Cumulative Photosynthetically Active Radiation Research Articles

PARADe: A low-cost open-source device for photosynthetically active radiation (PAR) measurements

In agriculture, proximal in-field information is crucial for a precise management of crop growth. A low-cost, miniaturized and innovative device, named PARADe (PAR Acquisition Device), is designed for in-field photosynthetically active radiation (PAR) acquisition. It combines an affordable PAR line quantum sensor (PAR/LE, SOLEMS) and an open source development platform (microcontroller) based on Arduino Integrated Development Environment. The quality of the measurement of the PARADe acquisition chain has been validated. The calibration is done by comparison (R2=0.99) with a robust acquisition chain composed of a certified PAR sensor (PQS1, Kipp and Zonen) and a data-logger (CR1000 Campbell Scientific). The accuracy of PARADe measurements is evaluated through three indicators, relative error, RMSE and normalized RMSE. They demonstrate that the PARADe system has certain operating limitations especially for low solar angles (sunset and sunrise) due to the choice of a line quantum sensor. This does not affect the accuracy and reliability of the results, but indicates that the PARADe device is specifically adapted to collect daily cumulative PAR values.

Seasonal dynamics of energy and nutrients of Pinus koraiensis seedlings in different successional stages of broadleaved Korean pine forest in Changbai Mountain, China.

We examined the seasonal variations of growth and nutrient accumulation of two-year-old Korean pine (Pinus koraiensis) seedlings in the primary broadleaved Korean pine forest (primary forest) and poplar birch forest (secondary forest) in Changbai Mountain. The seasonal changes of photosynthetically active radiation (PAR), seedling biomass, nonstructural carbohydrate (NSC), total nitrogen (N), and total phosphorus (P) were measured in both forests. The effects of understory light and its seasonal variations in both forests on the growth and nutrient accumulation of understory Korean pine seedlings were also explored. The results showed that the seasonal variations of monthly cumulative PAR under both forests showed a double-peak pattern. In summer, light conditions under the two forests were poor because of the closed canopy. In spring and autumn, light conditions were better due to the deciduous characteristics of canopy broadleaved trees, with light conditions under the secondary forest being obviously better than that of the primary forest. The seasonal variations of biomass, NSC, total N and total P concentrations of Korean pine seedlings in the primary and secondary forests were basically consistent with that of understory light. All those variables increased significantly in spring and autumn, and decreased in summer. In spring, starch concentration increased. In summer, starch and soluble sugar concentrations decreased gradually, and touched bottom in August. In autumn, soluble sugar concentration increased significantly. The biomass and NSC concentration of seedlings under secondary forest were significantly higher than those under primary forest in spring and autumn, but without difference in summer. Therefore, the diffe-rence of understory light conditions in spring and autumn between the two forests might be the key factor driving nutrient accumulation, growth and regeneration of Korean pine seedlings.

Novel technique for non-destructive LAI estimation by continuous measurement of NIR and PAR in rice canopy

Although leaf area index (LAI) is a useful index of crop growth, evaluating LAI by destructive sampling is laborious and time-consuming. In the present study, we developed a novel technique to determine rice LAI non-destructively and accurately throughout growth period in paddy field. Near-infrared radiation (NIR) and photosynthetically active radiation (PAR) were measured by a pair of optical sensors inside and outside rice canopy at 1-min intervals. Simple ratio (SR) and NIR to PAR ratio (N/P) were measured depending on growth stage, and the criteria to extract valid SR and N/P that represent LAI accurately were determined. SR and N/P obtained under cloudy conditions were found to represent LAI with high accuracy (R2 = 0.91), and seasonal dynamics of canopy growth curves were successfully drawn for two rice cultivars at two experimental fields. Mathematical analysis revealed that not cumulative PAR but cumulative temperature could trigger exponential canopy growth. The present technique can be used as a powerful tool for evaluating, modelling, and phenotyping crop growth characteristics in various rice cultivars as well as other crops.

