Annals of Botany 77: 35–45, 1996 Radiation Interception, Partitioning and Use in Grass–Clover Mixtures O.FAURIE*, J.F.SOUSSANA* and H.SINOQUET† *Fonctionnement et Gestion de l’Ecosyste[ me Prairial, INRA-Agronomie. 12 A. du BreU zet, and † PIAF, INRA-Bioclimatologie, Domaine de Crouelle, 63039 Clermont-Ferrand Cedex 2, France Received: 24 May 1995 Accepted: 29 August 1995 Mixed swards of perennial ryegrass}white clover were grown in competition under controlled environmental conditions, at two temperatures and with different inorganic nitrogen supplies. The swards were studied after canopy closure, from 800 to 1200 mC d cumulative temperatures. Clover contents did not vary significantly during the period. A simulation model of light interception was used to calculate light partitioning coefficients and radiation use efficiencies for both components of the mixture in this controlled environment experiment. Additionally, this same radiative transfer model was applied to the field data from Woledge (1988) (Annals of Applied Biology 112: 175–186) and from Woledge, Davidson and Dennis (1992) (Grass and Forage Science 47: 230–238). The measured and simulated values of light transmission, at different depths in the mixed canopy, were highly correlated (P ! 0±001) with more than 80% of the total variance explained. The daily average of photosynthetically active radiation (PAR) interception in a natural environment was estimated from simulations, for the field and controlled environment data. Under these conditions, white clover captured significantly more light per unit leaf area than perennial ryegrass at low, but not at high, nitrogen supply. In the controlled environment experiment, the radiation use efficiency of the legume was lower than that of its companion grass. For both species, radiation use efficiency was negatively correlated with the mean irradiance of the leaf. The role of a compensation between light interception and light use for stabilizing the botanical composition of dense grass–clover swards is discussed. # 1996 Annals of Botany Company Key words: Light interception, radiation transfer model, growth analysis, radiation use efficiency, grassland, white clover, perennial ryegrass, Trifolium repens L., Lolium perenne L. calculate light partitioning between grass and clover and INTRODUCTION thus to compare their light interception efficiencies in the Temperate perennial grasses and legumes differ with respect mixed canopy. to radiation interception and conversion efficiencies. In White clover leaves have high photosynthetic capacities monocultures, light is fully intercepted at a lower leaf area (Woledge, Dennis and Davidson, 1984; Dennis and index with legumes than with grasses (Brougham, 1958); on Woledge, 1985) and even in situ in the sward, where grass the other hand, the radiation use efficiency of C$ grasses was dominant (nitrogen fertilized plots), clover laminae had "% tends to be higher than that of legumes (Gosse et al., 1986). a greater assimilation rate of CO# per unit leaf area than Competition for light is considered to be important in grass (Woledge, 1988). This was also reported for simulated determining whether grass or legumes dominate in mixed mixed swards grown in a controlled environment (Davidson swards (Haynes, 1980). With perennial ryegrass (Lolium and Robson, 1985). perenne L.) and white clover (Trifolium repens L.) mixtures, The balance between species within the mixed sward long regrowth periods in conditions more favourable to depends upon the relative growth rate of each component grass growth (low temperatures, high availability of mineral (Woledge, 1988). The net assimilation rate of the leaf nitrogen) lead to strong competition for light and quite surface is not the only factor determining the plant relative often to a decline in the proportion of clover (Frame and growth rate; respiratory losses (Haystead et al., 1980; Ryle, Newbould, 1986). Arnott and Powell, 1981) and biomass partitioning between White clover has a greater proportion of its leaf lamina in organs (Ko$ rner, 1990) can be of overwhelming importance. the upper, well lit, layers of the canopy than grass (Dennis As a result, clover radiation use efficiency may be lower, in and Woledge, 1985; Woledge, 1988; Woledge et al., 1992). mixed swards, than that of the grass. Yet, there is little It seems therefore unlikely, even in nitrogen fertilized evidence of a lower radiation use efficiency of the legume in mixtures (Woledge, 1988), that the enhancement of grass a mixed sward. Models of radiation interception are needed growth could cause it to overtop and shade the clover. Yet, to calculate the efficiencies of photosynthetically active in nitrogen fertilized permanent grasslands, the small radiation (PAR) capture and of PAR use for species grown proportion of white clover was thought to be due to the in mixture. By using such a model, Sinoquet et al. (1990) large leaf area of the other species at heights which white calculated that the radiation use efficiency of white clover clover could not attain (Schwank, Blum and No$ sberger, was lower than that of the tall fescue in a nitrogen-fertilized 1986). However, in these reports, no attempt was made to mixture. However, some simplifications were made: (a) the 0305-7364}96}010035­11 $12.00}0 # 1996 Annals of Botany Company 36 Faurie et al.