Abstract
Exposure of vegetable seedlings to lowered temperature affects their growth and the content of certain chemical constituents. Chilling activates defensive mechanisms against stress and leads to acclimatization which increases plant ability to withstand harsh field conditions. Thus, seedlings subjected to low-temperature treatment had altered metabolic pathways, and these changes can persist until harvest. We therefore assessed: (i) the direct response of broccoli seedlings to 1 week or 2 weeks at lowered temperatures (6 °C, 10 °C, 14 °C, and 18 °C—control); and (ii) the long-term effects of the latter treatments on phytochemical components level in mature heads of broccoli cultivated in the field. Chilling stress decreased seedling shoot and root fresh and dry weights, plant height, number of leaves, leaf area, leaf perimeter, and leaf width. The most spectacular reductions in these parameters were observed at 6 °C and 10 °C. Longer exposure to lowered temperature resulted in greater reduction in the values of morphological traits. Chilling led to reduction in L-ascorbic acid content in broccoli seedlings, while a 6 °C temperature caused an increase in soluble sugars. The highest content of dry weight, soluble sugars, and L-ascorbic acid were observed in the heads of plants exposed to 14 °C; however, the content of dry weight (at 10 °C) and L-ascorbic acid (at 6 °C and 10 °C) also increased in broccoli heads in comparison with the control. Longer chilling (for 2 weeks) generally increased the content of these constituents in mature broccoli. Lower temperature (6–14 °C) applied at seedling stage increased P and Zn contents in broccoli heads in comparison to the control, whereas plants treated with 10 °C had more K, Fe, and, together with 6 °C treatment, Cu. The lowest temperature applied to the seedlings (6 °C) caused an increase in Mn content, while no effects of seedling chilling was noted for Ca levels. Significant linear correlations were noted and regression models were developed for the content of dry weight, soluble sugars, and L-ascorbic acid in the broccoli heads based on the chosen seedling parameters. The results show that the effect of lowered temperature to which the seedlings were subjected persists also in the further stages of plant ontogenesis, causing permanent changes in the chemical composition of mature broccoli heads.
Highlights
Broccoli, which belongs to the group of important vegetable crops, is widely cultivated in the world and is rich in bioactive compounds and minerals [1,2,3]
Morphological traits of broccoli seedlings were highly affected by temperature treatment (Table 1)
These observations were confirmed by the analysis of main effects of temperature duration: broccoli plants treated with the two lowest temperatures for 2 weeks were smaller than those exposed to the same temperatures for 1 week, while the control ones were the biggest
Summary
Broccoli, which belongs to the group of important vegetable crops, is widely cultivated in the world and is rich in bioactive compounds and minerals [1,2,3]. Health benefits from broccoli are associated with high amounts of glucosinolates, isothiocyanates, phenolic compounds, ascorbic acid, tocopherols, carotenoids, and minerals including calcium, magnesium, sulphur, and selenium [4,5]. Low-temperature treatment of juvenile plants via acclimatization processes induces modification of genome and transcriptome what is related to changes in primary and secondary metabolism which shapes stress response of the plants [7,8]. These metabolites are essential for plant to survive environmental changes, including low temperature conditions. Several reports describe the short-term effects of low-temperature treatments on vegetable seedlings, i.e., [14,15,16], but there are evidences for long-lasting chilling influence [16,17,18,19]
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