3D city models for urban farming site identification in buildings

Studies have suggested that there is farming potential in urban residential buildings. However, these studies are limited in scope, require field visits and time-consuming measurements. Furthermore, they have not suggested ways to identify suitable sites on a larger scale let alone means of surveying numerous micro-locations across the same building. Using a case study area focused on high-rise buildings in Singapore, this paper examines a novel application of three-dimensional (3D) city models to identify suitable farming micro-locations (level and orientation) in residential buildings. We specifically investigate whether the vertical spaces of these buildings comprising outdoor corridors, façades and windows receive sufficient photosynthetically active radiation (PAR) for growing food crops and do so at a high resolution. We also analyze the spatio-temporal characteristics of PAR, and the impact of shadows and different weather conditions on PAR in the building. Environmental simulations on the 3D model of the study area indicated that the cumulative daily PAR or Daily Light Integral (DLI) at a location in the building was dependent on its orientation and shape, sun's diurnal and annual motion, weather conditions, and shadowing effects of the building's own façades and surrounding buildings. The DLI in the study area generally increased with building's levels and, depending on the particular micro-location, was found suitable for growing moderately light-demanding crops such as lettuce and sweet pepper. These variations in DLI at different locations of the same building affirmed the need for such simulations. The simulations were validated with field measurements of PAR, and correlation coefficients between them exceeded 0.5 in most cases thus, making a case that 3D city models offer a promising practical solution to identifying suitable farming locations in residential buildings, and have the potential for urban-scale applications.

Study on the characteristics of climate change in Changbai Mountain National Natural Reserve from 1958 to 2017

Climate change is an important factor affecting the ecological environment of regions, and the local climate is closely related to the ecological environment. Taking the Changbai Mountain National Natural Reserve as an example, the regional climate change was studied by analyzing the data from the years 1953–2017. It is found that the temperature of Changbai Mountain is on the rise; the maximum monthly mean temperature difference is approximately 35 °C and the annual mean temperature is 2–5 °C. The monthly mean wind speed in this area has a wavy distribution and obvious seasonal variation. The correlation analysis shows that the correlation coefficient of the monthly mean value of cumulative photosynthetically active radiation (PAR) and total solar radiation (TSR), cumulative evaporation and cumulative PAR, cumulative evaporation and TSR, temperature and cumulative PAR, and temperature and cumulative evaporation were more than 0.9, indicating a significant correlation. The annual cumulative precipitation in this area is between 600 and 1000 mm, the annual cumulative evaporation is between 1000 and 1500 mm, the annual cumulative PAR is between 6000 and 10,000 mol m−2 s−1, and the annual TSR is between 4000 and 6000 MJ m−2. The annual cumulative precipitation, annual cumulative evaporation, annual mean wind speed, annual cumulative PAR, and annual TSR show a downward trend. The climate change in this area will not have a greater negative impact on the forest, but it still needs to pay close attention to the further climate change.

Characterization of ambient conditions inside greenhouses of papaya (Carica papaya L.) crops in the Canary Islands

Quality problems observed at certain times of year, related to low soluble solids content (°Brix), have restricted the exportation market and, thus, the expansion of papaya cultivation in the Canary Islands. In order to develop new management strategies oriented to maintain a steady marketable fruit supply all year around, it is necessary to improve our knowledge of crop phenology under the specific climatic conditions of the islands. The present study was conducted during a 2-year commercial crop cycle, from April 2015 to March 2017. Two commercial papaya fields under plastic-cover greenhouses were selected; one on the south (SP) and the other on the north (NP) shore of Tenerife. Meteorological variables [air temperature, relative humidity and photosynthetically active radiation (PAR)] were logged inside the selected greenhouses. Production and total soluble solids (TSS) content of the fruit, as a commercial quality parameter, were measured weekly. In winter, the daily average of growing degree days (GDD) and cumulative PAR were significantly higher in SP. However, in spring, GDD was higher in NP, while, in summer and winter, no significant differences were found between orchards. As a consequence of such differences in meteorological conditions, the papaya production periods were different at each greenhouse: from October to March in SP and from February to August in NP. Moreover, fruits from the SP greenhouse presented higher TSS (above 12 °Brix all year round) than in the NP, where TSS dropped off to below 12 °Brix from January to June. Scheduled cultivation on both shores of the island could improve the availability of papaya to supply the market throughout the year. Although the study suggests that the SP greenhouse produced fruit with better quality, further research is necessary in papaya crop management to increase fruit quality in the NP area, especially for fruit that is going to be harvested from January to June.

