Response of photosynthesis to genetic differences in sink-source ratios



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Texas Tech University


Photosynthetic rates, whether measured on single leaves or whole plants, varied in response to environmental parameters, plant age and sink:source (weight of fruit and stem per unit leaf area) ratios. In this study, five cotton (Gossypium hirsutum L.) and five sorghum (Sorghum bicolor L. Moench) strains, genetically different for sink: source ratios were grown in a common environment. This common environment surrounding each plant provided a standardized environment for photosynthesis across all plants within a sample time and allowed any photosynthetic rate differences to be in response to genetic differences in sink:source ratios. Photos3nithetic rates were measured using 1^C02 gas exchange rates on the uppermost fully expanded main stem leaf of cotton and, prior to flag leaf exsertion, the uppermost collared leaf of sorghum. After flag leaf exsertion, the next lower leaf was used as the sample leaf. Seven measurements were made on cotton commencing 51 days after planting (DAP) and terminating 104 DAP. Four measurements were made on sorghum beginning 40 DAP and terminating 70 DAP.

Parameters measured at each sampling date included solar irradiance, leaf and air temperature, and leaf conductance. Correlations between these parameters and photosynthetic rates at each sampling date for both cotton and sorghum were low.

Photosynthetic rates for cotton were low at the first sampling date and increased thereafter with significant differences among fruiting types. Following fruit formation, the rate of photosynthesis for the fruiting strains did not change significantly indicating that the chloroplasts were operating at their peak capacity within the surrounding environment. The sink:source ratios increased significantly at each sampling date with significant differences among strains. Photosynthetic rates did not respond to changes in sink:source ratios within each fruiting strain following fruit formation across the season but there was a significant difference in photosynthetic rates among the strains with different sink:source ratios.

Photosynthetic rates for sorghum were initially high and declined at each succeeding sampling date in response to leaf age. Sink:source ratios increased significantly across the season with significant differences between strains at each sampling date. With the declining photosynthetic rates across the season, it cannot be concluded that there was a positive photosynthetic rate response to the changing sink: source ratio. At the last sampling date, the fruiting strains with the highest sink:source ratio had the highest photosynthetic rate and, likewise, the nonfruiting strain with the lowest sink:source ratio had the lowest photosjmthetic rate.

Among cotton and sorghum strains, there was a photosynthetic rate response to changes in sink:source ratios. This consistency in response suggests that both species reacted similarly to changes in sink:source ratios and that some residual potential for increased photosynthetic rates existed when the plants were in the vegetative stage of development. This residual potential for increased photosynthetic rate increases the difficulty in practicing plant breeding for germplasm with enhanced photos3mthetic potential. The results dictate that many growth parameters must be monitored prior to selection of germplasm with enhanced photosynthetic potential. This may explain some of the failures in past attempts to select for this physiological trait.