Chlorophyll fluorescence measurements can be used as a non-destructive, non-intrusive estimate of the chilling resistance of the cultivated tomato and related species. The chlorophyll fluorescence parameter Fo:Fp is the least variable and most precise indicator of low temperature tolerance for the selection of breeding material and for the screening of segregating populations (Walker et al., 1990). The screening procedure utilizes detached leaves chilled on an aluminum plate for 24 h at 2 C with a 16-h photoperiod and light intensity of 250 mol m^-2^s^-1. Non-chilled control values range from 0.6 to 0.7 and chilling causes the Fo:Fp ratio to rise approaching 0.9 to 1.0. Chilling-tolerant lines are selected based upon their ability to maintain a relatively low Fo:Fp value.
Exposing Lycopersicon esculentum Mill cv H722, a chilling-sensitive tomato, for 3 days to 2 C and a similar photoperiod caused an increase in the production of free radicals within thylakoid membranes as estimated by electron spin resonance. Chilled thylakoid membranes had electron transport rates higher than non-chilled membranes when measurements were made below 10 C in an oxygen electrode. Therefore, the increase in electron transport activity may result in an overproduction of free radicals. This chilling-induced oxidative stress also resulted in a decline in the antioxidant capacity of the chloroplast and decreased photosynthetic activity. In comparison, chilling-resistant L. hirsutum Humb. and Bonpl. (LA 1363, elevation 3100 m, Alta Fortaleza, Ancash, Peru) showed enhanced antioxidant capacity after the 3-day chill through the increased synthesis of ascorbate, glutathione and carotenoids (Table 1). Photosynthetic activity also recovered at a faster rate in L. hirsutum compared to L. esculentum cv H722. The higher antioxidant capacity was coupled with a decrease in electron transport rates of approximately 70%, thereby reducing the potential for an injurious overproduction of free radicals.
Table 1. Effects of chilling on the levels of ascorbate, glutathione and carotenoids in chloroplasts of L. esculentum cv H722 and L. hirsutum.
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Treatment Ascorbate Glutathione Carotenoids
--------- ug g dw^-1 --- mg gdw^-1
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H722 control 5.46 a 743 b 75 c
chill 3.52 b 752 b 68 c
L. hirsutum control 4.40 b 657 c 95 b
chill 5.81 a 846 a 130 a
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Values not followed by the same letter are significantly different at the 5%
level.Literature cited:
Walker, M.A., D.M. Smith, K.P. Pauls and B.D. McKersie. 1990. HortScience 25:334-339.