Daily Light Integral Requirement of a Creeping Bentgrass Putting Green as Affected by Shade, Trinexapac‐ethyl, and a Plant Colorant

ABSTRACTTurfgrass quality is often limited by the availability of photosynthetically active radiation (PAR) delivered to the turf surface. The cumulative PAR at a given site during 1 d is termed the daily light integral (DLI) and is an effective measurement for quantifying light requirements for various turfgrass systems. The minimum DLI necessary to maintain acceptable quality on creeping bentgrass (Agrostis stolonifera L.) putting greens has yet to be determined. Shade timing, trinexapac‐ethyl growth regulator, and ultraviolet‐absorbing plant colorant applications may affect the DLI requirement of creeping bentgrass putting greens. The objective of this research was to determine the DLI requirement of a creeping bentgrass putting green by evaluating the effects of shade intensity and timing, as well as the influence of trinexapac‐ethyl and a plant colorant on turfgrass quality and coverage. Intensity treatments of 0, 70, 80, and 90% shade were applied during either morning (sunrise to solar noon) or afternoon (solar noon to sunset) hours from May through October 2016 and 2017 on a ‘Tyee’ creeping bentgrass, sand‐based putting green. Evaluations of visual turf quality and turf coverage were performed every 2 wk. After 2 yr, the DLI requirement to maintain acceptable putting green quality was estimated to be 30 mol m−2 d−1. Although effective in improving turf quality at times during summer months, trinexapac‐ethyl and the plant colorant did not significantly reduce the DLI requirement.

Impact of solar radiation exposure on phyllosphere bacterial community of red-pigmented baby leaf lettuce

Solar radiation has been identified as a stress factor affecting phyllosphere associated bacteria colonization and survival during primary production. In the present study, the impact of different solar radiation doses on the phyllosphere microbiota of red-pigmented baby leaf lettuce cultivated in open field under commercial conditions was evaluated. Four weeks before harvest, the growing field was divided into four plots; each one was consecutively covered with one-week-interval with a light-excluding plastic to reduce the sunlight exposure. Four different solar radiation treatments were generated and cumulative photosynthetically active radiation (PAR) was used to differentiate treatments as follows: 4889 ± 428 μmol/m2/s (uncovered), 4265 ± 356 μmol/m2/s (covered for 1 week), 3602 ± 225 μmol/m2/s (covered for 2 weeks) and 3115 ± 313 μmol/m2/s (covered for 3 weeks). The size and composition of the phyllosphere bacterial community were determined by cultivation-depended (plate count) and independent (qPCR) techniques. Exposure to decreased levels of cumulative PAR did not produce significant differences in total bacterial community size, regardless of the chosen quantification techniques. However, total bacteria size quantified by qPCR was around 3.5 orders of magnitude higher than those obtained by plate count. The observed differences between cultivation-depended and independent techniques could be attributed to the presence of non-viable or viable but non-culturable (VBNC) bacteria. The bacterial community structure was analyzed using temperature gradient gel electrophoresis (TGGE), and significant differences were detected when the four solar treatment were compared. A qPCR approach was applied to the quantification of specific bacterial phyla and classes, previously identified in the phyllosphere of plants available literature, confirming that Proteobacteria, Bacteroidetes, Actinobacterias and Firmicutes were the most abundantly represented phyla in lettuce. Treatment comparison revealed higher proportions of Gammaproteobacteria as opposed to the Betaproteobacteria on the lettuce exposed to the lowest cumulative PAR dose (3115 ± 313 μmol/m2/s). The obtained results demonstrated that the solar radiation is a relevant environmental factor influencing the relative abundance of specific-groups of phyllosphere-associated bacteria in pigmented baby leaf lettuce.

