Photosynthesis and Production in a Changing Environment: A field and laboratory manual

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The remainder is made up from remobilized storage carbohydrate in leaf sheaths and culms laid down before anthesis Yoshida, ; Watanabe et al. Since the majority of leaf N is found in chloroplasts, there is a potential conflict between the maintenance of photosynthate supply and the dismantling of photosynthetic proteins into component amino acids for translocation.

There is, therefore, a need to examine the timing of grain filling and photosynthetic capacity more closely. A recent study Yang et al. Fast, synchronized filling is associated with higher yields and a higher percentage of grains completely filled. Moreover, the rates of grain filling correlated well with cytokinin content of grains and roots. The periods of rapid grain filling can, therefore, influence leaf photosynthesis in two ways. Secondly, the hormonal changes associated with the RGFP within the plant may influence photosynthesis by inducing senescence.

Chlorophyll loss in leaves has been correlated with leaf cytokinin flux Soejima et al. Understanding the factors that regulate grain filling is important for improving rice yield potential.

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However, there is disagreement as to whether improving sink size alone in rice crops will also result in an increase in leaf and whole plant photosynthesis Horton, ; Reynolds et al. It is generally recognized that the photosynthetic performance of crop plants needs to be improved in order to increase the rate of biomass production. Because canopy structure has been greatly improved and leaf area index is very high, the increase in assimilation will probably have to arise from the leaf level.

In rice there is evidence that panicle removal has no effect upon P max Nakano et al. In this paper, several cultivars of rice have been used which differ according to the rates and patterns of grain filling as identified by Yang et al. It was determined whether a photosynthetic capacity in the flag leaves is related to the timing and magnitude of RGFP implying sink regulation of photosynthesis and b whether the onset of senescence is consistent with the timing of the RGFP. Rice Oryza sativa L. Measurements were made in the dry season of For the period 22 February 5 d before the commencement of measurements to 16 March end of the experiment , average radiation was Panicles that headed on the second day from the onset of heading were selected and tagged.


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Previous data showed no significant differences between the main stem and primary tiller in either final panicle weight or the timing or rate of grain filling within a line or cultivar if their panicles headed on the same day Yang et al. Only tagged tillers were used for measurements of photosynthesis, sample collection 1 cm 2 leaf discs per sample and panicle dry weight DW. Measurements were started on the day after the onset of flowering day one and repeated at intervals of 3 d.

On each tiller, one measurement of photosynthesis was made in the morning and leaf discs for protein, carbohydrate and Chl analysis were cut from that tiller and frozen in liquid N 2 immediately. Three of these panicles were then removed for immediate assay of DW. Leaf temperature was typically slightly below that of block temperature. The two systems provided photosynthetic data that closely matched. Leaves were held in the chamber until values of photosynthesis were observed to be as constant as possible, i. Leaf discs 1 cm 2 per sample were excised in the field and frozen immediately in liquid N 2 for assays of Chl, carbohydrate, protein, and Rubisco content.

Total leaf protein content was assayed by the binding of Coomassie and measurement of absorbance at nm Bradford, This was left for 15 min and absorbance at nm was measured. Gaussian transformations were performed for the peaks in each gel lane, and the area under each peak compared to that of the purified standards.

When tests were carried out using varying amounts of either purified Rubisco or crude extract, a linear relationship was seen between amount per lane and area under each Rubisco peak for the range 0. Contamination of the insoluble fraction was negligible data not shown. Due to the sensitivity of photosynthetic parameters to temperature, irradiance and humidity, the data were carefully checked against environmental data provided by the IRRI weather station.

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Figure 1 shows the pattern of DW accumulation in panicles of each variety. However, due to the biphasic nature of grain filling in NPT varieties, the superior spikelets will have been completely filled during this period Yang et al. Spikelets were not categorized during weighing and so it was not possible to distinguish filling of inferior and superior spikelets, although some biphasic behaviour could be seen IR The most important parameters derived from these data are a the maximum rate of grain filling and b the time point at which this occurred.

The RGFPs of the panicle were clearly identified in all varieties except NPT1 and allowed good comparison with leaf biochemical parameters. These data are similar to those of Yang et al. P max was measured and results are shown in Fig. There were only small changes over the 3 week period. Measurements on leaves penultimate to the flag leaf were also carried out concurrently with flag leaf measurements and showed similar patterns although rates of P max were lower data not shown. Chlorophyll fluorescence yield, measured under ambient conditions and natural sunlight confirmed the absence of significant changes in flag leaf photosynthesis Fig.

