Yvan Fracheboud

Effect of growing season on the photosynthetic apparatus and leaf antioxidative defenses in two maize genotypes of different chilling tolerance

Abstract The effect of the growing season on the photosynthetic apparatus and on leaf antioxidants was investigated in a chilling-tolerant genotype (Z7) and a chilling-sensitive genotype (Penjalinan) of maize (Zea mays). Maize was sown in the field in Switzerland in 3 consecutive weeks, so plants sown early were exposed to chilling conditions, whereas those sown later developed under more favorable growth conditions. Measurements of the quantum efficiency of CO2 fixation (ΦCO2) and the photosystem II (ΦPSII) were made simultaneously on the third fully expanded leaves. The activity of scavenging enzymes and the content of pigments and antioxidants were also determined. Leaves that developed under chilling conditions showed typical chill-induced alterations, namely low photosynthetic capacity and efficiency, reduction in the pigment contents, and a decrease in catalase activity. These alterations were more pronounced in the chilling-sensitive than in the chilling-tolerant genotype. Determining the ΦPSII/ΦCO2 ratio indicated that it was very unlikely that alternative electron sinks, such as the Mehler reaction, were activated to a significant extent in either of the genotypes. This was supported by the measurements of the activity of enzymes involved in the Mehler ascorbate peroxidase reaction. However, a comparison of the genotypes showed that chilling tolerance might be correlated with an increase in the α-tocopherol and glutathione contents as well as in the activity of glutathione reductase.

Genetic analysis of cold-tolerance of photosynthesis in maize

The genetic basis of cold-tolerance was investigated by analyzing the quantitative trait loci (QTL) of an F2:3 population derived from a cross between two lines bred for contrasting cold-tolerance using chlorophyll fluorescence as a selection tool. Chlorophyll fluorescence parameters, CO2 exchange rate, leaf greenness, shoot dry matter and shoot nitrogen content were determined in plants grown under controlled conditions at 25/22 °C or 15/13 °C (day/night). The analysis revealed the presence of 18 and 19 QTLs (LOD > 3.5) significantly involved in the variation of nine target traits in plants grown at 25/22 °C and 15/13 °C, respectively. Only four QTLs were clearly identified in both temperatures regimes for the same traits, demonstrating that the genetic control of the performance of the photosynthetic apparatus differed, depending on the temperature regime. A major QTL for the cold-tolerance of photosynthesis was identified on chromosome 6. This QTL alone explained 37.4 of the phenotypic variance in the chronic photoinhibition at low temperature and was significantly involved in the expression of six other traits, including the rate of carbon fixation and shoot dry matter accumulation, indicating that the tolerance to photoinhibition is a key factor in the tolerance of maize to low growth temperature. An additional QTL on chromosomes 2 corresponded to a QTL identified previously in another population, suggesting some common genetic basis of the cold-tolerance of photosynthesis in different maize germplasms.

Quantitative trait loci for yield and correlated traits under high and low soil nitrogen conditions in tropical maize

The first objective of this study was to map and characterize quantitative trait loci (QTL) for grain yield (GY) and for secondary traits under varying nitrogen (N) supply. To achieve this objective, a segregating F2:3 population previously developed for QTL mapping under water-limited conditions was used. The population was evaluated in Mexico under low N conditions in the dry winter season and under low and high N conditions in the wet summer season. From eight QTLs identified for GY under low N conditions, two were also detected under high N conditions. Five QTLs were stable across the two low N environments and five co-localized with QTLs identified for the anthesis-silking interval (ASI) or for the number of ears per plant (ENO) under low N conditions. The percentage of the phenotypic variance expressed by all QTLs for ASI and ENO was quite different when evaluated under low N conditions during the dry winter (40% for ASI and 22% for ENO) and the wet summer seasons (22% for ASI and 46% for ENO). The results suggest optimizing different breeding strategies based on selection index depending on the growing season. Good QTL colocalization was observed for ASI (four QTLs) and ENO (three QTLs) when looking at QTL identified under low N and water-limited conditions in the same population. The results suggest that that both secondary traits can be used in breeding programs for simultaneous improvement of maize against low N and drought stresses.

The Application of Chlorophyll Fluorescence to Study Light, Temperature, and Drought Stress

Chlorophyll a fluorescence was studied for many years in specialized laboratories by biophysicists and molecular plant physiologists to obtain a mechanistic understanding of the functioning of the photosynthetic apparatus. In the last 15 years, however, this technique has been increasingly used by other plant scientists, in particular by plant physiologists for investigating stress responses. One reason for this has been the development of user-friendly instruments, enabling the recording of the chlorophyll fluorescence of intact leaves in their natural environment. Another reason was the development of mathematical models and parameters to interpret the results for scientists, who are not experts in photobiology or biophysics.