Insect-proof nets affect paddy field microclimate parameters and grain quality of different japonica rice varieties

Insect-proof nets (IPN) are widely used to protect crops from invertebrate pests. The effects of IPN on the main microclimate parameters of paddy fields, as well as grain quality, were investigated for three japonica Oryza sativa L. varieties (Nanjing 44, Nanjing 45, and Suxiangjing 3) that are widely grown in China. The environmental temperature of IPN-treated fields increased slightly, whereas the maximum value of photosynthetically active radiation (PAR), daily cumulative PAR, and daily cumulative global radiation all decreased. However, the ratio of PAR to global radiation increased significantly, from 15.08 to 45.48%. Compared to the no insect-proof net (NIPN) treatment, the head rice percentage increased significantly for all three varieties; however, the chalky grain percentage, area and degree, and the amylose content all decreased significantly. The response of gel consistency, protein concentration, and the main parameters of the rapid visco analyzer (RVA) profile of rice starch to IPN treatment differed between varieties. The peak viscosity and breakdown were increased compared to the NIPN treatment, whereas the final viscosity and setback were decreased. However, no significant differences were found between the IPN and NIPN treatments. Furthermore, statistical analysis indicated no interaction between rice planting pattern (RPP) and rice variety (RV) for the rice grain quality parameters. These results suggest that IPN treatment improved rice grain quality for all japonica rice varieties studied, possibly due to the slight increase in environmental temperature and increased ratio of PAR to global radiation in IPN treated paddy fields.

Effects of cumulus clouds on microclimate and shoot-level photosynthetic gas exchange in Picea engelmannii and Abies lasiocarpa at treeline, Medicine Bow Mountains, Wyoming, USA

Here we describe the dynamic effects of cumulus clouds on microclimate and shoot-level photosynthetic gas exchange in saplings of two treeline conifer species—Picea engelmannii and Abies lasiocarpa. Measurements were made during both clear-sky and partly cloudy conditions (∼10–70% cumulus cloud cover) throughout the 2012 growing season within an alpine-treeline ecotone. Cumulus clouds generated dynamic fluctuations in photosynthetically active radiation (PAR), higher maximum PAR (>2500μmolm−2s−1), 2–4 fold increases in diffuse PAR, reduced daily mean and cumulative PAR, lower needle temperatures, and reduced leaf-to-air vapor pressure differences relative to clear-sky conditions. Onset of cloud-shade corresponded with declines in photosynthesis, needle temperatures, and evapotranspiration, which were proportional to cloud duration and opacity. Despite increased diffuse light and greater sunlight intensity during cloud-gaps, photosynthesis was never higher on partly cloudy days compared to clear days in either species, during cloud-gaps or cloud-shade. However, reduced transpiration paired with photosynthesis comparable with clear-sky levels during cloud-gaps resulted in greater instantaneous water use efficiency relative to clear-sky measurements. There was no apparent photoinhibition of photosynthesis reflected in gas exchange measurements in response to abrupt and dramatic changes in PAR levels caused by cumulus clouds. We conclude that cumulus clouds reduce instantaneous and daily carbon gain, although lower needle temperatures and associated reductions in transpirational water loss may alleviate daily and seasonal water stress, thereby enhancing carbon gain over the growing season.

FLOWER ABSCISSION IN PEPPER PLANTS GROWN UNDER DIFFERENT REGIMES OF NITROGEN FERTILIZATION AND PHOTOSYNTHETICALLY ACTIVE RADIATION

The effect of nitrogen fertilization on flower abscission in pepper was studied under different growth regimes. The pepper plants were irrigated with 4, 9, and 14 meq L−1of nitrate (N4, N9, and N14). The plants were grown in winter under a low level (639 mol m−2) of cumulative photosynthetically active radiation (PAR) (LPAR), and in spring under a high level (1074 mol m−2) of cumulative PAR (HPAR). The number of flowers and flower abscission were higher under HPAR than under LPAR. Flower abscission was higher in response to treatment N4, than in response to treatments N9 and N14, while the flower number was significantly lower. Flower abscission was strongly correlated with growth-related parameters as well as with the carbon or nitrogen contents in plants. Neither the sucrose fluxes nor the amino acid fluxes through the flower pedicels were affected by nitrogen supply. The sucrose fluxes were strongly correlated with air temperature.

