Changes in the Photosynthetic Properties of Australian Wheat Cultivars Over the Last Century

Abstract

A selection of Australian wheat cultivars, representative of those widely cultivated over the last century, was grown in the glasshouse. Photosynthetic properties of the flag leaves were determined as well as the chlorophyll, soluble protein, nitrogen and photosystem II content of the leaves. The rate of electron transport was calculated from gas exchange measurements at high CO₂ partial pressures or from chlorophyll fluorescence. We found a trend towards higher rates of CO₂ assimilation, higher chlorophyll and nitrogen contents per unit leaf area over time. The increase in leaf nitrogen and photosynthetic rate is most probably due to the introduction of the Rht dwarfing genes. Counter to this we observed declines in rate of electron transport per unit chlorophyll, chlorophyll a/b ratio and photosystem II content per unit chlorophyll. Because the chlorophyll to nitrogen ratio was similar between cultivars, lowering of the rate of electron transport per unit chlorophyll in modern cultivars results in a fall in the rate of electron transport per unit of nitrogen. There was no variation in the ratio of intercellular to ambient CO₂ partial pressures, or leaf mass per unit leaf area. The comparison of a near isogenic pair, with or without a portion of a rye chromosome that is associated with high yield, did not reveal any association with photosynthetic characteristics. It is suggested that selection for higher chlorophyll a/b ratios for a given leaf chlorophyll content may provide a feasible surrogate for selecting higher photosynthetic capacity.