Photosynthetic Activity and N Metabolism of Lettuce as Affected by Humic Acid

The mechanism by which humic substances stimulate plant biological activities is not clear. To study effects of humic acid on N metabolism and growth of lettuce (Lactuca sativa L.) a hydroponic experiment was conducted under greenhouse conditions. Humic acid (HA) at 0, 100, or 1000 mg·L−1 was added with a nutrient solution to plants. HA accelerated uptake of N and NO3 and accelerated N metabolism by enhancing nitrate reductase activity, which resulted in production of protein. HA did not affect P absorption and proline content of leaves. The photosynthetic activity of lettuce increased with all levels of HA due to enhancement of chlorophyll content and mesophyll conductance. HA can stimulate N metabolism and photosynthesis activity of lettuce to improve yield.