Photosynthetically active radiation (PAR) × ultraviolet radiation (UV) interact to initiate solar injury in apple

Solar injury (SI) in apple is associated with high temperature, high visible light and ultraviolet radiation (UV). Fruit surface temperature (FST) thresholds for SI related disorders have been developed but there are no thresholds established for solar radiation. The objective of the study was to measure the effect of different levels of FST, PAR and UV on quantum efficiency (Fv/Fm) alone and in combination to identify interactions affecting Fv/Fm recovery to serve as a guideline in evaluating SI-prevention technology. The variable transmission of UVb, UVa, PAR, and NIR through films of Surround™ and Raynox™ provided a wide range of radiation levels at the fruit surface to model Fv/Fm of photosystem II (PSII) in growth chamber and greenhouse studies. Increasing UV and PAR reduced Fv/Fm the greatest amount in ‘Gala’ while ‘Granny Smith’ had the least reduction suggesting that PSII in ‘Granny Smith’ has higher UV and PAR tolerance. All cultivars except ‘Fuji’ at harvest had some interaction component of PAR and UV that decreased Fv/Fm compared to untreated control fruit as both PAR and UV increased. When re-exposure was conducted on the 5 cultivars, Fv/Fm further decreased with the cumulative PAR and UV despite having a period of dark recovery between each 24h exposure. Cultivar differences have a strong influence on the sensitivity of the peel to FST and radiation levels. Cultivars differ in their response to FST, UV and PAR as they mature and also exhibit year-to-year variation. This inconsistency suggests that acclimation to the environment also varies between years. PAR×UV interactions are complex and include the acclimation of the cultivar to the stress but in the prevention of SI, reducing FST, PAR and UV simultaneously is likely the most effective strategy.

Effects of Planting Configuration and In-Row Plant Spacing on Photosynthetically Active Radiation Interception for Three Irrigated Potato Cultivars

Research studies have evaluated the production of potatoes (Solanum tuberosum L.) grown in conventional and bed planting configurations. However, intercepted photosynthetically active radiation (PAR) from these planting configurations has not been quantified. A study conducted in 2008 and 2009 quantified and compared the intercepted PAR from three planting configurations (four row conventional ridged-row [4RC], five row bed [5RB], and seven row bed [7RB]), and from different plant spacings of cvs Russet Burbank, Russet Norkotah, and Ranger Russet potatoes under sprinkler irrigation. A second study was conducted in 2007 to evaluate the relationship between PAR and leaf area of Russet Norkotah and Russet Burbank for the three planting configurations. These studies were conducted at the USDA-ARS Northwest Irrigation & Soils Research Lab in Kimberly, ID, on a Portneuf silt loam (coarse–silty mixed mesic Durixerollic Calciorthid). The canopy of Russet Norkotah and Ranger Russet potatoes grown in 5RB and 7RB planting configurations intercepted more PAR during the early vegetative and tuber initiation growth stages compared to the 4RC planting configuration at equal populations in 2008 and 2009 at all measurement dates. The canopy of Russet Burbank intercepted more PAR during the early growth stage in 2008 when planted in the bed planting configurations compared to the 4RC planting configuration, but not on the July 17, 2008 and July 9, 2009 dates. The canopy cover of Russet Burbank potatoes planted in the 4RC planting configuration tended to catch up with the bed planting configurations quicker than the other two cultivars. In general, the quantity of PAR intercepted as affected by planting configuration did not influence total tuber yield and other measured production variables. Cumulative PAR interception 0–72 days after planting (DAP) was increased 35%, 38%, and 32% for the 5RB and 65%, 69%, 23% for the 7RB relative to the 4RC planting configuration for Ranger Russet, Ranger Norkotah, and Russet Burbank, respectively. Cumulative PAR interception for the season was increased 15%, 16%, and 4% for the 5RB and 23%, 23%, 5% for the 7RB relative to the 4RC planting configuration for Ranger Russet, Ranger Norkotah, and Russet Burbank, respectively. The relationship between intercepted PAR and leaf area for Russet Norkotah during the early vegetative and tuber initiation growth stages was significantly different between the three planting configurations, with intercepted PAR at a given leaf area in the order of 7RB>5RB>4RC. For Russet Burbank, the relationship was significantly different for the 5RB and 7RB compared to 4RC planting configuration only, with intercepted PAR at a given leaf area in the order of 7RB=5RB>4RC.

