Photosynthetically Active Radiation Interception Research Articles

Estimation of absorbed PAR across Scandinavia from satellite measurements. Part II: Modeling and evaluating the fractional absorption

The aim of this study is to generate a quality-controlled sub-kilometer dataset of the fraction of absorbed photosynthetically active radiation (FAPAR) across Scandinavia from satellite. FAPAR is required for estimating the amount of PAR absorbed (APAR) by vegetation, which in turn allows for estimation of carbon uptake. In this study, FAPAR was modeled from normalized difference vegetation index (NDVI) which was obtained from the MODIS VI product (MOD13Q1) at 250 m spatial resolution. Modeled FAPAR was evaluated against in-situ measurements of fractional interception of PAR (FIPAR) and FAPAR at nine plots in six forested sites across Sweden and Denmark from 2001 to 2005. High resolution remote sensing data were used to investigate the representativeness of the measurement areas. Furthermore, FAPAR from the MODIS LAI/FPAR product at 1 km spatial resolution (MOD15A2) was investigated and compared the measured and modeled FAPAR. There was good agreement between modeled and measured FAPAR (6.9% average RMSE of the means). A linear relationship between daily values of NDVI and FAPAR was found ( R 2 = 0.82), and it is concluded that seasonally adjusted NDVI can be used for accurate FAPAR estimations over forested areas in Scandinavia. However, it was found that the error was correlated with average FAPAR and that it is important to take the understory vegetation into account when measuring FAPAR in open canopies. The observed difference between FIPAR and FAPAR was 2.3 and 1.4 percentage units for coniferous and deciduous stands, respectively. MODIS FAPAR performed well although a few unrealistic values were present, highlighting the necessity to filter out low quality values using the quality-control datasets.

Classes de perfilhos na composição do índice de área foliar em pastos de capim-elefante

O trabalho foi conduzido de outubro de 2002 a abril de 2003, em Coronel Pacheco, MG, com o objetivo de avaliar os efeitos da altura do resíduo pós-pastejo (50 e 100 cm) sobre a radiação fotossinteticamente ativa (RFA) interceptada pelo dossel e o coeficiente de extinção luminosa; a contribuição relativa de classes de perfilhos (basais e aéreos) sobre o índice de área foliar (IAF) e o acúmulo de forragem (AF), em pastos de capim-elefante (Pennisetum purpureum Schum.) cv. Napier manejados com intervalos de 30 dias entre pastejos. Foi usado o delineamento blocos completos ao acaso e um arranjo de parcelas subdivididas com quatro repetições, com alturas de resíduo alocadas nas parcelas e as classes de perfilhos nas subparcelas. O IAF, a interceptação da RFA e o coeficiente de extinção luminosa foram influenciados pela altura do resíduo pós-pastejo. As interceptações da RFA foram maiores no verão do que na primavera, em pastos manejados com resíduo de 100 cm. As variações do IAF e da interceptação da RFA não estiveram associados, durante todos os ciclos de pastejo. O IAF e o AF apresentaram variações sazonais, relacionadas à participação de perfilhos basais e aéreos, na população de perfilhos do pasto.

Changing scale in ecological modelling: A bottom up approach with an individual based vegetation model

Net primary production (NPP) depends crucially on the sun radiation absorption, and hence its prediction on the modelling of the radiation. In heterogeneous systems such as savannas, the photosynthetically active radiation (PAR) absorption is more difficult to predict than in homogeneous systems, due to the presence of tree clumps and open grass areas. Using a detailed 3D model, we propose new formulations of PAR absorption derived from the Beer–Lambert law for heterogeneous vegetation. To account for intra-plot vegetation heterogeneity we assume an intra-plot partition of space into two or three zones, based on tree location. We develop two models derived model from the Beer–Lambert law: (1) a simple partition of space below and outside trees, (2) a partition in three zones, including the tree shading area. The only model to predict PAR interception for all types of spatial patterns was the most complex one (including tree + tree shading zone + open grass areas). In this model, extinction coefficients can be derived from vegetation structural parameters like tree cover, and tree shading area. Tree shading area could be computed from geometric site parameters (latitude, longitude, tree height). With increasing satellite resolution it can be possible to obtain these structural parameters of vegetation (cover, overlap, tree height) needed by this Beer–Lambert derived model. Such an absorption model could then be used as a basis for a Monteith-style model of vegetation functioning for heterogeneous vegetation.

