Impacts of chilling on photosynthesis and chlorophyll pigment content in juvenile basil cultivars

Abstract

The objective of this study was to examine several cultivars of Ocimum basilicum L. (green, red, cinnamon, lettuce leaf, lemon, and Thai basils) for photosynthetic performance, chlorophyll a fluorescence, and chlorophyll content under chilling stress conditions of 6°C in comparison to non-stressed controls (18°C). The basil plants were grown in a peat substrate for 8 weeks and then exposed to chilling for 8 or 16 days, under a 300 μmol•m-2•s-1 photosynthetic photon flux. After chilling, significant reductions in both the transpiration (E) and net photosynthetic (PN) rates were observed in basil plants, while the intercellular CO2 concentration (Ci) was higher in the plants treated with 6°C in comparison to the controls. The decrease in PN and E was associated with decreased water use efficiency (WUE) and stomatal conductance (gs). The greatest impairment of photosynthesis for Thai basil leaves was observed after 8 days of chilling, and for green basil after the 16-day low temperature treatment. The photosystem II (PSII) activity (Fv/Fm) and variable-to-initial chlorophyll fluorescence (Fv/F0) were decreased after chilling. PSII activity was most affected in lettuce leaf basil after 8 days, and in Thai and red basil plants after the prolonged temperature treatment. Low temperatures did not significantly alter the chlorophyll concentration but did increase the Chl a/b ratio in leaves of basil. The results indicated that the decrease in photosynthesis was not attributable mainly to damage to PSII, but rather to chilling-induced photoinhibition of PSI. The knowledge gained in this study on the genotypic variation in basil response should be helpful for future selection of plants with low chilling sensitivity.