Photoperiod

Photoperiod (day length) is an important environmental signal that influences the rate and duration of phenological development in the Lentil model. Some lentil cultivars are photoperiod-sensitive, responding to changes in day length.

Overview

The daily photoperiod is calculated using the PhotoperiodFunction based on latitude and the day of year.

  • Photoperiod is the duration of daylight hours on a given day
  • It increases from winter solstice (minimum) to summer solstice (maximum)
  • The calculation uses sunrise and sunset times based on latitude and solar declination
  • Twilight angle of -6 degrees is used (civil twilight)

Calculation Method

The photoperiod is calculated from astronomical principles:

  1. Solar declination is calculated from the day of year
  2. Sunset hour angle is calculated using the solar declination and latitude
  3. Photoperiod in hours is derived from the hour angle
  4. Twilight adjustment of -6 degrees expands the photoperiod period slightly

Photoperiod Sensitivity in Lentils

Some lentil cultivars are photoperiod-sensitive, meaning their phenological development is influenced by day length:

Photoperiod-Sensitive Cultivars

  • Flowering and pod initiation timing may be delayed under long-day conditions
  • Reproductive development progresses more rapidly under short-day conditions
  • The sensitivity varies by cultivar (higher or lower photoperiod sensitivity values)
  • Parameter: [Phenology].InductivePpSensitivity controls this sensitivity

Photoperiod-Insensitive Cultivars

  • Development proceeds relatively normally regardless of day length
  • These cultivars are adapted to a wider range of latitudes
  • Parameter: [Phenology].InductivePpSensitivity will be low or zero for these cultivars

Cultivar-Specific Parameters

Parameter Description Value Range
[Phenology].InductivePpSensitivity Photoperiod sensitivity during inductive phase 0.0 - 1.0
[Phenology].FlowerAbortionPpSensitivity Photoperiod sensitivity for flower abortion 0.0 - 1.0

Geographic and Seasonal Effects

Photoperiod effects are particularly important for:

  1. Latitude adaptation: Cultivars are adapted to specific latitude ranges based on photoperiod sensitivity
  2. Sowing date effects: Early vs. late sowing can change experienced photoperiods during critical phases
  3. Flowering timing: Photoperiod can delay or advance flowering relative to thermal time alone
  4. Latitude consideration: Long-day regions (high latitude) vs. short-day regions (low latitude) favor different cultivars

Examples

  • High-latitude (cool season region): Days are long in summer. Photoperiod-sensitive cultivars delay flowering until day length shortens, potentially pushing flowering into unfavorable conditions
  • Low-latitude (tropical region): Days stay relatively short year-round. Day length changes are minimal, so photoperiod sensitivity is less important
  • Sowing date effect: Early-sown crops experience longer photoperiods during flowering; late-sown crops experience declining photoperiods

Practical Implications

Understanding photoperiod effects is important for:

  1. Cultivar selection: Choosing cultivars matched to local photoperiod conditions
  2. Sowing date determination: Optimizing sowing date to match cultivar photoperiod sensitivity
  3. Predicting flowering: Combining thermal time and photoperiod effects for accurate predictions
  4. Yield forecasting: Photoperiod-induced changes in phasis can affect grain yield

The APSIM Lentil model incorporates photoperiod effects to accurately predict flowering and reproductive development across different latitudes and sowing dates.