Modelling the interception of photosynthetically active radiation by evergreen subtropical hedgerows

Horticultural tree crop yields tend to be linearly correlated with the percentage of photosynthetically active radiation (PAR) intercepted by the canopies, at least for part of the PAR interception range. Models of PAR interception by hedgerows have been used in the design of orchards for temperate tree crops, especially apples, but not for subtropical tree crops, such as lychee and macadamia. Subtropical crops need special consideration because of the latitudes at which they are grown, the specific shapes and dimensions of the hedgerows, and the evergreen habit, which requires an understanding of the entire annual cycle. We present outputs from a PAR interception model for solid rectangular and tapered hedgerows, based on a model of irradiation beneath blue skies. Annual PAR interception tends to decline as row orientation rotates from north–south to east–west, but with some exceptions for particular tree geometries, and declines slightly with decreasing latitude. Daily PAR interception is also affected by row orientation, with little seasonal variation for north–south rows but large fluctuations for east–west rows, including very high interception in winter and low interception in summer. Row orientation and tree shape greatly affect the distribution of PAR over the surface of the canopy. For example, the side faces of evenly spaced, symmetrical, identical north–south hedgerows are equally irradiated throughout the year, but there can be large seasonal differences in the relative irradiance of the north and south faces of the same hedgerows aligned east–west. The solid tapered hedgerow model tended to overestimate measured PAR interception by ~6% overall, but the percent overestimation seemed to vary with PAR interception, being greater at lower levels of PAR interception. A curvilinear relationship was found between the yield of macadamia in the Northern Rivers area of NSW in 1997 and the measured PAR intercepted by the trees, with an explained variance of 50%. Maximum yield occurred at ~86% PAR interception. Using modelled PAR interception the explained variance of the yield was 34%. Model estimates of PAR interception were close to those measured and might be used to address a range of physiological questions concerning the canopy development of subtropical hedgerows.

Is barley yield in the UK sink limited?: II. Factors affecting potential grain size

In the absence of serious post-anthesis stress, the potential supply of assimilates for grain filling in barley appears to exceed the storage (sink) capacity of the grains. The sink capacity is a function of the number of grains per unit land area and their potential size. In order to investigate the contribution of pre and early post-anthesis factors in the control of potential grain weight, regression analysis has been used to analyse data from field experiments conducted on winter barley crops cv. Pearl grown at six sites across the UK between 2002 and 2004. Crops were grown under a comparable husbandry regime with a full crop protection, growth regulator and fertilizer programme. The mean grain weight (MGW) varied from 35 to 46 mg at 100% dry matter over 17 site/year combinations. The variation in MGW between site/years was associated with differences in the rate rather than the duration of grain filling. MGW did not relate well to any measure of above ground growth or crop structure at ear emergence (Zadoks GS59). There was a significant positive linear relationship between MGW and the amount of radiation (PAR) intercepted per unit grain number between ear emergence and the start of rapid grain filling ( r 2 = 0.32, P = 0.023), but not over the whole grain filling period. This is consistent with the view that MGW is determined by potential grain size. A multiple regression model comprising five explanatory variables accounted for a much larger proportion of the total variation in MGW ( r 2 = 0.72, P = 0.013) than PAR interception on its own. There were significant positive associations of MGW with post-anthesis radiation use efficiency and PAR intercepted from GS59 to the start of rapid grain growth (RGG), and significant negative associations with the shoot number per plant, mean air temperature from GS39–59 and mean daily rainfall from GS59-RGG. The results suggest that both pre and post-anthesis conditions operate in concert to determine the potential grain weight of barley in temperate climates. The dry weight and water content of grains and ears followed a common pattern during grain growth for all site/years when the percentage moisture content was used as an index to normalise different rates of development. Use of the above data for predicting potential grain weight of barley prior to harvest is discussed.

