Phenotypic Plasticity for Growth and Nutrient Uptake in Milk Thistle under Salt Stress: Modulatory Role of Soil Supplementations with Plant Growth Promoters

Abstract

Salinity stress negatively affects key physiological phenomena in plants while plants show great variability and respond differentially for tolerance to salt stress. Usually, nutrients imbalances affect specific plant tissues and physiological processes which are requisite for normal plant growth and development. The aim of this two-year (2017 and 2018) simulated field study was to investigate phenotypic plasticity for growth, relative leaf water content (RLWC) and nutrient status in milk thistle [Silybum marianum (L.) Gaertn.] ecotypes and the potential role of soil supplementation with pre-optimized levels of plant growth promoters (PGPs) in modulating these attributes under control and salinity (12 dS/m) stress. Four ecotypes of milk thistle were collected from three ecologically distinct zones including Faisalabad (FSD) and Kalar Kahar (KK) – semi-arid zone, Gujranwala (GUJ) – hot semi-arid zone and Quetta (QTA) – cool semi-arid zone. The studied nutrients were nitrate-N, phosphate-P, sulfate-S, sodium (Na), potassium (K) and calcium (Ca). The soil supplemented PGPs, applied with irrigation water, were ascorbic acid (AsA), thiourea (TU) and moringa leaf extract (MLE) at 250 µM, 500 µM and 3%, respectively of soil moisture content at field capacity. Results indicated that soil supplementation of PGPs in the field conditions is a feasible approach for enhancing nutrient uptake of milk thistle ecotypes under salt stress, while the effect of salinity stress restricted the uptake of the studied nutrients and caused their imbalance. Although the salinity stress reduced shoot and root dry matter, RLWC and restricted the uptake of these nutrients irrespective of ecotypes, the levels of nitrate-N, phosphate-P, K, sulfate-S, Ca, and RWC contents increased more with the soil supplementation of AsA followed by MLE as compared to other soil supplements in both the study years. Among the ecotypes, QTA followed by KK and FSD ecotypes gained more dry weight with greater leaf RWC and higher tissue nutrient contents due to PGPs under salt stress. The principal component analysis and correlation data revealed the existence of distinct phenotypic plasticity in the milk thistle ecotypes for nutrient acquisition with soil supplementation of PGPs under salinity stress. To conclude, ecotypes from QTA and KK were more promising than the others while AsA and MLE were better soil supplements in improving shoot and root nutrients under salt stress.