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Changes in total leaf protein content are shown in Fig. The changes in the content of Rubisco were broadly similar to those for total protein content, with some differences: whilst the total protein over the first 7 DAF either declined or remained unchanged, the Rubisco content was maintained at the same level or increased slightly during this period.

The reductions in Rubisco content were greater than those for total protein.

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There were only small changes in Chl content of the leaves during grain filling. In the case of NPT1, the decline occurred much earlier. Measurements were made to determine whether the changes in sink activity were reflected in altered leaf carbohydrate content Fig.

Photosynthetic rate, transpiration rate and stomatal conductance of the regenerated plants in both date palm genotypes, were decreased whereas water use efficiency was increased with an increase in water stress level. Shamia genotype obtained higher values than Amri genotype in this respect. Carbon isotope discrimination was higher more negative values in shoots than in the roots and decreased with decreasing water supply. The values of maximum Fm and variable Fv fluorescence of the chlorophyll showed a significant reducing trend in both genotypes under stressed condition.

It could be concluded that both date palm genotypes studied, with the superiority of Shamia, have an ability to tolerate water stress under in vitro system. Chlorophyll fluorescence can be used as a tool, with other examined useful physiological characters, to screening date palm genotypes for drought resistant. How to cite this article: M.

Hanan El-Hosieny, American Journal of Plant Physiology, 6: DOI: The answers to these two questions holds the key to increasing plant productivity. Answering these two questions are extremely challenging. More than one hundred proteins are involved in photosynthesis. Even if manipulation is limited to simply altering the amount of protein, rather than its properties, the potential permutations will run into the millions. Considering that multiple proteins might need to be engineered at the same time, it quickly becomes apparent that experimentally up-regulating or down-regulating the amount of each protein and examining its impacts on productivity is inefficient.

One potential way to solve this problem is to take a systems approach. The systems approach is to develop mathematical models, which can faithfully simulate all of the biophysical and biochemical processes involved in photosynthesis, then, to conduct numerical experiments to identify targets to engineer for higher productivity. I will use a model of photosynthetic carbon metabolism to illustrate the basic methodology of developing systems models, and using an evolutionary algorithm to identify the optimal nitrogen allocation in metabolism for a higher photosynthetic rate of CO 2 uptake.

I will also briefly describe the other major applications of systems models of photosynthesis, which include linking genomic data to physiological data, studying dynamic signals in photosynthesis, studying the significance of changes in the photosynthetic apparatus to increase photosynthetic efficiency, and working as the basis for ecophysiological models.

One of the key targets for plant biology research is to increase photosynthesis During the past four decades, plant breeding has produced remarkable increases in the potential yields of many crops. Potential yield is defined as the yield that a genotype can achieve without biotic and abiotic stresses. S t is determined by the site and year. P n is the primary productivity, i. But, how can we increase Y further?

The harvest index for grain appears to be approaching an upper limit, given that a minimum quantity of biomass must remain in the plant body, to ensure that vital nutrients and reserves can be translocated into the grain, and to account for cell wall materials that cannot be degraded.

The harvest index for biomass crops is even higher.

Photosynthesis and Production in a Changing Environment

Figure 1. Minimum energy losses calculated for kJ of incident solar radiation, at each discrete step of the plant photosynthetic process from interception of radiation to the formation of stored chemical energy in biomass. The arrows indicate the amount of energy losses for different processes Zhu et al. For C3 crops, the highest short-term efficiencies are about 0.

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Direct evidence was provided by plants grown under Free Air CO 2 Enrichment, where both photosynthetic rate and plant productivity are increased Long et al. How can we identify new ways to engineer for higher productivity? As discussed earlier, experimental approaches become inefficient in identifying new engineering targets. Here I will use the model of photosynthetic carbon metabolism to illustrate a systems approach to identify new targets to engineer higher photosynthetic energy conversion efficiency.

Systems biology approach to identify new opportunities to engineer for higher productivity In this section, I will briefly describe the basic procedure for developing a systems model, and then describe the method of combining the systems model with evolutionary algorithms to identify new targets to engineer for higher productivity.