A simple approach to modeling radiation interception by fruit-tree orchards

The formulation and evaluation of a model of radiation interception by fruit-tree orchards is presented. The model computes radiation interception based on the proportion of the orchard floor that is shaded by the trees at any given time, calculated from geometric relationships of the length of the shadow cast by the trees and the orchard configuration. Inputs include tree dimensions, orchard configuration, daily global solar radiation and a correction factor that accounts for within canopy gaps. Evaluation of the internal consistency of the model and a sensitivity analysis were carried out. Then, a test of the performance of the model was done, comparing simulated values of fractional PAR interception with field measurements from a range of orchard conditions (species, orchard design, tree dimensions, and location), including orchards on steep terrain. Despite simplifications made to reduce input data requirements as well as the inherent variability among the orchards, the model simulated reasonably well the fractional interception of radiation on an hourly time step (mean absolute error, MAE = 10.08%; Willmott index of agreement, D = 0.94). When evaluated on a daily basis, the performance of the model improved considerably (MAE = 4.47%; D = 0.98). In both time scales, the model performed similarly for the different orchard conditions considered. The model can be used to estimate both PAR and global solar radiation interception, including different types of orchard configuration such as hedgerow, overhead trained, or isolated trees planted in rectangular patterns.

Row Width and Maize Grain Yield

Maize (Zea mays L.) grain yield increase in narrow rows (0.35–0.50 m) may be related to the improvement of light interception around silking, but percentages of grain yield increase are generally lower than those of light interception, suggesting a lower efficiency to convert the amount of intercepted photosynthetic active radiation (IPAR) into aboveground phytomass. We analyzed the effects of plant population and row spacing on grain yield and its components (kernel number and kernel weight) and on the underlying processes, the IPAR around silking and during the effective grain filling period, and radiation use efficiency (RUE) during both periods. Field experiments were conducted in Argentina from 1997 to 2001. Five hybrids were cultivated at a wide range of plant population densities (3, 4.5, 9, and 12 plants m−2) and row spacings (0.35, 0.50, 0.70, and 1 m) without water and nutrient limitations. Row spacing reduction increased IPAR around silking at low plant densities (≈8 and 4% for 3–4.5 and 9–12 plants m−2, respectively) but did not modify RUE during this period. Morphogenetic limitations in the reproductive organs of plants (number of florets per ear) cultivated at low stand densities, suppressed the slight benefits of enhanced light capture under narrow rows, yielding similar kernel numbers at any row spacing. Contrarily, a postsilking RUE reduction (≈13–16%) of crops in narrow rows compared to those in wide rows minimized or counterbalanced any positive effect on IPAR during the grain‐filling period. Hence, for the tested growing conditions, no benefits could be expected in terms of grain yield by reducing row spacing from the present 0.7‐ to 0.8‐m inter‐row distance.

Genetic and Environmental Effects on Water Use Efficiency in Peach

Two unpruned narrow-leaf and two unpruned standard-leaf peach [Prunus persica (L.) Batsch.] selections were evaluated for physiological components related to water use efficiency {WUE [carbon assimilation (A) per unit of transpiration (T)]}. The purpose of the study was to assess the value of narrow-leaf phenotypes to improve WUE in peach and separate the environmental component of canopy geometry from the genetic components. The narrow-leaf characteristic itself did not confer improved WUE. The interception of light was a key determinant of WUE in these genotypes. Internal shading of the tree by excessive leaf area reduced daily WUE measured in gas exchange studies. Canopies that intercepted more than 75% of the photosynthetically active radiation (PAR) had reduced daily WUE. Dormant season pruning of the four genotypes lowered isotopic carbon discrimination and therefore increased seasonal WUE compared to unpruned trees. None of the genotypes had a significant correlation of seasonal WUE with leaf and fruit weight. Analysis of covariance indicated that `Bounty' and both narrow-leaf genotypes had greater leaf and fruit weight than `Redhaven' for a given level of PAR interception. `Bounty' had the least internal canopy shading of the four genotypes. Genetic differences in peach growth types can be selected for factors increasing WUE as well as increased productivity. Future work in peach breeding to improve WUE and productivity must take into consideration light interception, productivity, and WUE in an integrated manner to make real progress in the efficient use of water and light in the orchard environment.

