A Model for Leaf Photosynthesis by C3 Plant Species

A mathematical model for leaf photosynthesis is constructed.The model relates the net rate of carbon dioxide exchange perunit leaf area to the ambient carbon dioxide and oxygen concentrationsand the light-flux density incident on the leaf surface. The behaviour of the model under steady-state conditions isexamined and discussed. Simulated carbon dioxide and light-responsecurves for the net carbon dioxide exchange rate accord wellwith experimental observation. The model describes an inhibitionof the net exchange rate by increasing oxygen concentrationswhich is similar to the commonly observed inhibition.     

An Analysis of Spatial Variation in the Nitrogen Content of Leaves from Different Horizons Within a Canopy

An analysis of spatial variation in the specific nitrogen contentof leaves from different positions within a canopy is developed.It is used to examine data obtained for contrasting crop species;the legume crop guar (Cyamopsis tetragonoloba), and crops ofa forage sorghum. The analysis distinguishes two componentsof the leaf nitrogen content: one associated with the metabolicapparatus within the leaf tissues and the other with structuralelements of the leaf. The analysis allows the spatial variationin specific leaf nitrogen content to be quantitatively described. Leaf nitrogen content, crop growth, light interception     

The Influence of Temperature on Photosynthesis and Transpiration in ten Temperate Grass Varieties Grown in four Different Environments

The influence of temperature on photosynthesis and transpirationwas studied in ten varieties of Lolium perenne, L. multiflorum,Dactylis glomerata, and Festuca arundinacea from three climaticorigins grown in three different controlled environments (15?C, 72 W m^-2 visible irradiation, 16-h photoperiod; 25 ?C, 72W m^-2 visible irradiation, 16-h photoperiod; and 25 ?C, 180W m^-2 visible irradiation, 16-h photoperiod) and in the glasshousein July/August. The optimum temperature for photosynthesis was influenced primarilyby growth environment; growth at low temperature (15 ?C) resultedin a low optimum temperature, which differed little from varietyto variety. The maximum CO₂-exchange rate was influenced bygrowth environment and by variety. Within a variety, plantsgrown at higher light intensity or lower temperature had a greaterCO₂-exchange rate. Seven varieties showed a negative correlationbetween the optimum leaf temperature and the maximum CO₂-exchangerate. Activation energies for photosynthesis were influencedby growth environment only. There were marked varietal differences in the values of leafresistances (r_a + r_t) obtained from transpiration data at theoptimum leaf temperature for CO₂ exchange. In Lolium, and Dactylisthe Mediterranean varieties had higher leaf resistances thanthe Northern varieties with the maritime varieties intermediate.In general the Dactylis varieties had higher resistances thanthe corresponding Lolium and Festuca varieties. Only at highgrowth temperatures was (r_a+r_l) insensitive to temperature;otherwise an activation energy of about 10 kcal/mole was observed.A negative correlation was found between mean varietal diffusionresistances (r_a+r_l), and corresponding maximum CO₂-exchangerates.     

Light interception by grain legume row crops

Abstract. Four contrasting grain legume species ( Glycine max, Vigna radiata. Vigna mungo and Vigna angularis ) were grown as row crops with both 0.5 m and 1.0 m spacings between row centres. Light transmission profiles, at ground level, across rows of plants, were obtained for each crop on a number of occasions during growth. The proportion of the incident downward light flux density intercepted by each crop at solar noon was found to be simply and directly related to the product of the proportion of the ground area covered by the crop's leaf canopy and the proportion of the downward light flux density incident at the row centre that was intercepted by the crop. The average proportion of the incident light energy intercepted over the whole day could be related to the proportion intercepted at solar noon.     

A Physiological Approach to the Analysis of Crop Growth Data. II. Growth of Stylosanthes humilis

Crops of three ecotypes of the pasture legume Stylosanthes humiliswere grown during different lengths of growing season at Katherine,N T, Australia The ecotypes had contrasting flowering behaviour,and the interaction between their relative periods of vegetativeand reproductive growth and the length of the growing seasonwas studied A preliminary analysis, employing the techniques of traditionalgrowth analysis, was made of the data. The results were equivocaland a physiological approach to the analysis of the data wastaken From this second analysis it was concluded that the mainphysiological differences in the growth of the crops was theirflowering date and that differences in forage yield, and theyield of reproductive parts, at the end of the growing seasoncould be attributed to this factor No other consistent significantdifferences in the physiological behaviour of the crops wereobserved crop growth, mathematical model, Stylosanthes humilis, legume     

Leaf Carbon Dioxide Compensation Points at High Light Flux Densities

A model for leaf photosynthesis by C₃ plant species is usedto examine the inter-relations between carbon dioxide and oxygenconcentrations, and dark respiration rates at the leaf compensationpoint. The model is applied to data published on a range ofC₃ plant types. These data indicate a remarkable constancy inthe slope of linear relationship between the CO₂ and O₂ concentrationsat the leaf compensation point, indicating a constancy in theratio of photorespiration to photosynthesis across C₃ planttypes. The model is extended to deal with C₄ plants and used to interpretthe differences in the compensation point relationships of C₃and C₄ plants.     