Cumulative solar irradiance and potential large-scale sea ice algae distribution off East Antarctica (30°E–150°E)

We present a computational model of the large-scale cumulative light exposure of sea ice in the Southern Ocean off East Antarctica (30°E–150°E). The model uses remotely sensed or modelled sea ice concentration, snow depth over sea ice, and solar irradiance data, and tracks sea ice motion over the season of interest in order to calculate the cumulative exposure of the ice field to photosynthetically active radiation (PAR). Light is the limiting factor to sea ice algal growth over winter and early spring, and so the results have implications for the estimation of algal biomass in East Antarctica. The model results indicate that highly light-exposed ice is restricted to within a few degrees of the coast in the eastern part of the study region, but extends much further north in the 30°E–100°E sector. The relative influences of sea ice motion, solar flux, and snow depth variations on interannual variations in model predictions were evaluated. The model estimates of cumulative PAR were found to correlate with satellite estimates of subsequent open-water chlorophyll-a concentration, consistent with the notion that sea ice algae can provide inocula for phytoplankton blooms.

Tomato quality is more dependent on temperature than on photosynthetically active radiation

AbstractBACKGROUND: A greenhouse experiment was performed to study the effect of cumulative air temperature and photosynthetically active radiation (PAR) on tomato quality. Tomato plants were subjected to two different shading treatments causing a 30 and 50% reduction in incoming PAR respectively (plants were exposed to 70 and 50% of incoming PAR respectively). Control plants (exposed to 100% of incoming PAR) were also included in the experiment. The experiment was carried out under unheated greenhouse conditions. To minimise the dependence of temperature on PAR, only a small area inside a large greenhouse was shaded, thereby allowing air currents to homogenise temperature all over the enclosure. Parameters of tomato quality were correlated with cumulative temperature (Tcum) and cumulative PAR (PARcum) for a period of 45 days before harvest.RESULTS: Tcum was strongly correlated with firmness, electrical conductivity, soluble solids content and total phenolic compounds and weakly correlated with pH, dry weight, titratable acidity and vitamin C content. PARcum was only weakly correlated with firmness, dry weight, soluble solids content and total phenolic compounds.CONCLUSION: Tcum has a stronger influence on tomato quality than PARcum. Growers could obtain tomatoes of similar quality under lower PAR than that provided by natural sunlight. Copyright © 2007 Society of Chemical Industry

Effects of nitrogen and radiation on dry matter and nitrogen accumulation in the spike of winter wheat