Response of Corn (Zea mays) and Weeds to Planting Pattern and Herbicide Use

ABSTRACT The use of narrow row arrangement in corn (Zea mays) has been suggested as a technological alternative to obtain grain yield increases through a better use of the resources. The regular pattern could diminish intraspecific competition while favouring interspecific competition with weeds. The objective of this study was to analyse the effect of corn planting pattern and herbicide application on (i) soil water content and photosynthetically active radiation (PAR) dynamics, (ii) below ground and above ground corn biomass and grain yield and (iii) weed suppressive ability of dryland corn sown at square and rectangular plant arrangement. Field experiments were conducted during the growing seasons 2002/03 and 2003/04. Three corn hybrids with two plant arrangement (rectangular: 0.70 × 0.20 m and square: 0.35 × 0.35 m) and herbicide (2.25 kg ai ha−1 of atrazine and 2.32 kg ai ha−1 of metolachlor) use compared with no herbicide use were tested. In the initial stages of the crop there was greater moisture content in the row of corn sowed at square planting pattern while a lower soil water content at maturity was observed. Conversely, between rows, the square plant arrangement registered lower soil water content during the growing cycle of the crop. This soil water profile was a consequence of the uniform corn below ground dry matter distribution. A greater PAR interception with a lower weed above ground dry matter was obtained in square plant arrangement. Corn grain yield was greater in the square-planting pattern than in the rectangular ones. This increase in grain yield was related to better resources use i.e. soil moisture and PAR interception while reduces the competition from weeds. The use of equidistant plant arrangement appeared as an interesting alternative to increase both the grain yield potential and the corn suppressive ability against weeds in dryland Argentinean production systems.

Crescimento e desenvolvimento do dossel de Panicum maximum cv. Mombaça

Em duas áreas de 1.200m² de capim Panicum maximum cv. Mombaça, acompanhou-se a evolução do perfilhamento, da biomassa, do índice de área foliar (IAF), da interceptação da radiação fotossinteticamente ativa (IRFA) e da eficiência de uso da radiação (EUR) no verão e outono. Para melhor compreesão do crescimento do dossel, realizou-se também análise do crescimento, estimando-se a taxa de crescimento relativo (TCR), a taxa assimilatória líquida (TAL) e a razão de área foliar (RAF) da gramínea. Para todas as variáveis, foram consideradas estimativas semanais, realizadas aos 7º, 14º, 21º, 28º, 35º, 42º e 49º dias de rebrotação da gramínea, após pastejo de uniformização. Utilizou-se delineamento experimental inteiramente casualizado, com quatro repetições e duas áreas experimetais: uma no verão e outra no outono. No verão e no outono, foram visualizados, respectivamente, 28 e 24 pontos, sorteados pelos tratamentos experimentais, semelhantes quanto à altura do dossel e à cobertura do solo, após intenso pastejo de uniformização. O perfilhamento da gramínea, intenso na primeira semana, decresceu nas duas semanas seguintes e praticamente cessou a partir da quarta semana. A IRFA evoluiu assintoticamente, atingindo valores de 96% em ambas as estações. O IAF apresentou o mesmo padrão de resposta, com valores de 8 e 4 no verão e no outono, respectivamente. Em ambas as estações, a biomassa evoluiu de forma quadrática com o decorrer da rebrotação. A EUR, mais alta no verão que no outono, decreceu e seus respectivos valores foram 1,76 e 0,54gMS/MJ, em razão da menor precipitação pluviométrica desse período. A eficiência de crescimento do dossel, medida pela TCR, mais alta no verão que no outono e decresceu assintoticamente durante a rebrotação, em ambos os períodos. Enquanto a TAL reduziu durante o período de rebrotação, a RAF aumentou até um ponto de máximo, tanto no verão como no outono, decrescendo posteriormente.

