Castor bean cake increases osmoprotection and oil production in basil (Ocimum basilicum) under saline stress

Abstract

Water salinity is a major abiotic stress that impacts plant growth and metabolism, reducing crop production. However, the adequate use of organic fertilizers rich in nitrogen (N) can attenuate the damages caused by saline stress. In this context, the castor bean cake (CBC), a co-product of the castor oil production chain, has shown great potential as organic fertilizer and stress mitigator, however, the mechanisms involved in these responses remain unclear. Thus, this study aimed to investigate the effects of different doses of CBC and electrical conductivities of irrigation water (EC) on the photochemical efficiency of photosystem II, the accumulation of organic solutes, and the oil production in basil. The experimental design was randomized blocks in a 5 × 5 factorial scheme, combining levels of EC (0.5, 1.2, 3.0, 4.7, 5.5 dS m−1) with doses of CBC (0.0, 1.4, 5.0, 8.5 and 10 t ha−1), in a matrix generated by the Box Central Compound. The contents of proline, glycine-betaine, proteins, amino acids, reducing sugars, non-reducing sugars, total sugars, photosynthetic pigments, electrolyte leakage, chlorophyll a fluorescence, and oil production were evaluated. The highest photochemical efficiency and oil content was found in basil irrigated with EC up to 3 dS m−1 and supplemented with 5 t ha−1 of CBC, which was followed by lower electrolyte leakage. In addition, CBC induced osmoregulation under saline conditions, as shown by the increase in free amino acids, proteins, proline, glycine-betaine, and sugars. However, high doses of CBC combined with high EC reduced photosynthetic pigment content, chlorophyll a fluorescence, and oil content. In conclusion, CBC induced osmoprotection, maintaining cell membrane integrity and increasing photochemical efficiency of basil. In addition, the use of 5 t ha−1 of CBC combined with low levels of water salinity can be employed to enhance oil production in basil.