Compensation of differences in light absorption at the levels of photosynthetic primary processes, CO2 uptake and growth of tobacco plants

Photosynthesis and growth were studied in tobacco (Nicotiana tabacum L.) plants exhibiting gradually reduced chlorophyll levels as a result of a glutamate 1-semialdehyde aminotransferase antisense transformation. Measurements of leaf pigmentation, spectral absorptance, chlorophyll fluorescence, CO2 gas exchange, and leaf area growth were carried out on leaves with an equal state of leaf and plant ontogeny, i.e., the 8th to 12th leaf from the top of plants with a total of 22 to 26 leaves. The combination of different growth light intensities (300 and 30 μmol · m−2 · s−1) and of differences in chlorophyll accumulation resulted in photon absorption rates of the leaves differing by a factor of 31. As a result of the gradually reduced energy input, rates of electron transport, net CO2 assimilation, and growth were lowered. However, the extent of this reduction varied between the hierarchic process levels: In relation to leaves of the experimental variant with the highest energy input, leaves with 10 % of the rates of photon absorption and electron transport reached 13 % of the assimilation rate and 53 % of the maximum leaf area growth rate. The data show that the electron transport rate was adjusted to a level slightly in excess of the energy demand of carbon assimilation, and the rate of net CO2 uptake was in excess of the carbohydrate demand of leaf area growth. The effort of these excessive flows of energy and carbon is the increased stability of the system under conditions of varying resource availability. As a result of excessive flows, differences in energy input restriction are compensated to a certain extent at higher hierarchic levels in leaves. Regarding the whole plant, another compensating mechanism plays a role: The differences between the variants were further diminished, as plants growing under severe energy input limitation reached total leaf areas equal to those of control plants with a delay of 40 days.