In wheat, spike growth prior to anthesis is a key period influencing kernel set. Incident radiation and crop N status affect the accumulation of dry matter (DM) and nitrogen (N) in the spike, as observed at anthesis. However, the influence of these two factors on the dynamics of spike growth throughout the pre-anthesis period is not fully understood. Our objective was to investigate the effects of incident radiation and crop N nutrition on the dynamics of spike dry matter, N concentration and N content throughout the pre-anthesis period and to propose models describing spike DM and N content during spike growth, per unit area. We also investigated whether kernel number was related to spike N content at anthesis independently of spike DM, a controversial issue. Field experiments were conducted over two seasons with ‘Trémie’ winter wheat and over an additional season with ‘Arche’ and ‘Soissons’. Treatments involved the application of N fertiliser at different stages and rates to achieve various levels of N deficit and the use of shading nets at various periods during spike growth to reduce incident radiation. Shading reduced spike DM but increased spike N concentration; it reduced spike N content whenever there was a large decrease in incident photosynthetically active radiation (PAR). All these effects occurred only during the second half of the spike growth period, even if shading was applied from the onset of spike growth. Crop N deficit reduced spike DM, N concentration and N accumulation. Spike DM and N content were affected throughout the spike growth period if the deficit started before the beginning of spike growth, and was severe. Continuous and temporary N deficits induced similar responses. The interaction between shading and N fertilisation was not significant for spike DM and N content, but was significant for spike N concentration. Two models, one predicting spike DM and the other predicting spike N content at any time during spike growth were developed. They were validated with an independent data set and by cross-validation with the initial database. Both models fitted the observed data well. The input variables, cumulative PAR after sowing, nitrogen nutrition index (NNI) of the crop and cumulative thermal time after sowing are easy to measure or to simulate. Kernel number was correlated with both spike DM and spike N content at anthesis, demonstrating that N content played no role in determining kernel number independently of DM.

Dry matter production and partitioning of Chenopodium album in contrasting competitive environments

Simple models were developed to quantitatively describe (a) dry matter production and (b) the effects of competition on dry matter partitioning of Chenopodium album L. Data on total biomass and its allocation to roots, stems and leaves were obtained from field experiments with C. album planted at two densities in pure and mixed stands with either cauliflower (Brassica oleracea L. convar. botrytis var. botrytis) or faba beans (Vicia faba L.). After germination, C. album produced biomass rapidly; weeds planted at low density accumulated 20–30% less dry matter than plants growing at a 2.5‐fold higher density per m2. A close correlation between the transmission of photosynthetically active radiation (PAR) and leaf area index was found. Biomass production was linearly related to cumulative PAR intercepted, but a seasonal variation of the radiation‐use efficiency could not entirely be explained. The root:shoot ratio was constant, whereas the level of competitive stress changed the distribution pattern between stems and leaves. With increasing competition in the cauliflower experiments, C. album allocated relatively more biomass to stems than to leaves; this was less evident in mixtures with faba beans. In field vegetable production with an abundant water and nutrient supply, the growth processes of C. album may be described quantitatively using simple functional relationships.

Grain legume species in low rainfall Mediterranean-type environments II. Canopy development, radiation interception, and dry-matter production

The adaptation of a wide range of grain legume species ( Lupinus albus L. cv. Kiev mutant, L. angustifolius L. cv. Yorrel, L. atlanticus L. accs. P22924 and P22927, L. pilosus Murr. acc. P23030, Cicer arietinum L. acc. T1587, Lens culinaris Med. acc. ILL6002 and cv. Digger, Vicia faba L. cv. Fiord, V. narbonensis L. acc. ACT60104, Lathyrus cicera L. acc. 495, L. ochrus (L.) DC acc. 537, L. sativus L. acc. 453, P. sativum L. cv. Dundale, V. benghalensis L. cv. Early purple, and V. sativa L. cv. Languedoc) to low-rainfall Mediterranean-type environments of southwestern Australia was examined in relation to canopy development, radiation interception, and dry-matter production. Species were grown at one location, and in two consecutive growing seasons (1993 and 1994). Dry-matter production was large for most species, particularly in 1993 (maximum biomass > 580 g/m 2 for all species except L. pilosus) which had above-average rainfall. V. faba and P. sativum developed greater maximum biomass than other species, and this was associated with high crop growth rates (CGR) during the early part of the growing season and large maximum CGR. Other species which produced large maximum biomass ( V. narbonensis and L. ochrus in 1993) developed dry matter more slowly during the early stages of plant growth, but achieved maximum CGR similar to those obtained for V. faba and P. sativum. CGR of these species increased rapidly close to flowering as air temperatures began to rise. Species which produced large maximum biomass intercepted a greater amount of photosynthetically active radiation (PAR) ( r = 0.73* and 0.70** in 1993 and 1994, respectively). There were differences among species in the efficiency with which intercepted PAR was converted into biomass (radiation-use efficiency), but these differences did not closely reflect differences in dry-matter production. Cumulative intercepted PAR was positively correlated with the fraction of incident PAR intercepted by species ( r = 0.87** and 0.96** in 1993 and 1994, respectively), but was poorly correlated with cumulative incident PAR. Differences among species in intercepted PAR were therefore not related to differences in the length of time species were intercepting incident PAR (a function of crop phenology). Greater fraction of incident PAR intercepted by species was closely related to green (photosynthetic) area duration (GAD) ( r = 0.60* and 0.78* in 1993 and 1994, respectively), but was poorly correlated with the efficiency with which green area intercepted PAR (described by the extinction coefficient). Maximum biomass of species was also closely related to GAD ( r = 0.78** and 0.84** in 1993 an 1994, respectively). These results indicate that in low-rainfall Mediterranean-type environments, grain legume species should be selected for the development of large green area index (GAI), which should maximise the interception of PAR, production of dry matter, and consequently seed yield of the crop.

