Response of evapotranspiration and yield to planting density of solar greenhouse grown tomato in northwest China

• Seasonal total ETc of tomato increased linearly as planting density increased from 3.1 to 5.6 plants m−2.
• The estimated Kc values from Allen and Pereira equations were higher than the calculated from ETc/ET0.
• Tomato is suggested to be planted at 3.7–4.4 plants m−2 in solar greenhouses in northwest China.

Selecting proper planting density can optimize light interception and increase crop water productivity. This research investigated the planting density effect on the evapotranspiration (ETc) and yield of greenhouse grown tomato in northwest China during December 2010 to June 2011 (first season), and September 2011 to January 2012 (second season). Five planting density treatments of 3.1, 3.7, 4.4, 5.0 and 5.6 plants m−2 were used and each treatment was replicated three times. The fifteen plots were randomized complete block design. The irrigation scheduling adopted was the same for all treatments and referred to a soil water depletion of 25 ± 2% below field capacity as observed for the 4.4 plants m−2 treatment. Water was applied beneath the plastic mulch covering both the bed and the furrow. The FAO 56 Penman–Monteith equation with a fixed aerodynamic resistance of 295 s m−1 was used to estimate the reference evapotranspiration (ET0) in the greenhouse. The ETc was calculated by the water balance method according to the change in soil water content over a period of time. The crop coefficients (Kc) at crop development stage in both growth seasons were calculated from ETc/ET0 and estimated by the equations proposed by Allen and Pereira (2009) that consider a plant density coefficient. Tomato yield and fruit number were also measured to assess the planting density effect on water productivity. Results showed that the seasonal total ETc of the greenhouse grown tomato increased, while the seasonal total ETc per plant decreased, linearly as planting density increased in both growth seasons. The calculated middle season Kc values from ETc/ET0 ranged from 0.77 to 0.83 in the first season and from 0.94 to 0.97 in the second season for different planting densities, which were lower than the Kc values estimated by Allen and Pereira's approach. Fruit number per plant, average single fruit weight and yield per plant decreased, but total yield increased, as planting density increased. From economical perspective, the recommended planting densities of tomato grown in northwest China's greenhouses are 3.7–4.4 plants m−2.