Effects of temperature regimes under low light conditions on growth rate and phytochemical composition of lettuce and carrot plants

Abstract

Environmental conditions such as temperature have a large impact on the growth and development of plants. Lettuce and carrot contain phytochemicals (secondary metabolites) with high nutritional value. The temperature conditions that the plant is grown in may affect the content of phytochemicals, which in turn affects the quality of crops. In view of climate change this may be important for the supply of human food resources for a growing population, if the increase of temperature systematically affects food quality.

The objective of this study was to determine the effects of different growth temperatures on plant growth parameters and nutritionally relevant phytochemicals in different varieties of lettuce and carrots. Plants of two varieties each of carrots and lettuce were grown in controlled environments at different day/night temperatures: 12/8, 17/13, 22/18, 27/23 and 32/28 ℃ for lettuce and 12/8, 17/13, 22/18 and 27/23 ℃ for carrots. Each temperature treatment was applied at a separate time and the plants were harvested after having produced 10-12 leaves (lettuce) and 6-8 leaves (carrots). The light was constant 150 2 µmol /m /s with an 11-hr-light/13-hr-dark cycle, approximately corresponding to winter outdoor conditions in Southern Europe.

Growth parameters such as number of leaves, plant height, leaf area of lettuce, fresh and dry weight were recorded. The sugar levels, nitrate concentration, phenolic compounds, carotenoids and chlorophyll pigments were determined in leaves and root of both species, in addition to polyacetylene compounds in carrot roots.

The results showed that temperature had a highly significant effect on growth parameters. The lowest temperature (12/8 °C) produced the highest dry matter content of both shoots and roots; the biomass of both species peaked at 17/13 ℃ and the leaf area of lettuce was greatest at 22/18 ℃. The rate of leaf production was more rapid at higher temperatures than below 18 °C. Lettuce varieties responded more positively to high temperature than the carrots. Carrot root growth was inhibited with increasing temperatures above (22/18 ℃). Concentrations of phytochemicals were also affected significantly by growth temperature. Polyacetylenes in carrot roots increased by at least 50 % when grown at the lowest temperature of 12/8 ℃. The accumulation of phenolic compounds in both carrots and lettuce leaves was associated with both low and high temperatures over the range tested, while the levels at 22/18 ℃ were only 3-50 % of the highest values. High temperatures (17, 22, 27 ℃) also more than doubled nitrate contents, particularly in lettuce. In contrast there were higher levels of sugar in plants grown at low temperature compared with high temperature. Temperatures above 22 °C increased accumulation of chlorophylls in lettuce leaves. Contents of carotenoids (lutein, α-carotene and β- carotene) were relatively unaffected by temperature, although contents of β-carotene and lutein in lettuce leaves were slightly higher at 22/18 ℃ than other regimes. Growth temperature had a greater effect on the composition of carrots and lettuce than variety (genotype).

The directions of the effects on composition were as expected from the physiological mechanisms involved. However for most of the measured compounds, the present study is the first to determine magnitudes of the effects. Such information could be useful to growers as it may help them to make decisions about variety choice or fertilizer application rates dependent on the temperature regimes experienced or applied during growth.

These effects could become increasingly important aspects as climate change and global warming proceeds with potentially substantial implications for the links between diet and human health.