Influence of Elevated CO2 and Temperature on the Photosynthesis and Respiration of White Clover Dependent on N2 Fixation

Single clonal plants of white clover (Trifolium repens L) grownfrom explants in a Perlite rooting medium, and dependent fornitrogen on N₂ fixation in root nodules, were grown for severalweeks in controlled environments which provided two regimesof CO₂, and temperature 23/18 °C day/night temperaturesat 680 µmol mol^–1 CO₂, (C680), and 20/15 °Cday/night temperatures at 340 µmol mol^–1 CO₂ (C340)After 3–4 weeks of growth, when the plants were acclimatedto the environmental regimes, leaf and whole-plant photosynthesisand respiration were measured using conventional infra-red gasanalysis techniques Elevated CO₂ and temperature increased ratesof photosynthesis of young, fully expanded leaves at the growthirradiance by 17–29%, despite decreased stomatal conductancesand transpiration rates Water use efficiency (mol CO₂ mol H₂O^–1)was also significantly increased Plants acclimated to elevatedCO₂, and temperature exhibited rates of leaf photosynthesisvery similar to those of C340 leaves ‘instantaneously’exposed to the C680 regime However, leaves developed in theC680 regime photosynthesised less rapidly than C340 leaves whenboth were exposed to a normal CO₂, and temperature environmentIn measurements where irradiance was varied, the enhancementof photosynthesis in elevated CO₂ at 23 °C increased graduallyfrom approx 10 % at 100 µmol m^–1 s^–1 to >27 % at 1170 µmol m^–2 s^–1 In parallel, wateruse efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 In parallel,water use efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 to approx100 % at the highest irradiance Elevated CO₂, and temperatureincreased whole-plant photosynthesis by > 40 %, when expressedin terms of shoot surface area or shoot weight No effects ofelevated CO₂ and temperature on rate of tissue respiration,either during growth or measurement, were established for singleleaves or for whole plants Dependence on N₂, fixation in rootnodules appeared to have no detrimental effect on photosyntheticperformance in elevated CO₂, and temperature Trifolium repens, white clover, photosynthesis, respiration, elevated CO₂, elevated temperature, water use efficiency, N₂ fixation     

Single Leaf and Canopy Photosynthesis in a Ryegrass Sward

Swards of S24 perennial ryegrass (Lolium perenne L.) were grownin the field and harvested by repeated cutting. The rate ofphotosynthesis of the swards and of single leaves of plantstaken from them was measured throughout the growing season.Leaf area and light interception of the sward were also measured. In each growth period, sward photosynthesis increased with increasingleaf area and then declined. In the first growth period, duringwhich the crop flowered, the maximum rate of sward photosynthesiswas higher and was reached later than in succeeding vegetativegrowth periods. The photosynthetic capacity of successive youngest expandedleaves was high throughout the first growth period, and onlydeclined due to leaf age when no further leaves were producedafter flowering. However, in the succeeding vegetative growthperiods, although the first leaves to expand had high photosyntheticcapacities, later leaves had progressively lower capacities. The extent to which changes in the photosynthetic capacity ofsingle leaves accounts for the changes in sward photosynthesisobserved in the field is examined using a mathematical model.     

The Effect of Temperature on Photosynthesis of Ryegrass and White Clover Leaves

The rate of photosynthesis of leaves of perennial ryegrass (Loliumperenne L.) and white clover (Trifollum pratense L.) grown atdifferent temperatures was measured at a range of temperatures.There was a small effect of the temperature at which a leafhad grown on its photosynthetic rate, but a large effect ofmeasurement temperature, especially in bright light, where photosyntheticrates at 15°C were about twice those at 5°C. It appearsthat temperature could affect sward photosynthesis in the field.Ryegrass and clover had similar photosynthetic rates which respondedsimilarly to temperature. Lolium perenne L., ryegrass, Trifolium pratense L., white clover, photosynthesis, temperature, irradiance     

The effect of ryegrass mosaic virus on carbon assimilation and growth of ryegrasses

The growth of simulated swards of Italian and perennial ryegrass, artificially infected with ryegrass mosaic virus (RMV) or RMV-free, was investigated. During the build up of RMV infection, dry-matter yield was usually decreased in infected swards, and the effect was confirmed when single vegetative regrowths of swards were investigated in detail. The primary cause of lower yields in RMV-infected swards appears to be a decrease in net canopy photosynthesis (maximum decrease about 50%) and an associated increase in dark respiration (maximum increase about 50%). The decrease in net canopy photosynthesis is due primarily to a lowered rate of net photosynthesis of constituent leaves within the canopy. A secondary effect of RMV infection is to decrease tillering (maximum decrease about 30%) which results in a change in canopy structure and in particular a lower leaf area index. In the present experiments, lower light utilization by the swards is less important than the decrease in leaf photo-synthetic efficiency in lowering yield.     