Production Efficiency and Radiation Use Efficiency of Four Tree Species Receiving Irrigation and Fertilization

Abstract To determine the effect of resource availability on efficiency of stemwood production, we calculated radiation use efficiency (Εstem = stem production/photosynthetically active radiation [PAR] interception) and production efficiency (PEstem = stem production/leaf area index [LAI]) from measurements of LAI, intercepted photosynthetically active radiation (IPAR), and stem production on stands of Pinus taeda, Pinus elliottii, Liquidambar styraciflua, and Platanus occidentalis during the sixth growing season. Treatments were control, irrigation only, and irrigation plus 57, 85, or 114 kg N ha−1 y−1. Compared to the control, irrigation plus 85 kg N ha−1 increased stem production: 8.7–15.0 Mg ha−1 y−1 (loblolly), 6.0–10.7 Mg ha−1 y−1 (slash), 1.2–12.8 Mg ha−1 y−1 (sweetgum), and 0.5–9.6 Mg ha−1 y−1 (sycamore). Annual PAR capture and LAI were correlated with stem production (r2 = 0.80, r2 = 0.66, respectively, average of 4 species). Fertilization and irrigation increased Εstem and PEstem for sycamore, whereas only fertilization increased efficiencies for sweetgum. Irrigation increased PEstem for the pines. Foliar [N], often a surrogate for photosynthetic efficiency, was positively correlated with both efficiency variables for the hardwoods (r2 from 0.65 to 0.79). For the pines, canopy size was the primary growth determinant. FOR. SCI. 51(6):556–569.

(352) Genetic and Environmental Effects Related to Water Use Efficiency in Peach

Two unpruned willow leaf and two unpruned standard leaf peach [Prunuspersica(L.) Batsch.] selections were evaluated for physiological components related to water use efficiency (WUE). The purpose of the study was to assess the value of willow leaf phenotypes to improve water use efficiency in peach and separate the environmental from the genetic components. The willow leaf characteristic itself did not confer improved water use efficiency. Light interception was a key determinant of WUE in these genotypes and the relationship of WUE with intercepted photosynthetically active radiation (PAR) by the entire canopy indicated a significant negative correlation. Internal shading of the tree by excessive leaf area reduced WUE and canopies that intercept more than 60% of the PAR have reduced WUE. While WUE is improved by reducing the amount of PAR interception of the canopy, productivity is reduced. Neither of the willow leaf genotypes had a significant correlation of WUE with yield (leaf and fruit weight); however, the standard leaf type cultivars, `Bounty' and `Redhaven', had significantly different regressions that indicate greater productivity in `Bounty' for a given level of WUE. `Redhaven' was the least productive cultivar; `Bounty' was the most productive, and the two willow leaf genotypes were intermediate in the relationship of intercepted PAR with yield. Therefore, genetic differences in peach growth types can be selected for both increased WUE as well as increased productivity. Future work in peach breeding to improve WUE and productivity must take into consideration light interception, productivity, and WUE in an integrated manner to make progress in the efficient use of water and light.

Duração do período de descanso e crescimento do dossel de Panicum maximum cv. Mombaça sob lotação intermitente

O crescimento do dossel do capim-Mombaça foi analisado sob lotação intermitente com três períodos de descanso (PD), tratamentos definidos em função do tempo necessário para a expansão de 2,5; 3,5 e 4,5 novas folhas por perfilho e período de pastejo de seis dias. A área experimental foi dividida em cinco ou seis piquetes por tratamento. Utilizaram-se cinco novilhos mestiços (holandês x zebu) por tratamento, como animais de prova, além de outros, como animais de equilíbrio, que foram adicionados, quando necessário, para garantir índice de área foliar (IAF) residual de 2,0, ao final do 6(0) dia de cada período de pastejo. O estudo constou de duas fases de crescimento: 1) após roçada de uniformização, no início da estação de pastejo e 2) após alguns ciclos de pastejo, sob efeito dos três PDs. Os piquetes receberam adubação em cobertura de N, P2O5 e K2O, na dose de 50; 12,5 e 50 kg/ha, no início dos crescimentos 1 e 2. A intervalo médio de seis dias, durante 45 dias, estimaram-se a intercepção da radiação fotossinteticamente ativa (RFA), o IAF e a massa seca de forragem verde (MSFV). Aos dados de MSFV e de área foliar foram ajustados polinômios de segundo grau em função da idade, a partir dos quais foram estimados os valores instantâneos de biomassa e IAF e, a partir destes, as taxas de crescimento relativo (TCR) e de assimilação líquida (TAL) e a razão de área foliar (RAF), para cada idade. A intercepção da RFA pelo dossel evoluiu assintoticamente, alcançando valores máximos entre 95 e 98%, enquanto os valores de IAF e MSFV experimentaram incrementos ainda na faixa de estabilidade da intercepção da RFA. Os valores de TCR, TAL e RAF exibiram evolução assintótica decrescente. A grande proporção de colmo do dossel, ao final do PD mais longo, favoreceu a TCR e a TAL.