Agronomic studies on soybean (Glycine max (L.) Merrill) in the dry seasons of the tropics. I. Limits to yield imposed by phenology

An analysis was undertaken of the development, growth and seed yield of irrigated soybean crops grown during the dry season of the semi-arid tropics in north-western Australia, to establish yield potentials and identify major climatic or physiological constraints. Ten tropically adapted genotypes were grown at three sowing times, using agronomic management practices designed to maximize productivity and minimize constraints due to water supply, fertility, weeds and insects. In addition to phenology, seed yield, dry matter (DM) accumulation, and seed and plant morphological traits, measurements were made at the beginning and end of flowering of DM accumulation, leaf area development and interception of photosynthetically active radiation (PAR). Harvest indices were generally large, but maximum seed yields were only c. 3 t ha-1, apparently because of inadequate biomass production. The analysis of growth and development suggested that DM accumulation during the vegetative phase was limited primarily by cumulative PAR interception by the crop canopy rather than the efficiency of conversion of intercepted PAR. In turn, both cumulative PAR interception, and canopy leaf area development, were constrained by precocious flowering, induced by the comparatively short-day/warm temperature conditions of the dry season. It was concluded that yield improvement strategies for the dry season will need to be based on agronomic and/or breeding strategies to enhance canopy development and improve biomass production.

Agronomic studies on soybean (Glycine max (L.) Merrill) in the dry season of the tropics. III. Effect of artificial photoperiod extension on phenology, growth and seed yield

Soybean cultivars developed for the tropical wet season performed poorly when grown in the dry season in north-west Australia. The proposition that breeding for later flowering time might enhance yields was tested by using artificial photoperiod extension (14 h day-1 for 28 days post-emergence) to delay flowering of field plots of agronomically improved genotypes. Canopy development and interception of photosynthetically active radiation (PAR), dry matter (DM) accumulation, seed yield and seed composition were examined, and compared with that from plants grown under natural photoperiods. Photoperiod extension delayed flowering an average 24 days, and maturity by an average 14.5 days, the effect being greater in the earlier-flowering genotypes. Differences among genotypes and photoperiod treatments in above-ground DM at the beginning and end of flowering were almost entirely due to the consequences of differences in phenology for cumulative PAR interception. DM at maturity was a simple linear function of crop duration (r2 = 0.95**), while seed yield exhibited an optimum-type response with DM ( R2 = 0.79**). The net consequence was that photoperiod extension increased DM production by an average 2.23 t ha-1 and seed yield by an average 0.65 t ha-1. The analyses suggested that a crop duration of c. 143-146 days would be needed to maximize seed yield under the agronomic conditions of the study, whereas the longest duration among the agronomically improved genotypes under natural photoperiod conditions was 136 days. It was concluded that breeding to constrain precocious flowering under short day conditions would be a viable strategy to improve the yield potential of soybean in the dry season.