Foliar-sprayed calcium-tryptophan mediated improvement in physio-biochemical attributes and nutritional profile of salt stressed Brassica oleracea var. italica

Abstract

Chelated mineral supplements are a combination of minerals with amino acids (AAs), putatively more efficient as fertilizers, especially for enhancing nutrient absorption. This study aimed at investigating the efficacy of foliar-applied calcium (Ca) and its complex with tryptophan to improve the vegetative growth, biochemical attributes and nutritional value of salt stressed broccoli (Brassica oleracea L. var. italica) plants. Treatments comprised of 2 mM Ca [as CaCl2], tryptophan 1% [Trp] and Ca-chelated with tryptophan 1% [Ca-Trp] at four salinity stress levels, i.e., S1 [0 mM NaCl], S2 [50 mM NaCl], S3 [100 mM NaCl] and S4 [150 mM NaCl]. With increasing salinity, growth indices decreased, however a significant improvement was witnessed under Ca-Trp-treatment for S2 and S3 levels. Coherently, leaf nitrogen (N) concentration was higher than urea and nitrate (NO3), with S1 and S2 displaying comparatively higher N levels than S3 and S4. Both Ca-Trp and tryptophan alone slightly increased nitrate reductase (NR) activity under non-saline treatment, while an inversely proportional relation between NO3 and NR was found in S2 and S4. Chlorophyll index values, internal CO2 of stomata and photosynthesis enhanced under Ca-Trp with increasing salinity levels (from S2 to S3) when compared to control plants. As opposed to Ca-Trp, the highest abscisic acid contents were seen in S3xTrp followed by S4 × Ca. The stress indices increased with increasing salinity, especially under alone Ca and Trp-treatments; moreover, growth attributes thrived in the absence of salinity, particularly under Ca-Trp. Enrichment of glucobrassicin and progoitrin amino acids represented improved nutritional characteristics of broccoli, which increased in low salinity levels with Ca-Trp application. It is, therefore advisable to apply Ca-tryptophan chelate to broccoli plants under low (S2) and moderate (S3) salt stress for a burgeoning effect and enhanced nutritional value.