Photosynthesis, transpiration, and primary productivity: Scaling up from leaves to canopies and regions using process models and remotely sensed data

Biophysical and physiological processes in plants and ecosystems occur over a wide range of spatial and temporal scales. Our knowledge (or models) of these processes is largely at small scales. It is, however, difficult to directly apply mechanistic process‐oriented models over large scales due to heterogeneities in the distributions of processes, and nonlinearities in the functional responses of processes to environmental variables. On the other hand, simple parametric/empirical models in which system complexity is lumped into a small number of parameters have been widely employed to describe processes at larger scales. The variation of these parameters in these simple parametric/empirical models depends on the underlying biophysical processes. In this work, we showed that detailed process models and simple parametric models for primary production and transpiration could be effectively combined to scale leaf photosynthesis and transpiration up to large spatial scales. The integrated process model, General Energy Mass Transfer Model (GEMTM), was used to identify major factors contributing to the variability of the parameters in the parametric models for regional transpiration and primary production and quantify their responses to these factors. Simulations with the GEMTM showed that net carbon assimilation was proportional to intercepted photosynthetically active radiation (IPAR), but the radiation use efficiency (RUE) changed with leaf N concentration, temperature, and atmospheric CO2 concentration; transpiration was linearly correlated with the product of net primary production (NPP) and atmospheric water vapor pressure deficit (VPD), and the slope varied with leaf N concentration. RUE increased with leaf N content asymptotically, and responded to temperature in an asymmetric bell shape pattern with a 22°C and 26°C optimal temperature under current ambient and doubled CO2 concentration, respectively. A simple parametric NPP model and a regional transpiration model (Tr model) were developed from the relationships and parameter values obtained using the GEMTM. The NPP model reasonably simulated the seasonal and interannual variations of accumulated NPP estimated from field data. Simulated regional distribution of NPP over the Central Grassland Region of the United States was consistent with estimates obtained using other models. NPP increased from 120 gC/m2/year in the northwest to 956 gC/m2/year in the southeast. Simulated regional transpiration had a similar spatial distribution pattern as NPP, ranging from about 16 cmH2O/year to 136 cmH2O/year. The transpiration model introduced in this study provides a mechanism to explicitly couple transpiration and NPP in large‐scale analyses, although more complete analysis and validation are required.

Plant Density Influences Marketable Yield of Directly Seeded `Piquillo' Pimiento Pepper

Pimiento pepper (Capsicum annuum var. annuum L. `Piquillo') was directly seeded on raised beds in double rows 0.35 cm apart. Plants were thinned within the row to establish densities ranging from 13,333 to 186,667 plants/ha. Marketable yield of pepper increased as plant density increased to 100,000 plants/ha. However, with plant densities >100,000 plants/ha, marketable yield did not increase (2001) or slightly decreased (2000). Plant density affected pepper yield only in the first harvest date. Fruit number and fruit weight per plant decreased with increasing plant populations. Weight per fruit decreased slightly at densities >100,000 plants/ha. The increase in yield per hectare as plant density increased was mainly a result of increased number of fruit per hectare. The intercepted PAR by the pepper canopy increased with increases in plant density to about 100,000 plants/ha and this increased interception of PAR resulted in an increase in yield. Plant densities in the range of 100,000 to 120,000 plants/ha were optimal in terms of marketable yield.