QTLs for the elongation of axile and lateral roots of maize in response to low water potential.

Changes in root architecture and the maintenance of root growth in drying soil are key traits for the adaptation of maize (Zea mays L.) to drought environments. The goal of this study was to map quantitative trait loci (QTLs) for root growth and its response to dehydration in a population of 208 recombinant inbred lines from the International Maize and Wheat Improvement Center (CIMMYT). The parents, Ac7643 and Ac7729/TZSRW, are known to be drought-tolerant and drought-sensitive, respectively. Roots were grown in pouches under well-watered conditions or at low water potential induced by the osmolyte polyethylene glycol (PEG 8000). Axile root length (LAx) increased linearly, while lateral root length (LLat) increased exponentially over time. Thirteen QTLs were identified for six seedling traits: elongation rates of axile roots (ERAx), the rate constant of lateral root elongation (kLat), the final respective lengths (LAx and LLat), and the ratios kLat/ERAx and LLat/LAx. While QTLs for lateral root traits were constitutively expressed, most QTLs for axile root traits responded to water stress. For axile roots, common QTLs existed for ERAx and LAx. Quantitative trait loci for the elongation rates of axile roots responded more clearly to water stress compared to root length. Two major QTLs were detected: a QTL for general vigor in bin 2.02, affecting most of the traits, and a QTL for the constitutive increase in kLat and kLat/ERAx in bins 6.04–6.05. The latter co-located with a major QTL for the anthesis-silking interval (ASI) reported in published field experiments, suggesting an involvement of root morphology in drought tolerance. Rapid seedling tests are feasible for elucidating the genetic response of root growth to low water potential. Some loci may even have pleiotropic effects on yield-related traits under drought stress.

Artificially increased ascorbate content affects zeaxanthin formation but not thermal energy dissipation or degradation of antioxidants during cold-induced photooxidative stress in maize leaves

Abstract. Infiltrating detached maize (Zeamays L.) leaves with L-galactono-1,4-lactone (L-GAL) resulted in a 4-fold increase in the content of leaf ascorbate. Upon exposure to high irradiance (1000 μmol photons m−2 s−1) at 5 °C, L-GAL leaves de-epoxidized the xanthophyll-cycle pigments faster than the control leaves; the maximal ratio of de-epoxidized xanthophyll-cycle pigments to the whole xanthophyll-cycle pool was the same in both leaf types. The elevated ascorbate content, together with the faster violaxanthin de-epoxidation, did not affect the degree of photoinhibition and the kinetics of the recovery from photoinhibition, assayed by monitoring the maximum quantum efficiency of photosystem II primary photochemistry (Fv/Fm). Under the experimental conditions, the thermal energy dissipation seems to be zeaxanthin-independent since, in contrast to the de-epoxidation, the decrease in the efficiency of excitation-energy capture by open photosystem II reaction centers (Fv′/Fm′) during the high-irradiance treatment at low temperature showed the same kinetic in both leaf types. This was also observed for the recovery of the maximal fluorescence after stress. Furthermore, the elevated ascorbate content did not diminish the degradation of pigments or α-tocopherol when leaves were exposed for up to 24 h to high irradiance at low temperature. Moreover, a higher content of ascorbate appeared to increase the requirement for reduced glutathione.

QTL studies reveal little relevance of chilling-related seedling traits for yield in maize.

Prolonged low temperature phases and short-term cold spells often occur in spring during the crucial stages of early maize (Zea mays L.) development. The effect of low temperature-induced growth retardation at the seedling stage on final yield is poorly studied. Therefore, the aim was to identify genomic regions associated with morpho-physiological traits at flowering and harvest stage and their relationship to previously identified quantitative trait loci (QTLs) for photosynthesis and morpho-physiological traits from the same plants at seedling stage. Flowering time, plant height and shoot biomass components at harvest were measured in a dent mapping population for cold tolerance studies, which was sown in the Swiss Midlands in early and late spring in two consecutive years. Early-sown plants exhibited chilling stress during seedling stage, whereas late-sown plants grew under favorable conditions. Significant QTLs, which were stable across environments, were found for plant height and for the time of flowering. The QTLs for flowering were frequently co-localized with QTLs for plant height or ear dry weight. The comparison with QTLs detected at seedling stage revealed only few common QTLs. A pleiotropic effect was found on chromosome 3 which revealed that a good photosynthetic performance of the seedling under warm conditions had a beneficial effect on plant height and partially on biomass at harvest. However, a high chilling tolerance of the seedling seemingly had an insignificant or small negative effect on the yield.