—Light Partitioning and Use in Grass–Cloer Swards canopy was only one horizontal layer, which did not (NIR, above 700 nm)—in which optical properties are account for differences in the vertical distribution of leaf assumed to be constant and incident radiation equally area between grass and clover; (b) leaf lamina angles were distributed (50% in each band, Varlet-Grancher, 1975, assumed to be constant. Moreover, as with most field amongst others). In this study, leaf transmittance was studies, the radiation use efficiency was presumably under- assumed to be 0±10 for both grass and legume species in the estimated as the root biomass was not taken into account PAR band, and 0±47 and 0±49 in the NIR band for tall fescue (Russell, 1993). and white clover, respectively (Nijs and Impens, 1993; In the present study, a similar light model, but using Varlet-Grancher, pers. comm.). several horizontal layers of leaves, was applied to the field data reported by Woledge (1988) and by Woledge et al. Controlled enironment experiment (1992) and to results from a controlled environment experiment with mixed grass-clover swards. Light par- Plant cultiation. Seeds of white clover (Trifolium repens titioning was calculated and, for the controlled environment L., cv. Grasslands Huı$ a) and perennial ryegrass (Lolium experiment, a growth analysis technique based on the whole perenne L., cv. Pre! fe! rence) were germinated in the dark at plant growth rate, allowed separate calculation of grass and 20 mC. After 3 d, the seedlings were transferred to an aerated clover radiation use efficiencies. basal nutrient solution (Faurie and Soussana, 1993) and grown in a controlled environment cabinet at 500 µmol # " PAR m− s− with a 14-h photoperiod at 20}16 mC, day}night MATERIALS AND METHODS temperature, respectively. Ten days after sowing, the clover seedlings were inoculated with Rhizobium leguminosarum Radiation interception model bv. trifolii USDA 2063. One week later, simulated grass– The simulation model of radiation interception computes clover swards were made by transplanting 21 plants of each the terms of the radiation balance of horizontally homo- species (in six alternate grass–clover rows) to a container # geneous, mixed crops. Most details of the model have been (0±40¬0±60) m surrounded with reflective side panels. For previously given (Sinoquet et al., 1990), thus only the main each species, five development classes were made, according features are summarized here. The model is based on the to the leaf number and to the leaf length, and plants from turbid medium analogy (see Ross, 1981). In the version used the two medium classes were distributed at random among in the present study, the canopy is divided into horizontal the replicate containers. After transplantation, the simulated layers containing foliage of either a single or the two species. mixed swards were grown, under the same conditions, either Each layer is characterized by the leaf area index (LAI), at (20}16) mC(T­)orat(12}9) mC(T®) day}night mean inclination and leaf scattering coefficient of each temperature, until a cumulative temperature sum of 1200 mC species present in the layer. Other model inputs are the sun d was reached, 65 (T­) or 100 (T®) d after sowing. The elevation, the direct and diffuse radiation at the top of the swards were rotated twice weekly around the growth cabinet. canopy, and the soil reflectance. Radiation interception for Two successive experiments were carried in the same growth each sky direction is computed from Beer’s law adapted to cabinet, one at T­ and one at T®. partition light between species in mixed layers (Sinoquet The basal liquid medium was renewed twice weekly and and Bonhomme, 1991). This is used to derive interception of supplemented every 2 d with Ca(NO$)#. Two amounts of N direct radiation (i.e. coming from a single sun direction) supply (N®,N­) were compared at the two temperatures − and diffuse light (i.e. assumed to come from a finite set of (T®,T­). A total of 50 and 200 mg N-NO$ per plant at sky directions). Scattering is characterized by exchange N® and N­, respectively, was supplied to the mixed coefficients between each pair of vegetation layers, which swards during their growth after transplantation. As nitrate combines scattering on leaf surfaces, assumed to be supply was based on the thermal time, the amounts supplied lambertian, and interception of scattered radiation. The every 2 d were smaller at T® than T­. radiation balance of the canopy, i.e. coupling between To avoid inhibition of clover N# fixation by excess nitrate interception and multiple scattering is solved using a method (Faurie and Soussana, 1993), nitrate supply was adjusted to similar to the ‘radiosity’ method (Ozisik, 1981).