PEAR WATER RELATIONS UNDER PARTIAL ROOTZONE DRYING

The effects of partial rootzone drying (PRD) on microjet-irrigated pear trees on a loam soil in the Victorian Goulburn Valley, Australia were investigated during the 2002/03 season. PRD effects on soil water content, midday stem water potential (ψ ψ ψ ψ stem), leaf conductance (gl) and sap velocity were examined. Treatments included 50% (PRD50) and 100% (PRD100) of estimated crop water requirement (ETc) applied daily to one side of the tree on a 14-day alternating cycle while the Control treatment received 100% of ETc applied to both sides of the tree. ETc was calculated using reference crop evapotranspiration (ETo) and FAO crop coefficient for pear adjusted for effective canopy cover (ECC). ECC was measured by canopy PAR interception over the planting square at solar noon and remained at low levels (≤32 %) during the season. ECC did not differ between treatments. Irrigation inputs to harvest were 1.9 Ml ha -1 for PRD50, and 3.5 Ml ha -1 for the fully watered PRD100 and Control trees, compared to the calculated ETc of 3.1 Ml ha -1 over the same period. Crop water relations of the PRD100 trees during a 14-day PRD cycle, as measured by ψ ψ ψ ψ stem and gl, was similar to the Control. Decreased ψ ψ ψ ψ stem under PRD50 indicated water stress. Sap velocity, which was assumed to equate to transpiration, increased in a curvilinear fashion to instantaneous ETo in all trees. This sap velocity-ETo relationship was maintained throughout the 14-day PRD cycle. No evidence of transient changes in stomatal control of transpiration over a drying cycle occurred in the PRD trees. These results indicate that PRD50 led to water stress. Furthermore there was no evidence of the PRD stomatal control and maintenance of plant water status previously reported for grapevines (Dry and Loveys, 1999; Stoll, 2000). Thus, further work on crop species and soil type influence with respect to PRD is required. Also, research into the establishment of ψ stem baselines for fully watered crops would aid irrigation management of orchards.

Effects of planting density on canopy dynamics and stem growth for intensively managed loblolly pine stands

Leaf area index (LAI), intercepted photosynthetically active radiation (IPAR), and foliar nitrogen content are related to stand growth. However, they are all a function of canopy size, and their relative importance is hard to determine. We used intensively managed, 4-year-old loblolly pine (Pinus taeda L.) stands planted at densities ranging from 740 to 4440 trees ha −1 that possessed very different canopy structures to separate the relationships between IPAR, LAI, and foliar N content. Our objectives were to determine whether stem growth was best correlated with LAI, IPAR, or foliar N content and to determine how the relationship between LAI and IPAR varied among stands of different planting densities. While stem growth rates increased significantly with planting density, from 12.0 to 35.2 m 3 ha −1 y −1, increases were not proportional, indicating the presence of competition-induced limitations to growth. Annual IPAR (863–2345 MJ m −2 y −1), LAI (2.5–4.9), and foliar N content (67–122 kg ha −1) all increased as stand density increased from 740 to 4440 trees ha −1. Stem growth rates were better correlated with annual IPAR ( r 2 = 0.90), than LAI ( r 2 = 0.67) or foliar N content ( r 2 = 0.55). The efficiency of LAI to capture radiation (IPAR LAI −1) increased with stand density even though the higher density stands possessed greater LAI, which increased the likelihood of within canopy shading. The increase in IPAR LAI −1 corresponded to significantly decreased variation in light interception within plots as planting density increased. These results indicate that stem growth is best correlated with annual IPAR and underscore the importance of stand structure since the more even distribution of foliage within canopies of higher density stands increased the efficiency of light interception.

Crop responses to compacted soil: capture and efficiency in the use of water and radiation

