Features Fertilizer Inputs
Special Series mineral nutrition: Optimizing solar radiant energy

This feature is a continuation in a series of technical features on mineral nutrition in plants. As stated in the first installment (Oct. 2008, pgs. 20-22), this feature is part of a series exploring the forces at work that affect the mobilization and absorption of elements into the plant.

This feature is a continuation in a series of technical features on mineral nutrition in plants. As stated in the first instalment (Oct. 2008, pgs. 20-22), this feature is part of a series exploring the forces at work that affect the mobilization and absorption of elements into the plant.

Sunlight is the chief source of energy for plant growth. The term solar radiant energy, simply stated, refers to sunlight or daylight. We are not going to get into the physics behind radiant energy; and for all intensive purposes, all you need to understand is that sunlight is the driving force in regards to the uptake of nutrients.

Solar radiant energy is involved in so many phases of growth and development that, unquestionably, it is the most important of the many factors that make up the greenhouse environment. Green plants are completely dependent on sunlight for the energy and substances they require for growth.

This is a photo of a mum that was not spaced on time. The breaks had to compete for light at too close of a spacing. PHOTO COURTESY BRIAN CANTIN

DAY/NIGHT TEMPERATURES ARE ADJUSED TO SEASONAL CHANGES IN RADIANT ENERGY
Day/night temperatures are adjusted according to seasonal changes in radiant energy provided by the sun. The response to carbon dioxide enrichment is mainly limited by the level of available solar radiant energy. The frequency of irrigation varies with radiant energy, especially as it affects leaf and air temperatures and influences the rate of evapo-transpiration. The loss of water through the leaf is what triggers the mobilization of nutrients in the media and throughout the plant.

Fertilizer requirements are closely related to plant growth rates, which to a great degree are determined by the amount of sunlight energy emitted and received by the leaf surface. Consequently, adjustments in the programs for maintaining fertility levels for container-grown crops are frequently based on seasonal changes in solar radiant energy.

Cultural procedures also are influenced by radiant energy. Timing schedules for the start of short-day or long-day treatments are adjusted according to growth rates as they are influenced by radiant energy levels. The space between plants is increased during the winter months when sunlight energy is low. Growth retardants may be necessary to control the height of plants when seasonal levels of sunlight energy are high, whereas over-application of PGR (plant growth regulator) can be detrimental during low irradiance periods of the year. Successful crop production requires numerous decisions by management; many of them are influenced by solar radiant energy.

SOLAR RADIANT ENERGY HAS MAJOR EFFECT ON FINAL CROP YIELDS
Solar radiant energy has a major effect on the ultimate yield of a crop. Mineral elements, water, carbon dioxide and temperature also influence plant growth, but only to the extent to which they support and promote photosynthesis At this point in time, photosynthesis can be described as the process where leaves gather sunlight energy that is then converted into food compounds used as structural components and as sources of energy for growth.

The capacity of plants to intercept and use radiant energy is determined by the leaf surface area exposed to direct sunlight. As the sunlight warms up the leaf surface, certain physiological responses are put into action. The increase in leaf temperature that results from the absorption of this radiant energy has a major effect on transpiration, photosynthesis, respiration and other metabolic processes. Mineral elements are released from the exchange sites of the media colloids; water flows to the roots or roots come in contact with water as they elongate, and carbon dioxide diffuses through the stomates (leaf openings) from the atmosphere outside the leaf.

The individual plants on a greenhouse bench or within a container compete for radiant energy with adjacent plants. A competitive leaf area is of fundamental importance for obtaining high yields and optimum quality.

LEAVES ARE THE PRIMARY RADIANT ENERGY ABSORBING ORGANS
Leaves are the primary radiant energy absorbing organs of the plants. Cultural procedures that result in the optimum leaf area index in the shortest period of time are most likely to produce the best quality. Proper moisture and fertility levels are conducive to rapid leaf growth. Competition for space begins early in crop production and spacing on time is crucial to the ability of each plant to absorb sufficient light energy. Delayed spacing can create a weak and stretched plant that can be wrongly diagnosed as a nutrient problem or even a response to a poor growing media.

Enough said about this dominant factor that affects our watering and fertilizing practices. However, it would be remiss of me not to leave you with a few words of wisdom. Trying to diagnose a problem from the get-go can be confusing even on the best of days.

The common diagnostic approach would be to start with basic questions. Are the symptoms related to the media or to water management, or is it related to improper feeding? The quality and quantity of light, or the lack of or even too much light, are often overshadowed by the most common plant-related problems.

Knowing what you now know about solar radiant energy, take time to review topics such:
(1) Overhead shadows caused by hanging baskets or greenhouse structures.
(2) Type of greenhouse covering, i.e., glass or double poly.
(3) The age of the poly covering.
(4) Initial and final spacing of plants, and whether this spacing is adequate for the size of the plant?
(5) Upper leaf canopy and filtering of far red light to the lower leaf canopy that jeo-pardizes the overall strength of the plant.
(6) Transitional growing, i.e., taking into consideration the time of year and the quality of light that the plant perceives.

Can we relate solar radiant energy to mineral nutrition in plants? We certainly can!