Mechanism of soil moisture temporal variance affecting water use efficiency of romaine lettuce (Lactuca sativa L. var. longifolia)

Objective  Stable soil moisture is beneficial for improving crop water use efficiency. Here, we investigate the mechanism of how stable soil moisture affect water and nutrient utilization efficiency of romaine lettuce.

Method The pot experiment was conducted in a screen house using romaine lettuce (Lactuca sativa L. var. longifolia) as the experimental crop. Traditional irrigation (TI) and negative pressure irrigation (NPI) were set up to provide fluctuating and stable soil moisture. At the 4-leaf stage of romaine lettuce growth, the treatments were imposed. We used a soil moisture meter to measure the soil moisture content every two days. The crop was harvested 28 days after treatment. The plant height, leaf number, and the most extended leaf length and width were measured on the 1st, 11th, 21st, and 28th day of treatment imposition. The net photosynthetic rate (P_n), intercellular CO₂ concentration (C_i), transpiration rate (T_r), and stomatal conductance (G_s) of romaine lettuce leaves were measured via a Li-6400 portable photosynthesis system simultaneously. The free proline (Pro), malondialdehyde (MDA), abscisic acid (ABA), salicylic acid (SA), soluble protein (SP), soluble sugars (SS), and stable carbon isotope ratios (δ¹³C) and discrimination (Δ¹³C) in romaine lettuce leaves were determined.

Result The cumulative amount of irrigation and mean soil moisture content under NPI and TI were similar. However, the coefficient of variation (CV) of soil moisture content under NPI was 5.0%, indicating the stable soil moisture, and was 10.3% under TI, showing fluctuating soil moisture. Plant height, the most extended leaf length, and leaf width of romaine lettuce in NPI were 103.8%, 155.4%, and 62.5% higher than in TI. The P_n, C_i, T_r, and G_s values in NPI were higher than in TI. Similarly, the T_r and G_s on the 11th day and C_i on the 28th day in NPL were (P<0.05) higher than in TI. The yield, water-use-efficiency, leaf P content, the NPK uptake, and the δ¹³C were (P<0.05) higher in NPI than in TI at harvest, while root/shoot ratio, ABA, SS, and Δ13C were (P<0.05) lower than in TI. Δ¹³C was positively correlated with water-use efficiency and negatively correlated with the CV of soil moisture.

Conclusions Compared with the fluctuating soil moisture in TI, the negative pressure irrigation created stable soil moisture, thus avoiding the possible temporal drought stress for romaine lettuce. This promoted photosynthesis, nutrient uptake, and water-use-efficiency of romaine lettuce and achieved high shoot growth.