M. Reynolds, B. Skovmand, R. Trethowan, R. Singh, and M. van Ginkel Wheat Program, CIMMYT

Physiological basis of improved yield and biomass

Increases in both yield and biomass have been associated with the introgression of a chromosome segment containing Lr19 (Agropyron 7DL.7Ag). Theoretically higher biomass may be achieved by:

• Increased interception of radiation (e.g. improved ground cover & 'stay-green')
• Greater intrinsic radiation use efficiency (e.g. improve net photosynthesis & canopy architecture)
• Improved source-sink balance (e.g. increase potential grain number and weight)

Experiments were conducted to determine which of these mechanisms were associated with greater yield and biomass (Table 1) in near-isogenic lines for the Lr19 gene complex.

Radiation Interception

• No differences in early biomass or "stay-green" in response to Lr19
• Therefore, differences in final biomass not related to differences in ability to intercept light

Radiation Use Efficiency

• Biomass accumulation and photosynthesis were greater after flowering in Lr19 lines
• No differences were observed before flowering (Table 2)

Source-Sink Balance:

partitioning to spike, duration of spike growth

• Lr19 increased partitioning of assimilates to developing spike
• Lr19 did not effect duration of juvenile spike growth (Table 2)

Conclusions

• Increased biomass of Lr19 lines resulted from and improved source-sink balance at flowering
• This led to higher demand-driven photosynthetic rates during grainfilling
• Lr19 had no effect on light interception, photosynthesis pre-flowering, or phenological pattern

Exploiting genetic resources

Traits have been identified in CIMMYT's germplasm bank with potential to improve "source" and "sinks" to raise yield potential, and to improve stress tolerance.

• Traits are introgressed into good backgrounds to establish potential genetic gains
• "Source" and "sink" type traits are crossed together to obtain synergy

Traits to improve spike fertility ("sink")

• Large spikes. Good sources available but seed often shrivelled
• Multi-ovary florets. Trait expressed in high yield backgrounds
• Branched spikelets. Introgressed with good results in Yugoslavia
• Higher grain weight potential. Expressed when extra assimilates available in boot stage
• Phenology. Genetic variation exists for duration of juvenile spike growth

Traits to improve assimilate availability ("source")

• Green area duration. Rapid full light interception & stay-green sources identified
• Stem reserves. Significant variation in accumulation and utilization exists
• Erect leaf. Being introgressed into high biomass Baviacora

Traits to improve stress tolerance

Many traits have been postulated to confer stress tolerance in wheat, depending on specific environments. Germplasm is being screened for sources of these characters.

Physiological screening tools

Canopy Temperature Depression (CTD)

• Leaves are cooled when water evaporates from their surface, part of the process photosynthesis
• CTD affected by many physiological processes, indicates a genotype's fitness to its environment
• CTD predicts yield best in irrigated situations when measured on sunny days in grainfilling
• Under drought, morning measurements are recommended (Table 3)

Table 3. Correlation between CTD measured pre-heading and during grainfilling, on 25 sister lines of Seri82/Baviacora92, morning and afternoon in two environments in Mexico, 1999-00.   Correlation with yield Trait Obregon Tlaltizapan CTD AM prehead 0.82** 0.79** CTD AM grainfill 0.79** 0.68** CTD PM prehead 0.85** 0.72** CTD PM grainfill 0.37    0.06    ** Statistical significance at P>0.01.

Potential genetic gains by selecting for CTD

• CTD measured on F_5:8 sister lines explained over 40% of the variation in yield (Fig. 1)

• CTD  of advanced lines predicted yield in heat stressed target countries (Reynolds et al., 1998)

• Stomatal conductance measured on single F_2:5 plants predicted yield of F_5:7 lines

Aerial infra-red imagery

• CTD was estimated remotely using aerial IR imagery on relatively small yield plots (Table 4)

• Results validated the potential of aerial IR imagery to screen thousands of breeding plots in a day

Spectral reflectance

• Sunlight reflected from a plot can be measured with a radiometer (Araus et al., 2000)

• Spectral reflectance (SR) estimates a range of physiological traits: chlorophyll, biomass, water status

• The SR index NDVI was significantly correlated with biomass and yield of advanced lines

• NDVI is being evaluating as a fast screening tool for yield, NUE, and triticale forage production

Incorporating physiological selection traits into a breeding scheme

• Breeding strategies must take into account multiple factors in addition to physiological traits

• Table 5 shows where physiological criteria might fit into a breeding scheme

References

Araus, J.L., J. Casadesus, and J. Bort, 2000. A review of some rapid screening tools for physiological traits determining yield. In: Eds Reynolds, M.P., Ortiz-Monasterio, I., McNab, A. Application of Physiology in Wheat Breeding. CIMMYT, Mexico D.F. 

Reynolds, M.P., Singh, R.P., Ibrahim, A., Ageeb, O.A., Larqué-Saavedra, A., and Quick J.S., 1998. Evaluating physiological traits to complement empirical selection for wheat in warm environments. Euphytica 100:85-94.

Published on June 2001

August, 2004