The influence of elevated CO2 and simulated seasonal changes in temperature on tissue turnover in pasture turves dominated by perennial ryegrass (Lolium perenne) and white clover (Trifolium repens).
Tissue turnover, leaf morphology and population dynamics of perennial ryegrass and white clover were studied in pasture turves grown at ambient (350 µmol mol-1 ) or double ambient (700 µmol mol-1) CO2 concentrations for 217 days in controlled environment rooms. The turves were subjected sequentially to three day/night temperature regimes; 10/4°C, 16/10° and 22/16° and harvested at 3-week intervals. The photoperiod was 12 hours for all of the temperature treatments with a mean photon flux density of 480 µE m-2 s-1. Ryegrass leaf extension and leaf death rates did not differ between CO2 treatments and there was no effect of CO2 on rates of leaf appearance in white clover. Weight per unit length of ryegrass laminae was unaffected by elevated CO2 but lamina weight per unit area, lamina area and petiole weight per unit length in white clover showed a small positive response, especially at the two higher temperatures. Rates of growth and senescence per ryegrass tiller were therefore similar between CO2 treatments, but the rates of growth per white clover growing point were increased by 4, 23 and 13% at 10/4°, 16/10° and 22/16°, respectively, at elevated CO2. Responses to CO2 could not be attributed to any consistent change in morphological characteristics in either species and exposure to elevated concentrations of CO2 did not appear to change the relationship between growth and senescence per meristem. Total grass tiller populations were similar at both CO2 concentrations, but ryegrass tiller densities more than halved in both CO2 treatments as the temperature was increased. The fall was most severe at 700 µmol mol-1 and at the end of the experiment ryegrass tiller densities in this treatment were only 47% of those found at 350 µmol mol-1. There was no consistent effect of CO2 concentration on clover growing point numbers and they increased from 800 m-2 to over 3000 m-2 in both treatments with maximum densities occuring at 22/16°. It is suggested that, in swards dominated by ryegrass and white clover, exposure to elevated CO2 concentrations will alter the species composition in favour of white clover. Responses in aboveground DM yield to elevated CO2 will be a balance between the positive response shown by white clover and the negative response of perennial ryegrass. Temperature will have a major influence on the magnitude of this response since both the response of white clover to CO2 and the ratio of white clover growing points to ryegrass tillers are temperature-dependent.