We combined field and modelling experiments to investigate crop-level responses to soil compaction. Our working hypotheses are that the effect of soil compaction on crop growth is (i) primarily mediated by reduction in capture of water and photosynthetically active radiation (PAR), and (ii) secondarily affected by reduced transpiration efficiency (biomass per unit transpiration) and radiation-use efficiency (biomass per unit intercepted PAR). Three field experiments were carried out in the Mediterranean-type Mallee region of south Australia where the landscape alternates sand dunes (hills) and swales (flats) of sandy loam soil. All three experiments compared wheat crops grown in compacted (control) soils, and soils in which compaction was alleviated with deep tillage (ripped); additional sources of variation include season and soil type as related to topography. All soil and crop responses to ripping were more marked in sand hills than in sandy loam flats. Penetration resistance of undisturbed soil had a peak ∼2 MPa at 0.1–0.2 m depth in sandy loam flats and ∼3 MPa at 0.2–0.3 m depth in sand hills. Ripping dramatically reduced soil penetration resistance between 0.10 and 0.3–0.4 m. Control crops yielded between 1.2 and 2.9 t ha −1 and yield improvement attributable to alleviation of soil compaction ranged from nil to 43%; yield response to ripping remained for at least two cropping seasons. Increased transpiration and PAR interception fully accounted for the increase in crop growth associated with alleviation of soil compaction; ripping did not affect transpiration efficiency or radiation-use efficiency. The proportion of evapotranspiration accounted for by soil evaporation (E:ET) declined from 0.58 in controls to 0.36–0.45 in ripped sand hills. A limited modelling study showed that water availability, as characterised with the lower limit of plant available water, could partially account for the effect of soil compaction and deep tillage on crop growth and evapotranspiration. Long-term simulations indicated important changes in the fate of water in response to ripping in sandy soils, including a moderate increase in evapotranspiration, a substantial reduction in E:ET, and important reductions in the frequency and rate of drainage beyond the crop root zone.

Studies on the physiological effects of viruses on sweet potato yield in Kenya

SummaryExperiments to determine the effect of the three viruses most common on Kenyan sweet potato varieties were carried out at the University of Nairobi Kabete farm. The sweet potato varieties were Bungoma, Kemb 10 and Ex‐Shimba hills which were either virus‐free or infected with Sweet potato feathery mottle virus (SPFMV), Sweet potato mild mottle virus (SPMMV), Sweet potato chlorotic stunt virus (SPCSV) or a combination of SPFMV and SPCSV. Canopy photosynthetically active radiation (PAR) interception and the relative chlorophyll content of the plants were determined. At harvest, marketable tuber yield, harvest index (HI), leaf area index (LAI) and specific leaf weight (SLW) were measured.Infection with single viruses caused no or mild symptoms and had no significant effect on chlorophyll content of leaves of any variety. Dual infection with SPFMV and SPCSV resulted in severe symptoms characteristic of sweet potato virus disease complex (SPVD) and significantly reduced chlorophyll content. Only SPCSV and SPCSV + SPFMV reduced PAR interception, the latter combination especially. Generally, infection with single viruses increased the SLW but a decrease occurred in plants dually infected with SPFMV and SPCSV. Whereas SPVD significantly reduced the harvest index (78%) and tuber yield (98%) in the three varieties, infection with single viruses caused a reduction only in the varieties Bungoma and Kemb 10. SPVD reduced yield through reducing leaf chlorophyll content, PAR interception, LAI and HI. The single virus infections possibly reduced assimilate translocation from leaves to tubers as is inferred from high SLW. Effects of viruses on physiological processes and yield are discussed.

Light interception and utilization of four grain legumes sown at different plant populations and depths

Canopy development, radiation absorption and its utilization for yield was studied in four grain legume species Cicer arietinum, Lens culinaris, Lupinus angustifolius and Pisum sativum. The grain legumes were grown at different plant populations and sowing depths over two seasons in Canterbury, New Zealand. The green area index (GAI), intercepted radiation, radiation use efficiency (RUE) and total intercepted photosynthetically active radiation (PAR) increased significantly (P<0·001) with increased plant population. Narrow-leafed lupin produced the highest maximum biomass (878 and 972 g/m2, averaged over all populations during 1998/99 and 1999/2000, respectively) and intercepted more radiation (600 and 714 MJ/m2, averaged over all populations during 1998/99 and 1999/2000, respectively) than the other three legumes. In all four species, in both trials, the highest plant populations reached their peak GAI about 7–10 days earlier than legumes sown at low populations. Cumulative intercepted PAR was strongly associated with seed yield and crop harvest index (CHI).The RUE increased (from 1·10 to 1·46 and from 1·04 to 1·34 g/MJ during 1998/99 and 1999/2000, respectively) as plant population increased and was highest in the highest yielding species (e.g. 146 and 1·36 g/MJ for narrow-leafed lupin in both experiments). The larger leaf canopies produced at the higher plant populations reduced the extinction coefficient (k).The results suggest that in the subhumid temperate environment of Canterbury, grain legume species should be selected for the development of a large GAI. This should maximize PAR interception, DM production and, consequently, seed yield.