Lettuce plant growth and tipburn occurrence as affected by airflow using a multi-fan system in a plant factory with artificial light

A multi-fan system (MFS) for single culture beds was developed to improve the airflow in a plant factory with artificial light. The MFS had seven fans which were installed on both the front and back sides of culture beds to generate airflow from two opposite horizontal directions. The fans that push the air into the culture bed were air inlets while those that pull the air out of the culture bed were air outlets. In this study, three airflow patterns were evaluated: T₁, the front and back sides of the culture bed were air inlets; T₂, the front side was an air inlet and the backside was an air outlet; and T₃, both the front and back sides were air outlets. A culture bed with no MFS was used as a control (T₄). Lettuce growth and tipburn occurrence were evaluated and leaf boundary layer resistance (1/g_bv), sensible heat flux (S_h), and latent heat flux (L_h) of lettuce plants were estimated. The airflow pattern in T₁ improved the air velocity (V_a) by an average of 0.75 m s^-1 and a variation coefficient of 65%. The 1/g_bv decreased significantly with the increase in V_a, and the lowest value of 54.0 s m^-1 was observed in T₁. The low resistance to heat and moisture transfer enhanced the S_h and L_h of lettuce plants. The average S_h and L_h were 40% and 46% higher in T₁ compared with those in T₄. The fresh and dry weights of lettuce plants in T₁ were 1.13 and 1.06 higher than those in T₄, respectively. No tipburn occurrence was observed in lettuce plants grown under the MFS while five leaves per plant were injured with tipburn in T₄. The results indicated that improving the airflow can improve the growth of indoor cultured lettuce and alleviate the occurrence of tipburn due to the decrease in the 1/g_bv and the increase in the transpiration rate.