Effect of Flowering on the Photosynthetic Capacity of Ryegrass Leaves Grown With and Without Natural Shading

The photosynthetic capacity of newly expanded leaves of vernalizedor non-vernalized plants of S24 perennial ryegrass (Lolium perenneL.), grown in long or short photoperiods, was measured in twoexperiments. In the first, leaves were protected from shadingduring development, while in the second, the natural shade ofneighbouring tillers in a sward was allowed. In the first experiment there was little effect of vernalization,day length or flowering, and leaves in all treatments had photosyntheticrates at 250 W m^–2 of between 28 and 32 mg CO₂ dm^–2h_–1.In the second experiment the photosynthetic rate of successiveleaves fell as sward leaf area increased. This downward trendwas reversed, however, in flowering tillers in the vernalizedlong-day treatment, while in the other treatments, which didnot flower, photosynthetic capacity continued to fall. It isconcluded that the leaves of reproductive tillers have highphotosynthetic capacities because stem extension carries themto the top of the canopy where they are well illuminated duringexpansion. Lolium perenneL, ryegrass, photosynthetic capacity, flowering, shading, vernalization     

The Effect of Age on the Rate of Apparent Photosynthesis in Leaves of Tall Fescue (Festuca arundinaceaSchreb.)

n apparatus is described in which the rate of apparent photosynthesisof single attached or detached grass leaves was determined bymeasuring with an infra-red gas analyser the uptake of carbondioxide from a stream of air passed over the leaves. In one experiment the apparatus was used to determine the apparentphotosynthesis at one time of the three youngest leaves removedfrom a tiller of tall fescue plants grown in the field. Resultsshowed that each older leaf had a lower rate of apparent photosynthesisthan its successor. To determine the real effect of age of theleaf without any possible interaction with position of the leafor an after-effect of environment, measurements of apparentphotosynthesis were made in two further experiments on leavesattached to the main stem on plants grown in controlled environments.Results were obtained from samples of the fifth leaf of increasingage from full expansion. These showed a progressive declinein apparent photosynthesis with increasing age: in one experimentthis was preceded by a small initial rise after complete leafexpansion. There were no marked changes in the weight of theleaf as it aged and, in consequence, the form of the resultswere similar when expressed on either a leaf area or dry weightbasis.     

The Effect of Shading on the Photosynthetic Rate and Longevity of Grass Leaves

The rate at which the net photosynthesis of grass leaves grownin bright light (119 W m^–2) decreased as they aged wasincreased by severe shading (to 21 W m^–2 or less). However,less severe shading (light intensities of 36 W m^–2 ormore) had no effect. The decrease in photosynthesis was unaffectedby whether the whole plant was shaded or only the leaf whosephotosynthesis was measured. In both shaded and unshaded leaves, photosynthesis measuredin bright light fell faster as the leaf aged than did photosynthesisin dim light. Both mesophyll and stomatal diffusion resistancesrose as the leaf aged but the former rose faster. The chlorophyllcontent fell only towards the end of the life of the leaves.     

The Effect of Temperature during Growth on the Subsequent Rate of Photosynthesis in Leaves of Tall Fescue (Festuca arundinacea Schreb)

Two experiments are reported in which young plants of tall fescuewere grown in temperature regimes of 20 °C day/15 °Cnight or 10 °C day/5 °C night until the fourth leafon the main stem was fully expanded. These temperature regimeswere then either changed over for individual plants or continuedunchanged up to the seven-leaf stage. Photosynthesis and respirationrates were determined in the fourth and subsequent leaves andalso in ageing leaves, using an infra-red gas analyser in anopen system and at temperatures of 10 and 20 °C in one and10, 15, 20, and 25 °C in the other experiment. Rates of apparent photosynthesis per unit leaf area in fullyexpanded leaves differed little as a result of previous treatmentand were not greatly affected by temperature during measurement.However, the specific leaf area and the rate of apparent photosynthesisper unit dry weight were higher in plants grown at the hightemperature. Leaves from the high-temperature regime had a higheroptimum temperature for apparent photosynthesis, a shorter life,and a lower respiration rate at any one temperature of measurementthan did leaves from the low-temperature regime. After transfer from one temperature regime to the other, therate of apparent photosynthesis of the next leaf to become fullyexpanded was higher in plants transferred from low to high temperatureand lower in plants transferred from high to low than in plantsremaining in either temperature regime; the leaves which subsequentlyexpanded had rates similar to those of unchanged plants. Inleaves which were fully expanded at the time of transfer, therate of apparent photosynthesis rose after transfer to the high-temperatureregime and fell after transfer to the low-temperature regime. These results are discussed in relation to growth-analysis datafrom other plants grown in the same conditions.     

Competition between grass and clover in spring as affected by nitrogen fertiliser

Detailed measurements of irrigated ryegrass/white clover swards growing without interruption, with or without nitrogen fertiliser in spring, showed that the relative growth rate of clover was as great as that of grass, in the + N sward, and considerably greater than grass, in the – N sward. Clover leaves were not overtopped by grass leaves. Indeed, in both swards, clover had a greater proportion of its leaf lamina area in the upper, well lit, layers of the canopy than grass did. Consequently, clover had a greater mean rate of leaf photosynthesis in situ in the sward than grass. Clover's advantage in photosynthetic rate per leaf area was offset to some extent by its smaller ratio of leaf area to total above-ground dry weight than grass. The consequences of these results for our understanding of competition between grass and clover in mixed swards are discussed.     

The photosynthesis of ryegrass leaves grown in a simulated sward

Plants were taken from simulated swards of perennial ryegrass (Lolium perenne) grown in a controlled environment and the rates of photosynthesis of the youngest fully expanded leaves, and the second and third youngest leaves on the same tillers were measured. The youngest leaves had the highest rates and the third the lowest, with the second leaves intermediate. The rate of photosynthesis in bright light of successive youngest expanded leaves decreased as the swards increased in leaf area, but did not when plants were grown so that the main stem was not shaded. When plants were grown at different densities and the photosynthetic rates of leaves of a particular ontogenetic rank were measured, it was found that leaves on plants from higher densities had lower rates of photosynthesis. Also leaves on plants grown in bright light had higher photosynthetic rates than those on plants grown in dim light. It is concluded that the decline in the photosynthetic capacity of successive leaves in a rapidly growing simulated sward is due to the intense shading to which they are subjected during their expansion.