has proven to function as a source of assimilates for the growth of upright flower shoots, mainly in low light intensity conditions (Baille et al., 2006; González-Real et al., 2007). This technique can result in increases of the plant growth rate and the number and length of flower stems (Kool and Lenssen, 1997). According to Aikman (1989), plant productivity can be enhanced if light is uniformly provided along the vertical profile of the plant and penetration of light into the canopy is increased, preventing the lower and inner leaves being below the light compensation point and the upper and outer leaves approaching the saturation point. However, the final effect of the vertical light distribution on crop photosynthesis may depend on the season (Sarlikioti et al., 2011). Inter-lighting, performed by putting lamps in the row in between the canopy, leads to higher light intensities in the lower part of the canopy. This has been successfully used in a number of vegetable crops (Hovi-Pekkanen et al., 2006; Hovi-Pekkanen and Tahvonen, 2008; Trouwborst et al., 2009). This lighting strategy irradiates both the adaxial (upper) and abaxial (lower) leaf sides. In most of the plant species with upright growth, the adaxial and the abaxial leaf surfaces develop and function in different environments, with respect to the light intensity (Photosynthetic Photon Flux Density, PPFD) and quality (wavelength composition). Adaxial sides are exposed to more direct radiation (from solar or artificial lighting), with higher light intensity and broader spectra. Abaxial sides are shaded by the leaf blade itself and receive only about 10% of the light incident to the adaxial ones. This light reaches the Proc. 7 IS on Light in Horticultural Systems Eds.: S. Hemming and E. Heuvelink Acta Hort. 956, ISHS 2012 158 abaxial side after it has been transmitted through the mesophyll (self-transmitted light) or reflected from the surroundings (mainly by leaves), and contains relatively much green light (Pospíšilová and Solárová, 1987). However, when inter-lighting is used a considerable amount of direct light may reach the abaxial side of the leaves. Measurements of photosynthesis on plant crops mainly concern lighting from the adaxial leaf side. Most of the studies on the response to light stimuli to the different leaf sides have been carried out on C4 metabolism plants (Driscoll et al., 2006; Soares et al., 2008) more than on C3 species (Morr and O’Leary, 1984; Wang et al., 2008). Although, several researches have been reported on field grown plant species (Terashima, 1986; Postl and Bolhar-Nordenkampf, 1992), only little information seems to be available on greenhouse crops. In some hypostomatic species, like rose is (Pandey et al., 2007), under equal light intensity, photosynthesis is higher when light is provided from the adaxial rather than the abaxial side and the difference increases as the irradiance increases (Syvertsen and Cunningham, 1979; Proietti and Palliotti, 1997). However, as far as we know, no data on abaxial lighting of leaves is available on rose, particularly in greenhouse grown plants. The aim of the experiment was to investigate the optical properties and the light response of photosynthesis (instantaneous effects) in intact leaves of rose, hydroponically grown with the bending technique, in relation to the leaf side lighted (adaxial vs. abaxial). MATERIALS AND METHODS Plant Material and Growth Conditions The experiment was carried out in Wageningen (The Netherlands, 51°97’N; 5°67’E), in a heated experimental glasshouse. Rose plants (Rosa hybrida L.) cultivar ‘Akito’ for cut flowers were grown on rockwool slabs. Cuttings were transplanted on 25 February in double rows, at the plant density of 6.5 plants m. Plants were structured following the bent shoot technique, bending the weaker or blind stems down into the paths and leaving the harvestable flower stems to form the upright canopy (Kool, 1997). During the experimental period, from the beginning of October to the end of November, the temperature inside the greenhouse was 21.1°C on average during the day (heating set point 21°C) and 18.2°C during the night (heating set point 17.5°C). Supplemental lighting by HPS lamps (Philips SON-T Green Power 600 W; Koninklijke Philips Electronics N.V., The Netherlands) provided a light intensity of 150 μmol m s at the canopy level (switching off threshold 250 W m of outside global radiation), extending the natural day-length to 16 hours (3:00 till 19:00). During the daytime, the relative humidity was kept around 70%, by using a mist system, and the air CO2 concentration was enriched to 500 ppm approximately. Water and fertilizers were supplied via a drip-system, which was automatically controlled by a fertigation computer. Details on crop management are reported by Paradiso et al. (2011). Measurements Photosynthesis measurements were carried out during the sixth week of the growing cycle (starting from October, 6 when upright shoots were cut back), on stems with 13 to 16 leaves, with small flower bud already visible. Middle age leaves, pentafoliate, from the 6 to the 8 from the top of the upright stem, were considered. Net photosynthesis was measured on the top leaflet, with a portable photosynthesis open system (LCpro ADC, UK), connected to a 6.25 cm leaf chamber. The leaf chamber was lighted by a LED array of about 15% blue (around 465 nm) and about 85% red light (around 655 nm). Light saturation curves were performed at decreasing levels of light intensity, at the following theoretical PPFD values: 1500, 1000, 500, 250, 100, 50 and 0 mol m s, by using a LED array. The actual values recorded in the leaf chamber were: 1395, 930, 465, 232, 93, 46, 0 μmol m s. Measurements lasted