The Effects of Shade Treatment and Light Intensity on Rihulose-1,5-Diphosphate Carboxylase Activity and Fraction I Protein Level in the First Leaf of Barley
It has been confirmed that shading leaves from day 5 onwards lowers the rate of CO2 fixation when they are placed in saturating irradiances. The reduction due to shade treatment is about 46 per cent and a similar reduction in maximum chlorophyll content of the leaf follows shading. Maximum amounts of total soluble protein and of Fraction I protein are less in shaded leaves than in control leaves and prolonged treatment leads to a decline in leaf protein content. The relative amounts of different protein are also affected by treatment; in control leaves Fraction I protein accounts for about 45 per cent of the total but in shaded leaves the value is about 30 per cent. Increases and decreases in leaf protein amount, with concomitant changes in the ratio of Fraction I to total protein can be brought about by removing shades and re-applying them. Such changes can be induced even in fully expanded leaves in which net protein synthesis is not usually found. Maximal amounts of leaf protein are found in irradiances of 60 W m−2 or more, with lower values at lower light intensities. Where the first leaf is held in a stream of CO2-free air a lower level of protein is found. This, and the ratio of Fraction I to total protein, are similar to values for shaded leaves, and suggest the involvement of photosynthetic carbon fixation in determining leaf protein amount. A 1:1 linear correlation between amount of Fraction I protein and RuDP carboxylase activity is shown but the rate of CO2 incorporation by leaf extracts is 2–3 times greater than that of the intact leaf. The significance of this and the effect of irradiance on leaf protein amount are discussed.