We measured: a) the photochemical efficiency of photosystem (PS) II expressed as the ratio of the variable fluorescence (FV) to the maximum fluorescence yield (FM) (FV/FM ratio) and b) the injuries of the cell membranes of leaves during growth at 5 C or at 25 C by the electric conductivity of solutions due to electrolyte leakage of leaf slices in distilled water.
Maize seedlings were obtained by germination of seeds while Tripsacum and hybrid plants were grown from its rhizomes. When plants were at the four-leaf stage of development, chilling conditions (5 C) were applied in a growth chamber up to 3 days (16 h light - 8 h dark; 400 mol m-2 s-1 PAR at leaf level; 80% RH). Data were analyzed by ANOVA and differences among individual means were compared by the least significant difference (LSD) test.
Table 1. FV/FM ratios in leaves of control (25 C) and chilled (5 C)
|maize||Tripsacum||maize x Tripsacum|
|(2n = 40)||(2n = 72)||(2n = 56)|
|Control plants||0.796 a||0.727 a||0.764 a|
|5 C 1 day||0.547 b||0.513 b||0.593 b|
|5 C 2 days||0.297 cd||0.264 d||0.517 b|
|5 C 3 days||0.150 e||0.173 e||0.380 c|
Means followed by the same small letter are not significantly different (P<0.05) according to the LSD test.
Table 2. Electric conductivity (µS cm-1 g FM-1 hour-1) of leaf
slices suspension in distilled water.
|Genotypes||Stressed plants||Control plants|
|(5 C; 3 d)||(25 C)|
|Zea mays (2n = 40)||71.1 ab||36.7 d|
|Tripsacum dactyloides (2n = 72)||84.9 a||47.0 cd|
|Zea x Tripsacum (2n = 56)||61.5 c||35.9 d|
|Mean||72.5 A||39.9 B|
Results of both comparisons indicated that the hybrid was less affected
than its parents. After the measures, all chilled plants were cultured
at 25 C in the same place as the control plants. After one week, stressed
maize seedlings died and, also, leaves from chilled Tripsacum plants,
while stressed hybrid plants remained alive as they did in the field.
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