ALBERT

Notes: Available evidence suggests that the stress-induced increase in the activity of glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49), the key regulatory enzyme of the oxidative pentose phosphate pathway, might often be related to the presence of plant water deficit. The response of G6PDH to dark chilling in chilling sensitive plant species is still unknown. In this communication we report on this response and its dependence on the presence of chill-induced drought stress. A chilling sensitive soybean (Glycine max L. Merr.) genotype was exposed to dark chilling of the entire plant (whole-chilled) or only the shoots and leaves (shoot-chilled). The development of chill-induced drought stress upon illumination was quantified by measurement of proline and relative water content (RWC). Chill-induced drought stress (decrease in RWC and increase in proline content) developed with time in whole-chilled plants, but not in shoot-chilled plants. The response of the above-mentioned treatments on G6PDH activity in fully expanded leaves was assessed. In parallel, the effects on CO2 assimilation, PSII activity and chloroplast fructose-1,6-bisphosphatase (FBPase EC 3.1.3.11) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco EC 4.1.1.39) activity were quantified. A decrease in CO2 assimilation rate, FBPase activity and ribulose-1,5-bisphosphate (RuBP) content was observed in whole-chilled but not in shoot-chilled plants. However, in shoot-chilled plants regulation of diurnal PSII activity was altered. The increase in the activation state of NADP-dependent malate dehydrogenase (NADP-MDH EC 1.1.1.82) in shoot-chilled plants suggests an increase in stromal redox state. Although the two different dark chilling treatments resulted in distinct physiological and biochemical effects, both induced an increase in foliar G6PDH activity, suggesting an important role of this enzyme during and following dark chilling stress, irrespective of the presence of chill-induced drought stress.

Notes: Sub-optimal night temperatures below 15°C (dark chilling) frequently reduce soybean [Glycine max (L.) Merrill] production. Nitrate application is known to alleviate some of the negative effects of low root zone temperatures, probably by counteracting the inhibition caused by decreased symbiotic nitrogen fixation (SNF). Under field conditions, however, dark chilling is frequently not accompanied by low root zone temperatures. The possibility that nitrate might increase dark-chilling tolerance under these conditions is still largely unexplored. In addition to quantifying vegetative development by means of the plastochron index, O–J–I–P (O–I1–I2–P) chlorophyll a fluorescence transients were recorded in soybean genotypes of contrasting chilling tolerance during and following exposure to dark chilling in the absence of low root zone temperatures. Plants, inoculated with the N2-fixing bacteria, Bradyrhizobium japonicum, were grown with and without nitrate supplementation. The recorded O–J–I–P chlorophyll a fluorescence transients were analysed by the so-called JIP-test which translates stress-induced alterations in these transients to changes in biophysical parameters that quantifies the energy flow through photosystem II (PSII). One of these parameters, the performance index (PIABS), combines the three main functional steps (light energy absorption, excitation energy trapping, and conversion of excitation energy to electron transport) of photosynthetic activity by a PSII reaction centre complex into a single multiparametric expression. By using the PIABS we could convincingly show that nitrate supplementation considerably enhances dark-chilling tolerance and recovery capacity of plants in the absence of low root zone temperatures. This was especially true for the chilling-sensitive genotype (‘Java 29’), suggesting that the response of SNF to dark chilling might be an important factor contributing towards genotypic differences in chilling tolerance. Our results corroborated previous reports about the superior chilling tolerance of ‘Maple Arrow’, a chilling-tolerant genotype. The results obtained indicated that the PIABS is a far more sensitive indicator of dark-chilling stress than the maximum quantum yield of primary photochemistry (FV/FM).

Notes: The effects of dark chilling on CO2 assimilation, chlorophyll a fluorescence kinetics and nitrogen fixation were compared in two Glycine max (L.) Merr. genotypes. The aim was to elucidate the mechanisms by which photosynthesis was inhibited as well as identification of selection criteria for dark chilling tolerance. Seedlings were dark chilled (8°C) for 9 consecutive nights but kept at normal day temperatures (28°C). CO2 gas exchange analysis indicated that photosynthesis in Maple Arrow was inhibited largely as a result of stomatal limitation, while in Fiskeby V, it indicated inhibition of the mesophyll reactions. Increased intercellular CO2 concentration and decreased carboxylation efficiency suggested loss of Rubisco activity in Fiskeby V, although no effect on the KM (CO2) of Rubisco was observed. Quantification and deconvolution of the Chl a fluorescence transients into several phenomenological and biophysical parameters (JIP-test) revealed large genotypic differences in the response of PSII to dark chilling. These parameters differentially changed in the two genotypes during the progression of the chilling treatment. Among them, the performance index, reflecting several responses of the photochemical apparatus, provided the best preliminary overall assessment of the genotypes. In contrast, the quantum yield of primary photochemistry ϕPo (FV/FM) was quite insensitive. The recovery of most of the JIP-test parameters in Maple Arrow after 6 and 9 nights of dark chilling was a major genotypic difference. Genotypic differences were also observed with regard to the ureide response and N2 fixation appeared to be more sensitive to dark chilling than CO2 assimilation. The JIP-test provided information consistent with results derived from CO2 assimilation and N2 fixation studies suggesting that it can substitute the much more time-consuming methods for the detection of chilling stress and can well satisfy the requirements of a rapid and accurate screening method.