Influence of photosynthetic photon flux densities before and during long-term chilling on xanthophyll cycle and chlorophyll fluorescence quenching in leaves of tomato (Lycopersicon hirsutum)

A high-altitude ecotype of tomato ( Lycopersicon hirsutum f. typicum Humb. and Bonpl.) has previously been shown to resist further loss of photosynthetic function after three to four days of chilling stress. This study examined the influence of PPFD prior to, and during chilling on the development of protective zeaxanthin and energy-dependent quenching mechanisms in this ecotype. Five-week-old tomato plants were acclimated to either low PPFD (60 μmol m⁻² s⁻¹) or high PPFD (550 μmol m⁻² S⁻¹) at 25/20°C (day/night) for three days, and then exposed to a temperature of 5/5°C and a PPFD of either 60 or 550 μmol m⁻² s⁻¹ for three days. The plants acclimated to low PPFD had lower Chl a/b ratio, and lower level of total Chl per leaf area, total xanthophyll cycle pool and β-carotene. The capacity of their photosynthetic system to resist photoinhibition and to recover photosynthetic function was also lower compared to that of the plants acclimated at high PPFD but exposed to the same chilling stress. In the plants chilled at low PPFD, energy-dependent quenching preceded the formation of zeaxanthin on the first day of chilling and there was an overall reduction in the conversion of violaxanthin to zeaxanthin as compared to the plants chilled at high PPFD. During the last day of chilling-induced photoinhibition, energy-dependent quenching in any of the treatments did not increase, but zeaxanthin levels increased continuously throughout the three days of chilling. Our results suggest that light-acclimation before chilling affects the capacity of the plants to resist chilling-induced photoinhibition. In addition, photoinhibitory quenching appears to be a major component for quenching excessive energy at the latter stage of long-term chilling.