Annals of Botany

Copious carbon fails to influence sink so leaves linger on as usual

In spring ephemerals of northern temperate woodlands, aerial shoots emerge when conditions become warm enough, e.g. after the snow has melted; flowering and seed set are completed before the canopy closes over. During the same period, the underground perennial organs are replenished and then the aerial shoots die back. It had been assumed that shoot senescence was initiated by canopy closure but recently it has been suggested that completion of sink filling in the underground perennial organ is the main factor. This has been tested by Gutjahr and Lapointe at Québec (pp. 835–843), working with Erythronium americanum. The authors point out the advantages of this plant as a subject of study: non-flowering individuals consist of a single leaf and a single bulb; the root system develops during the cold stratification period before shoot emergence. Non-flowering plants were grown under ambient (400 ppm) or elevated (1100 ppm) CO2 concentrations. As expected, plants grown at 1100 ppm CO2 exhibited a significantly higher net assimilation rate and therefore fixed more C than those grown at 400 ppm. Despite this, there were no differences in bulb-filling rates, nor in the final size of the bulbs. This was reflected in the lack of difference in both bulb cell number and cell size between the two treatments. Similarly, starch deposition and final starch content were very similar in the two treatments. Neither were there differences in leaf dry weight and area, nor in leaf growth period. However, there was one major difference: both leaves and bulbs of plants grown under elevated CO2 exhibited much higher respiration rates than control plants. Overall then, elevated CO2 concentrations do not affect the sink (bulb) because of the over-riding controls of cell size and cell division, and it remains possible that leaf senescence is indeed tied to the filling of the sink. It appears that the extra carbon fixed under elevated CO2 levels is simply burned off.