Course Content
Learn Principles of Agronomy — B.SC agriculture
About Lesson
  1. Photoperiodic effect:
  • It has been observed that the initiation of the reproductive phase in the plants is initiated when certain duration of light period is obtained. Based on the duration of light period required, the plants are classified as a) short day plants b) long day plants and c) day neutral plants.
  • Short day plants are those whose reproductive phase is initiated when period of shorter light
  • duration (maximum of about 12 hours) is provided e.g. rice, soybean, sesame, cowpea etc.
  • Similarly, the long day plants require longer light duration (minimum of 13 hours) for the
  • initiation of reproductive phase e.g. wheat, barley, oat, lentil, chickpea etc.
  • Day neutral plants on the other hand do not have any specific requirements of duration of light period for the initiation of the reproductive phase e.g. buckwheat, sunflower, cotton, tobacco, majority of maize varieties and some varieties of cowpea.

  1. Photosynthetic effect:
  • In general, higher is the solar radiation higher is the photosynthetic rate.
  • Normally, higher solar radiation intensity is suitable for most of the crop plants but the requirement varies with plant to plant, with their varieties and with their stages of growth.
  • The radiation that utilized in the photosynthesis process includes the wave lengths ranging from 0.36- 0.76 micron known as photosynthetic active radiation.
  • For example, the radiation below 0.25 micron is harmful to the plants and that above 0.76 micron has almost heat or thermal effects only.
  • In the color spectrum of solar radiation wave of different color bands have different effects on the plants.
  • Yellow, orange and red bands are important in photoperiodism. Similarly, blue, orange and red rays are important in photosynthesis.

  1. Photothermic effect:
  • Light also influenced on stomata and their opening and closing also.
  • Usually leaves developing under full sunlight condition have reduced size and closer arrangement of stomata than the plants grown in shade.

  1. Seed dormancy and germination:
  • Solar radiation or light also influenced on seed dormancy and germination.
  • Depending upon the light requirement during seed germination, seeds of some plant species are classified as positively photoblastic and negatively photoblastic seed.
  • Seeds of some plant species like lettuce, tobacco etc will not germinate in the absence of light and are called positively photoblastic seed.
  • On the other hand, seeds of onions and amaranthus will remain dormant if exposed to light and
  • these are called negatively photoblastic seed.

  1. Chemical composition of crops
  • Cool bright conditions favor the conversion of starch into sugar. Crops grown in sufficient light
  • have decreased acidity of cell sap, high C:N ratio, low K, Ca and P content.
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