Citrulline spray treatments affect tomato and lettuce transplant production

Citrulline is a non-protein amino acid intermediate in the arginine biosynthetic pathway. It has been observed to accumulate in plants such as watermelon, melon, and chickpea under environmental stress. It has also been demonstrated that this amino acid has the capacity to neutralise hydroxyl radicals and protect cellular enzymes from oxidative damage. It has been hypothesised that endogenous citrulline plays a role in facilitating nitrogen assimilation, deposition, and transport in higher plants. However, there is limited information on the effect of exogenous citrulline supplementation on crop plants. This study aimed to evaluate the effect of citrulline spray treatments on the growth of lettuce and tomato seedlings in transplant nursery production. The experiment was repeated over two production cycles during autumn and spring. The seeds of lettuce and tomato were sown in trays filled with a commercial substrate, and the plants were then grown in a greenhouse with an ebb-and-flow system supplementing a complete nutrient solution. The seedlings were sprayed with citrulline at increasing concentrations (0, 10^-5, 10^-4, 10^-3, 10^-2, 10^-1 M) in three distinct growth stages: at the emission of the first true leaf and after 7 and 14 days. Some morpho-physiological and chemical parameters (height, stem size, fresh and dry biomass, number of leaves, leaf area, SLA, WUE, NUE) were evaluated when the seedlings were ready for transplant (5-6-true-leaf stage). The supply of exogenous citrulline via foliar application had a positive effect on the growth rates and biomass accumulation of the seedlings of both species, with some differences according to the citrulline concentration and growth season. The intermediate doses of citrulline (10^-4-10^-3 M) produced a moderate positive effect on these parameters compared with the untreated control (0 M), whereas the highest growth-promoting effect was found at the highest dose of citrulline (10^-1 M), especially during spring. Citrulline also positively affected the contents of mineral elements in the vegetable seedlings. Citrulline foliar treatments were found to exert a growth-promoting effect on tomato and lettuce plants, but further studies are necessary to fully understand its effect on plant growth and metabolism.