Interesting discussion, and good posts 419
, but are you sure that we are interested
only in IAA?
Going back a few posts, there is considerable confusion about the "dark cycle" reactions. The Calvin Cycle takes place in the stroma of the chloroplast, and is perhaps better referred to as the "light-independent process".
The
light-dependent reactions, which occur in the thylakoid membrane of the chloroplast, convert light energy to chemical bond energy of ATP & NADPH.
The
light-independent process uses these products of the light reactions to reduce CO2 to C6H12O6 (glucose).
The light-dependent reactions occur only in the presence of light. When light is removed, the reactions stop. The light-independent reactions also occur in the presence of light, but will continue to occur in the absence of light as long as there is enough ATP and NADPH left over after the light-dependent reactions shut down.
The "dark cycle" reactions do occur during daylight, and cannot proceed without the ATP & NADPH produced by the light reactions. Photosynthesis is not complete without the products of the dark cycle.
Both the light reactions and the dark cycle (anabolic processes) are part of photosynthesis, occur in the chloroplast, and thus occur only in photosynthesizing tissues. This produces the energy (stored as bond energy of carbohydrates) that fuels plant metabolism and growth.
Photorespiration is another metabolic (catabolic) pathway that also occurs in the chloroplast. It consumes O2, evolves CO2, produces no ATP or glucose, and reduces photosynthetic output. Photorespiration occurs when CO2 is not available -- due either to a low CO2 concentration because of insufficient ventilation, or to the stoma being closed (hot, dry, bright days). This is why high temperature and low RH inhibits the growth rate of C3 plants, and why CO2 enrichment allows for higher temperature and lower humidity.
None of that is the same as
cellular respiration that occurs in the mitochondria of cells of all tissue types, including non-phtosynthesizing tissues (e.g., roots). This is a catabolic process that breaks down glucose to produce O2, H2O, and energy (as ATP). Cellular respiration occurs regardless of whether it's light or dark, driven by carboyhdrates produces in the photosynthesizing tissues.
So hopefully that clarifies why the "dark cycle" is not really relevant to root growth.
Generally speaking, a longer period of photosynthesis will result in more energy available for plant growth and thus increase the rate of growth. This has been demonstrated with supplemental lighting in commercial greenhouses. However, the benefit of this increased growth rate must be weighed against the increased cost of electricity. There is a difference between maximum growth and optimized growth. Cost/ benefit analysis of supplemental lighting strategies for year-round greenhouse operators at high latitudes suggest that optimum results are obtained for day-nuetral crops using progressive supplemental lighting to maintain a 17/7 photoperiod. This optimum photoperiod is somewhat crop- and latitude-specific.
In the context of an indoor
garden that is performing at near-peak effciency, I am of the opinion that the photoperiod used for mother plants, clones, and vegetative growth of rooted clones is not a major factor affecting yield efficiency (g/w/m). Certainly there are many other factors that have a much more significant impact on efficiency, such as: light intensity & quality, canopy depth & density, temperature + relative humidity + CO2 concentration, oxygen availability in the root zone, water status, nutrient profile, etc.
It is important to understand that maximizing efficiency (highest g/w/m and lowest cost $/g) involves drastically reducing or eliminating the vegetative growth period entirely (i.e., growing from cuttings rather than seed).
In my setup, most of the veg side wattage has been devoted to mothers, upon which clone demand is not sufficient to require maxized growth rate. Less wattage goes to cuttings and veg plants. Therefore, any increased growth rate from a longer photoperiod does not translate to increased yield. For the past several years I have been using a 16/8 photoperiod in veg. In my own comparisons, I have not noticed any significant difference in rooting time for cuttings under various photoperiods from 24/0 to 16/8. I have also not measured any significant variation in yield efficiency (on a g/w/m basis) related to vegetative photoperiod. Increased phtooperiod costs more, so for the same g/w/m it is more costly $/g.
penguin