Photosynthesis is becoming one of the main targets for improving wheat yields, so a better understanding of techniques such as gas exchange and chlorophyll fluorescence is important. However, evaluating photosynthesis in different genotypes is not a trivial task. While theoretically we might expect a strong positive relationship between leaf photosynthesis and biomass and/or yield, this has not always been the case in field experiments. This lack of correlation is due to the fact that at any given period of ti me across several genotypes, photosynthetic rates change in parallel with leaf canopy size, architecture, leaf specific area and plant phenology. Further, translocation of assimilates from the leaf to stems, roots and reproductive structures (and maintenance of the plant as a living unit at minimum cost) are also important downstream contributors to yield. In other words, instantaneous measurements of leaf carbon dioxide (CO2) fixation represent only a snapshot in ti me and space of total canopy photosynthesis over a crop cycle. The difficulties related to the lack of plant uniformity when evaluating photosynthesis are aggravated when assessing the responses to drought, since stomatal conductance is low and highly variable. However, under well irrigated and warm conditions there are examples where photosynthesis capacity has shown important positive contributions to yield. Therefore, evaluating photosynthesis under well irrigated and warm environments may provide an appropriate measurement of tolerance to those environments. Photosynthesis can be measured using gas exchange and esti mated using chlorophyll fluorescence. A range of portable systems are now available allowing users to make real-ti me measurements of leaf photosynthetic CO2 uptake, transpiration, leaf conductance, intercellular CO2 mole fraction, efficiency of photosystem II photochemistry, photochemical and non-photochemical quenching and other chlorophyll fluorescence parameters that can be simultaneously determined. However, for appropriate use of these techniques, it is essential to have a basic knowledge of the principles, applications, measurements and limitations of both gas exchange and chlorophyll fluorescence systems. The principles and applications of these techniques are described in this chapter.

Global Wheat Program

Text in English

INT2835|INT3193

CIMMYT Staff Publications Collection