The Contribution of Leaves from Different Levels within a Tomato Crop to Canopy Net Photosynthesis: An Experimental Examination of Two Canopy Models

The rates of net photosynthesis per unit ground area by a closedcanopy of tomato plants were measured over a range of naturallight flux densities. The canopy, of leaf area index 8.6, wasdivided into three horizontal layers of equal depth. On successivedays the canopy was progressively defoliated in layers fromthe ground upwards, allowing the photosynthetic contributionfrom individual leaf layers to be determined. The uppermostlayer, 23% of the total leaf area, assimilated 66% of the netCO₂ fixed by the canopy and accounted for a similar percentageof the total leaf respiration. Net photosynthesis versus light response curves for individualleaves from different positions within the canopy were alsoobtained. Leaf conductances to CO₂ transfer and the dark respirationrates of leaves from the uppermost leaf layer were approximatelyten times those from the lowest layer. The canopy data were analysed using a simple model which assumedthat the canopy was composed of leaves with identical photosyntheticand respiratory characteristics. The model fitted the data andallowed the characteristics of an ‘idealized’ leafto be estimated. The estimated values of the leaf light utilizationefficiency, ,and the leaf conductance CO₂ transfer, , were similarto values directly determined for individual leaves in the uppermostleaf layer and the estimated rate of leaf dark respiration,R_d, corresponded to measured rates for leaves much lower inthe canopy. The simple model may be used to examine gross effectsof crop environment on the leaf photosynthetic characteristicof an ‘idealized’ leaf, but cannot be used to predictaccurately canopy net photosynthesis from the photosyntheticand respiratory characteristics of any single real leaf. A moredetailed model, developed to allow explicitly for the observedvariation in and R_d within the canopy is appropriate for thispurpose.     

Growth Response of a Chrysanthemum Crop to the Environment. III, Effects of Radiation and Temperature on Dry Matter Partitioning and Photosynthesis

Vegetative crops of chrysanthemum were grown for 5 or 6 weekperiods in daylit assimilation chambers. Crop responses to differentradiation levels and temperatures were analysed into effectson dry matter partitioning, specific leaf area, leaf photosynthesisand canopy light interception. The percentage of newly formed dry matter partitioned to theleaves was almost constant, although with increasing radiationor decreasing temperature, a greater percentage of dry matterwas partitioned to stem tissue at the expense of root tissue.There was a positive correlation between the percentage of drymatter in shoot material and the overall carbon: dry matterratio. Canopy photosynthesis was analysed assuming identical behaviourfor all leaves in the crop. Leaf photochemical efficiency wasonly slightly affected by crop environment. The rate of grossphotosynthesis per unit leaf area at light saturation, P_A (max),increased with increasing radiation integral, but the same parameterexpressed per unit leaf dry matter, P_w (max) was almost unaffectedby growth radiation. In contrast, P_A (max) was hardly affectedby temperature but P_w (max) increased with increasing growthtemperature. This was because specific leaf area decreased withdecreasing temperature and increased with decreasing radiation.There was a positive correlation between canopy respirationintegral and photosynthesis integral, and despite a four-foldchange in crop mass during the experiments, the maintenancecomponent of canopy respiration remained small and constant. Canopy extinction coefficient showed no consistent variationwith radiation integral but was negatively correlated with temperature.This decrease in the efficiency of the canopy at interceptingradiation exactly cancelled the increase in specific carbonassimilation rate that occurred with increasing growth temperature,giving a growth rate depending solely on the incident lightlevel. Chrysanthemum, dry matter partitioning, photosynthesis, specific leaf area     

Interception of Diffuse and Direct-beam Radiation by a Hedgerow Apple Orchard

A model for the interception of diffuse and direct-beam radiationby parallel rows of trees has been used to simulate the radiationtransmission profiles for row transects in a hedgerow appleorchard. The simulated profiles are in close agreement withexperimentally measured radiation transmission profiles foran apple orchard. The model has been used to predict average daily and seasonalvariations in the proportions and amounts of the incident radiationabsorbed by the orchard. The effects of row orientation androw spacing on the absorption of radiation by the orchard havealso been examined.     

A Model of the Partitioning of New Above-ground Dry Matter

A model for the partitioning of new above-ground dry matterof a vegetative dicotyledonous plant is developed. It assumestwo distinct functional components of the stem tissues. One,the primary stem tissue, includes physiologically active tissuessuch as xylem, phloem and meristematic tissues, and the other,secondary stem tissue, includes the main mechanical structures.The model is used to examine the partitioning of dry matterbetween leaf and stem tissues, and its behavour is comparedwith experimental observations. Partitioning, leaf, stem, secondary stem tissues