Hugh J. Earl

Stomatal and non-stomatal restrictions to carbon assimilation in soybean (Glycine max) lines differing in water use efficiency

Abstract Genetic variability for water use efficiency (WUE, the quantity of crop dry matter produced per unit water transpired) has been demonstrated in a wide range of crop species. In agreement with established theory, genotypes with higher WUE are often found to maintain lower leaf internal CO 2 concentration ( c i ), as estimated by carbon isotope discrimination. However, lower c i may result from reduced stomatal conductance, increased mesophyll (non-stomatal) conductance, or a combination of both. When genotypic variation for WUE is found, it may be important for plant breeding purposes to define whether such variation arises from differences in stomatal or non-stomatal restrictions to CO 2 uptake. The soybean cultivar Young was previously shown to have higher WUE than the soybean plant introduction PI416937 when both were grown under cyclic drought stress, but the relative importance of stomatal and non-stomatal factors was not known. In the present work, the difference in WUE between these soybean lines was found to be smaller than previously reported, and was also demonstrated to be constitutive in nature; that is, it occurred under both cyclic drought stress and water-replete conditions. Leaf gas exchange measurements revealed that cv. Young maintained lower c i and higher leaf-level WUE than PI416937, as expected. Sensitivity analysis indicated that the observed differences in c i under steady-state gas exchange conditions could be attributed entirely to differences in stomatal limitations to photosynthesis. Genotype differences in stomatal conductance and c i were also examined under fluctuating photosynthetically active radiation, and in some cases were found to be even more pronounced than under steady-state conditions.

Relationship between thylakoid electron transport and photosynthetic CO2 uptake in leaves of three maize (Zea mays L.) hybrids

The introduction of a more efficient means of measuring leaf photosynthetic rates under field conditions may help to clarify the relationship between single leaf photosynthesis and crop growth rates of commercial maize hybrids. A large body of evidence suggests that gross photosynthesis (AG) of maize leaves can be accurately estimated from measurements of thylakoid electron transport rates (ETR) using chlorophyll fluorescence techniques. However, before this technique can be adopted, it will first be necessary to determine how the relationship between chlorophyll fluorescence and CO2 assimilation is affected by the non-steady state PPFD conditions which predominate in the field. Also, it must be determined if the relationship is stable across different maize genotypes, and across phenological stages. In the present work, the relationship between ETR and AG was examined in leaves of three maize hybrids by making simultaneous measurements of leaf gas exchange and chlorophyll fluorescence, both under controlled environment conditions and in the field. Under steady-state conditions, a linear relationship between ETR and AG was observed, although a slight deviation from linearity was apparent at low AG. This deviation may arise from an error in the assumption that respiration in illuminated leaves is equivalent to respiration in darkened leaves. The relationship between chlorophyll fluorescence and photosynthetic CO2 assimilation was not stable during fluctuations in incident PPFD. Since even minor (e.g. 20%) fluctuations in incident PPFD can produce sustained ( > 20 s) departures from the mean relationship between ETR and AG, chlorophyll fluorometry can only provide an accurate estimate of actual CO2 assimilation rates under relatively stable PPFD conditions. In the field, the mean value of ETR / AG during the early part of the season (4.70 ± 0.07) was very similar to that observed in indoor-grown plants in the vegetative stage (4.60 ± 0.09); however, ETR / AG increased significantly over the growing season, reaching 5.00 ± 0.09 by the late grain-filling stage. Differences in ETR / AG among the three genotypes examined were small (less than 1% of the mean) and not statistically significant, suggesting that chlorophyll fluorometry can be used as the basis of a fair comparison of leaf photosynthetic rates among different maize cultivars.

Estimating photosynthetic electron transport via chlorophyll fluorometry without Photosystem II light saturation.

Estimates of thylakoid electron transport rates (Je) from chlorophyll fluorometry are often used in combination with leaf gas exchange measurements to provide detailed information about photosynthetic activity of leaves in situ. Estimating Je requires accurate determination of the quantum efficiency of Photosystem II (ΦP), which in turn requires momentary light saturation of the Photosystem II light harvesting complex to induce the maximum fluorescence signal (FM′). In practice, full saturation is often difficult to achieve, especially when incident photosynthetic photon flux density (Q) is high and energy is effectively dissipated by non-photochemical quenching. In the present work, a method for estimating the true FM′ under high Q was developed, using multiple light pulses of varying intensity (Q′). The form of the expected relationship between the apparent FM′ and Q′ was derived from theoretical considerations. This allowed the true FM′ at infinite Q′ to be estimated from linear regression. Using a commercially available leaf gas exchange/ chlorophyll fluorescence measurement system, Je was compared to gross photosynthetic CO2 assimilation (AG) under conditions where the relationship between Je and AG was expected to be linear. Both in C4 leaves (Zea mays) in ambient air and also in C3 leaves (Gossypium hirsutum) under non-photorespiratory conditions the apparent ratio between Je and AG declined at high Q when ΦP was calculated from FM′ measured simply using the highest available saturating pulse intensity. When FM′ was determined using the multiple pulse / linear regression technique, the expected relationship between Je and AG at high Q was restored, indicating that the ΦP estimate was improved. This method of determining FM′ should prove useful for verifying when saturating pulse intensities are sufficient, and for accurately determining ΦP when they are not.

Shared and genetically distinct Zea mays transcriptome responses to ongoing and past low temperature exposure

BackgroundCold temperatures and their alleviation affect many plant traits including the abundance of protein coding gene transcripts. Transcript level changes that occur in response to cold temperatures and their alleviation are shared or vary across genotypes. In this study we identify individual transcripts and groups of functionally related transcripts that consistently respond to cold and its alleviation. Genes that respond differently to temperature changes across genotypes may have limited functional importance. We investigate if these genes share functions, and if their genotype-specific gene expression levels change in magnitude or rank across temperatures.ResultsWe estimate transcript abundances from over 22,000 genes in two unrelated Zea mays inbred lines during and after cold temperature exposure. Genotype and temperature contribute to many genes’ abundances. Past cold exposure affects many fewer genes. Genes up-regulated in cold encode many cytokinin glucoside biosynthesis enzymes, transcription factors, signalling molecules, and proteins involved in diverse environmental responses. After cold exposure, protease inhibitors and cuticular wax genes are newly up-regulated, and environmentally responsive genes continue to be up-regulated. Genes down-regulated in response to cold include many photosynthesis, translation, and DNA replication associated genes. After cold exposure, DNA replication and translation genes are still preferentially downregulated. Lignin and suberin biosynthesis are newly down-regulated. DNA replication, reactive oxygen species response, and anthocyanin biosynthesis genes have strong, genotype-specific temperature responses. The ranks of genotypes’ transcript abundances often change across temperatures.ConclusionsWe report a large, core transcriptome response to cold and the alleviation of cold. In cold, many of the core suite of genes are up or downregulated to control plant growth and photosynthesis and limit cellular damage. In recovery, core responses are in part to prepare for future stress. Functionally related genes are consistently and greatly up-regulated in a single genotype in response to cold or its alleviation, suggesting positive selection has driven genotype-specific temperature responses in maize.