Effect of crop density on growth and light interception in greenhouse lettuce

Greenhouse experiments were carried out in February-April 1996 and in November 1999-February 2000 in Central Italy (Perugia, 43degreesN, elev. 165 in) on butterhead lettuce [Lactuca sativa L. var. capitata (L.) Janchen] grown at four different plant densities (5, 10, 15, 30 plants m(-2)) in order to evaluate crop growth and yield as related to the photosynthetically active radiation (PAR) intercepted by the crop. Lettuce was sown on 5.2.1996 and 5.11.1999. Water, nutrients, weeds, pests, and diseases were not limiting factors. PAR fluxes and air temperature both outside and inside the greenhouse were recorded. Crop growth was determined every 7-14 days until final harvest by destructive samplings of plants. Crop PAR interception was calculated on a crop ground cover basis. Crop radiation use efficiency (RUE) was calculated as the slope of the linear regression of dry weight on cumulative intercepted PAR. Increasing plant density increased LAI, ground cover, PAR interception and biomass yield. The reciprocal of plant weight decreased linearly with plant density. Growth parameters were related to each other according to relationships that are in line with those reported in literature for greenhouse and field grown lettuces. Results are discussed considering greenhouse light and temperature environments. The RUE was not significantly affected by plant density, and therefore the increase in biomass yield with density has mainly to be ascribed to increased PAR interception.