Sunflower Canopy Research Articles

Far‐red enrichment and photosynthetically active radiation level influence leaf senescence in field‐grown sunflower

Basal leaves frequently senesce before anthesis in high population density crops. This paper evaluates the hypothesis that quantitative and qualitative changes in the light environment associated with a high leaf area index (LAI) trigger leaf senescence in sunflower (Helianthus annuus L.) canopies. Mean leaf duration (LD, time from achievement of maximum leaf area) of leaf 8 was significantly (P < 0.05) reduced from 51 to 19 days as crop population density was increased from 0.47 to 4.76 plants m−2. High compared to low plant population density was associated with earlier reduction in the photosynthetically active radiation (PAR) and red/far‐red ratio (R/FR) reaching the target leaf. However the changes in R/FR preceded those in PAR. When the light environment of individual leaves of isolated plants growing under field conditions was manipulated using filters and FR‐reflecting mirrors, LD was positively and linearly related with the mean daily PAR (MDR) received in the FR‐ (no FR enrichment) treatments. FR enrichment of light reaching the abaxial surface of the leaf significantly (P < 0.05) reduced LD by 9 days at intermediate PAR levels with respect to FR‐controls, but did not affect LD at the maximum PAR used in these experiments. However, when light reaching both leaf surfaces was enriched with FR, LD (for leaves receiving maximum PAR) was 13 days shorter than that of the FR‐ control. These results show that basal leaf senescence in sunflower is enhanced both by a decrease in PAR and by a decrease in R/FR.

A model to simulate the development, growth and yield of irrigated sunflower

A simulation model of the development, growth and yield of sunflower cv. Hysun 30 is presented. In the model, crop growth rate is a function of radiation use efficiency, incident radiation, the canopy extinction coefficient and leaf area index. The radiation use efficiency of a sunflower canopy was measured and found to change during the growth of the crop. It is higher after floret initiation than before, and reaches a maximum at anthesis. The changing radiation use efficiency has been incorporated into the model. The leaf area index each day is a function of the number of leaves produced and the rate and duration of expansion of each leaf. A nitrogen and plant population factor modulates crop growth through its effect on the expansion rate of each leaf and the rate of leaf senescence after anthesis. In this way the nitrogen concentration in leaves at floret initiation and population density affect the dry-matter accumulation by the crop. Crop dry matter at the end of floret initiation and at physiological maturity are used to calculate the components of yield from relationships previously demonstrated in greenhouse experiments. The mean daily temperature affects the rate of leaf production and the radiation use efficiency, as well as determining the time of phenological stages through thermal time. Although the model was developed for the hybrid Hysun 30, it has been extended to some other hybrids by changing the size and activity of the generative area producing florets in the floral meristem. The simulated yields of four hybrids are close to the measured yields of irrigated crops.

A narrow-waveband spectral index that tracks diurnal changes in photosynthetic efficiency

We present a new “physiological reflectance index” (PRI) isolated from narrow waveband spectral measurements of sunflower canopies. This index correlates with the epoxidation state of the xanthophyll cycle pigments and with the efficiency of photosynthesis in control and nitrogen stress canopies, but not in water stress canopies undergoing midday wilting. It is analogous in formulation to the broadband normalized difference vegetation index (NDVI) and uses reflectance at 531 nm and at a reference wavelength to minimize complications associated with diurnal sun angle changes. In conjunction with other methods, this index may lead to improved remote and ground-based estimates of canopy photosynthetic function.

Anisotropy of thermal infrared exitance in sunflower canopies

Anisotropy of thermal infrared exitance above and within a relatively closed fully irrigated sunflower canopy is detailed. Azimuthal variation in thermal infrared exitance above canopies was weakly (statistically) related to solar position and was comparable to or larger than errors in satellite-based canopy estimates. Anisotropy within canopies was significantly lower and decreased with canopy closure and depth into the canopy. Measured azimuthal isotropy within canopies supports the use of this assumption in radiative transfer models. Significant differences in canopy temperature measurements were found depending upon whether the instruments were within or above the canopy. These differences could produce errors of 20–35% in latent energy estimates during periods of high evapotranspiration (ET) and greater errors in periods of restricted ET.

Simulating sunflower canopy temperatures to infer root-zone soil water potential

A soil—plant—atmosphere model for sunflower ( Helianthus annuus L.), together with clear sky weather data for several days, is used to study the relationship between canopy temperature and root-zone soil water potential. Considering the empirical dependence of stomatal resistance on insolation, air temperature and leaf water potential, a continuity equation for water flux in the soil—plant—atmosphere system is solved for the leaf water potential. The transpirational flux is calculated using Monteith's combination equation, while the canopy temperature is calculated from the energy balance equation. The simulation shows that, at high soil water potentials, canopy temperature is determined primarily by air and dew point temperatures. These results agree with an empirically derived linear regression equation relating canopy-air temperature differential to air vapor pressure deficit. The model predictions of leaf water potential are also in agreement with observations, indicating that measurements of canopy temperature together with a knowledge of air and dew point temperatures can provide a reliable estimate of the root-zone soil water potential.

The influence of penumbra on canopy photosynthesis II. Canopy of horizontal circular leaves

Rate of gross photosynthesis is calculated for canopies of randomly dispersed circular horizontal leaves irradiated by the sun's disk of finite angular radius, for one particular light response of photosynthesis. The theory developed in an earlier paper is used to take account of irradiance distribution in penumbral regions of canopies. Calculations are performed first for zenith sun and then for other solar elevation angles. A parameter, called characteristic leaf area index ( L r), occurs naturally in the theoretical development; L r incorporates leaf size and leaf number density. Probability of penumbra at a given depth, depth distribution of photosynthesis rate and hence canopy photosynthesis rate are found to depend on L r as well as on canopy leaf area index (LAI) and the light scattering coefficient of the leaves. Potential application of the results to problems involving different vertical distributions of foliage is illustrated. Rates thus calculated are compared with rates calculated from parallel solar beam geometry. The discrepancy between the results from the two irradiation geometries, computed from the present theory, is compared with an estimate in the literature based on irradiance measurements under a sunflower canopy of LAI = 1.88. The values compare well for mean photosynthesis rate per unit leaf area at that particular level, but it is shown that the discrepancy between canopy photosynthesis rates calculated for the two irradiation conditions is much smaller than that local rate.