Seed priming with potassium nitrate alleviates the high temperature stress by modulating growth and antioxidant potential in carrot seeds and seedlings

Abstract

Early season carrot (Daucus carota) production is being practiced in Punjab, Pakistan to meet the market demand but high temperature hampers the seed germination and seedling establishment which cause marked yield reduction. Seed priming with potassium nitrate breaks the seed dormancy and improves the seed germination and seedling growth potential but effects vary among the species and ecological conditions. The mechanism of KNO₃ priming in high temperature stress tolerance is poorly understood yet. Thus, present study aimed to evaluate high temperature stress tolerance potential of carrot seeds primed with potassium nitrate and impacts on growth, physiological, and antioxidant defense systems. Carrot seeds of a local cultivar (T-29) were primed with various concentration of KNO₃ (T₀: unprimed (negative control), T₁: hydroprimed (positive control), T₂: 50 mM, T₃:100mM, T₄: 150 mM, T₅: 200 mM, T₆: 250 mM and T₇: 300 mM) for 12 h each in darkness at 20 ± 2℃. Seed priming with 50 mM of KNO₃ significantly enhanced the seed germination (36%), seedling growth (28%) with maximum seedling vigor (55%) and also exhibited 16.75% more carrot root biomass under high temperature stress as compared to respective control. Moreover, enzymatic activities including peroxidase, catalase, superoxidase dismutase, total phenolic contents, total antioxidants contents and physiological responses of plants were also improved in response to seed priming under high temperature stress. By increasing the level of KNO₃, seed germination, growth and root biomass were reduced. These findings suggest that seed priming with 50 mM of KNO₃ can be an effective strategy to improve germination, growth and yield of carrot cultivar (T-29) under high temperature stress in early cropping. This study also proposes that KNO₃ may induces the stress memory by heritable modulations in chromosomal structure and methylation and acetylation of histones that may upregulate the hormonal and antioxidant activities to enhance the stress tolerance in plants.

Keywords: Carrot growth; Heat stress; Potassium nitrate; Reactive oxygen species; Seed treatments.

Fig. 1

Classified filled boxplots of (A) final seed germination (%), (B), time taken for 50% germination in days, (C) mean germination time in days, (D) germination energy (%), and (E) germination index of a carrot cultivar T-29. The eight priming treatments of KNO₃ from T0 to T7 represented by different color filled boxplots. Each boxplot is delimited 25th and 75th percentile with error bars represents four replicates from each treatment and include a solid line represents the mean value. One way ANOVA calculated significant differences among the treatment represented with *** (P < 0.0001), ** (P < 0.001), * (P < 0.05), and ns: non-significant (P > 0.05) notations

Fig. 2

Classified filled boxplots of (A) seedling length (cm), (B), seedling fresh weight (mg), (C) seedling dry weight (mg), (D) seedling vigour index of carrot seedling. The eight priming treatments of KNO₃ from T0 to T7 represented by different color filled boxplots. Each boxplot is delimited 25th and 75th percentile with error bars represents four replicates from each treatment and include a solid line represents the mean value. One way ANOVA calculated significant differences among the treatment represented with *** (P < 0.0001), ** (P < 0.001), * (P < 0.05), and ns: non-significant (P > 0.05) notations

Fig. 3

a.b.c Multivariate analysis of preliminary experiment (A) pearson’s correlation coefficients among pairs of response variables of carrot cultivar, (B) cluster heatmap of eight priming treatments and 10 response variables of seed germination and seedling establishment, (C) biplot of principal components of response variables. Purple dots represents negative while dark green dots represent positive correlation coefficients between response variables. Color intensity and size of dots represents the strength of dependency between pairs of response variable. Euclidian distance-based clustering in the heatmap exhibit three separate clusters for priming treatments and four separate clusters for response variables. All response variables were plotted in three directions in principal component based biplot

Fig. 4

Classified filled boxplots and barplots of (A, B) final seed emergence, (C, D), seedling vigour index, (E, F) transpiration rate, (G, H) stomatal conductance, (I, J) photosynthetic rate, (K, L) water use efficiency of carrot seedlings respectively. In boxplots, four optimized priming treatments (T0, T1, T2, and T4) of KNO₃ represented by different colors. Each boxplot is delimited 25th and 75th percentile with error bars represents four replicates from each treatment and include a solid line represents the mean value. Two way ANOVA (treatments and years) calculated significant differences, followed by LSD test based lettering are also presented, where same letter did not vary significantly. In barplots, two years represented by different colors, each barplot is presenting mean value and error bar represents standard error of four replicates

Fig. 5

Classified filled boxplots and barplots of (A, B) total phenolic contents, (C, D) total antioxidants, (E, F) activity of POD, (G, H) activity of CAT, (I, J) activity of SOD, (K, L) activity of MDA in carrot leaves respectively. In boxplots, four optimized priming treatments (T0, T1, T2, and T4) of KNO₃ represented by different colors. Each boxplot is delimited 25th and 75th percentile with error bars represents four replicates from each treatment and include a solid line represents the mean value. Two way ANOVA (treatments and years) calculated significant differences, followed by LSD test based lettering are also presented, where same letter did not vary significantly. In barplots, two years represented by different colors, each barplot is presenting mean value and error bar represents standard error of four replicates

Fig. 6

Classified filled boxplots and barplots of (A, B) root length, (C, D) root weight, (E, F) carrot root yield of carrot plants respectively. In boxplots, four optimized priming treatments (T0, T1, T2, and T4) of KNO₃ represented by different colors. Each boxplot is delimited 25th and 75th percentile with error bars represents four replicates from each treatment and include a solid line represents the mean value. Two way ANOVA (treatments and years) calculated significant differences, followed by LSD test based lettering are also presented, where same letter did not vary significantly. In barplots, two years represented by different colors, each barplot is presenting mean value and error bar represents standard error of four replicates

Fig. 7

Multivariate analysis of preliminary experiment (A) cluster heatmap of four optimized priming treatments and 15 response variables of carrot, (B) pearson’s correlation coefficients among pairs of response variables of carrot cultivar, (C) biplot of principal components of response variables. Purple dots represents negative while dark green dots represents positive correlation coefficients between response variables. Color intensity and size of dots represents the strength of dependency between pairs of response variable. Euclidian distance based clustering in the heatmap exhibit three separate clusters for priming treatments and four separate clusters for response variables. All response variables were plotted in three directions in principal component based biplot

Fig. 8

Proposed mechanistic model for the KNO₃-induced alleviation of adverse effects of high temperature stress in carrot seed