Correlated Colour Temperature of Sunlight

[Davey246]

Spectra from light sources rather depends on what the light source is.

Spectra from light sources such as SON and SOX and high pressure mercury are what they are within very minor variations in manufacture, as they produce light from electronic changes within a single element, sodium in SON and SOX, although with minute contirbutions from mercury and mercury-sodium pseudo-molecules in SON and argon in SOX.

Spectra from metal halides and phosphors vary as recipes will change, usually rather little, from manufacturer to manufacturer AND from wattage to wattage from any one manufacturer in some types of lamps, not least metal halide lamps as the spectrum depends on cold spot temperature, which controls halide vapour pressures in the operating lamps (metal halide lamp spectrum tends to change, often enormously in quartz, as the lamp ages too).

Photograhy is a total red herring. It is captuing and/or measuring REFLECTED radiation, which is why the CCT of subject matter that does not include any significant amount of sky, varies from what is radiated from the sun/reaches earth - it very much depends on what has been reflected and what has been absorbed. So far as cloud cover is concerned - there are almost endless online and printed sources that state that cloud cover reduces the CCT of light received on the earth's surface/the sky, not the opposite.

As before - recommended reading - https://www.amazon.co.uk/gp/product/076 ... UTF8&psc=1

It also reminded me that light has little or no effect, in terms of colour or anything else, on germination. The vast majority of seeds will germinate and grow in darkness and it take a good deal of growth/absorption of the stores within the seed before photosynthesis can or must provide a significant contribution.

[esp]

Photograhy is a total red herring. It is captuing and/or measuring REFLECTED radiation

Photography captures / measures whatever is in the field of vision of the camera lens. Point it at a light source, and that is what it will capture/ measure. Point it at a blue (or overcast) sky, it will measure the light from the blue (or overcast) sky. Which will be largely scattered light, I understand.

which is why the CCT of subject matter that does not include any significant amount of sky, varies from what is radiated from the sun/reaches earth

Does this make any sense? Light sources can have a CCT. "Subject matter", unless it is a light source do not.

there are almost endless online and printed sources that state that cloud cover reduces the CCT of light received on the earth's surface/the sky, not the opposite.

Rather than picking one of these "almost endless" sources, you previously chose to reference the wikipedia page on colour temperature. This gives the follow colour temperatures:

6500 K Daylight, overcast
15,000 – 27,000 K Clear blue poleward sky

Are you saying the Wikipedia article is wrong?

[SimonT]

As before - recommended reading - https://www.amazon.co.uk/gp/product/076 ... UTF8&psc=1

It also reminded me that light has little or no effect, in terms of colour or anything else, on germination. The vast majority of seeds will germinate and grow in darkness and it take a good deal of growth/absorption of the stores within the seed before photosynthesis can or must provide a significant contribution.

The introductory text book cited states that "Most seed germinate underground, in complete darkness" (p12).

I guess this might be true for most seeds in general. However, this is a specific forum for cacti and succulents. It is more informative to read the specific research papers or reviews about C&S eg

Barrios, Duniel, et al. "Seed traits and germination in the Cactaceae family: a review across the Americas." Botanical Sciences 98.3 (2020): 417-440.

This review summarised data from 275 cactus species and found that 80.9% were positively photoblastic and none were negatively photoblastic. Positive photoblastic seeds are seeds whose germination responds positively to light and negative photoblastic those that respond negatively to light. So you might think that light has little or no effect on seed germination, but data from over 220 cactus species disagrees with this suggestion!

[Davey246]

I have always buried all cactus seeds apart from really tiny ones, which have been sown into very fine gravel - Blossfeldia, Aztekium, Strombocactus for instance. Germination has been, in the main, excellent (dud batches happen) in all except a few genera - Blossfeldia and Turbinicarpus come immediately to mind, which have never produced great percentages of germinating seeds for me over at least a few 100 seeds of each of numerous species over the years.

A few batches over the years, of supposed "tricky" species have been treated no differently and produced good crops of seedlings - E. horizonthalonius comes to mind, which has led me to believe that seed "quality" is routinely poor for some species.

Apart from huge seeds - Dioscorea, Adenia, Adenium, Commiphora, Welwitschia, Sansevieria, some Aloes, especially if sold in small lots - I have always buried seed. Huge seeds are planted individually, usually worked just into the surface of the compost or, if flat, slotted into it in a vertical plane. Further exceptions would be some flake-like seeds which I have germinated floating/held in the surface film of a saucer of water, but they have all been bulbs.

Presumably photoblasticism is an imprecise state. Any comment in learned documents about colour/CCT?

[Pattock]

The document mentioned by SimonT is a review that does not mention all of the precise experimental methods of the articles that are included. You would have to follow the references for that. Here is a link to the review paper.

http://www.scielo.org.mx/scielo.php?pid ... xt&tlng=en

Here is another link to the same paper:

https://www.botanicalsciences.com.mx/in ... 1?inline=1

I have seen it suggested that very fresh seeds are not going to need the usual dormancy-breaking conditions that older seed that has had time to go dormant would need. Obviously, one of the conditions that might tell a seed that has been banked in the soil for a long time that it was in a good position to grow would be some light. For example, if the ground had been disturbed. A heavy drench of water, smoky water or actual exposure to heat works with other seeds.

[SimonT]

I have always buried all cactus seeds apart from really tiny ones, which have been sown into very fine gravel - Blossfeldia, Aztekium, Strombocactus for instance. Germination has been, in the main, excellent (dud batches happen) in all except a few genera - Blossfeldia and Turbinicarpus come immediately to mind, which have never produced great percentages of germinating seeds for me over at least a few 100 seeds of each of numerous species over the years.

A few batches over the years, of supposed "tricky" species have been treated no differently and produced good crops of seedlings - E. horizonthalonius comes to mind, which has led me to believe that seed "quality" is routinely poor for some species.

Apart from huge seeds - Dioscorea, Adenia, Adenium, Commiphora, Welwitschia, Sansevieria, some Aloes, especially if sold in small lots - I have always buried seed. Huge seeds are planted individually, usually worked just into the surface of the compost or, if flat, slotted into it in a vertical plane. Further exceptions would be some flake-like seeds which I have germinated floating/held in the surface film of a saucer of water, but they have all been bulbs.

Presumably photoblasticism is an imprecise state. Any comment in learned documents about colour/CCT?

Well, if you get more germination in the presence of light than in the absence of light then this germination is positive photoblastic. If you have more germination in the absence of light than in the presence of light then this is negative photoblastic. That's basically the definition. If you wanted to be precise you could measure germination under otherwise identical conditions in the presence of light and the absence of light multiple times and perform some basic statistics. This would tell you if your seed germination is affected by light.

In practical terms even if germination of a given seed is positively photoblastic, this does not mean that you will not get any germination in the absence of light, it just means you will expect more germination in the presence of light compared to the amount of germination you would expect in the absence of light. This information is useful to know though- who does not want better germination of their seed?

Given these definitions, nothing that you say about the germination of buried seed argues against most cacti seed being positively photoblastic!

[Davey246]

Agreed, but, without reading the paper that you mentioned, the tone of your post implied that germination at least could be night and day (pardon the pun), covered or not.
For any plant that doesn't habitually grow in terrain that isn't routinely/frequently disturbed, it is difficult to imagine a positive advantage to having positively photoblastic seed, the more so for plants growing in harsh, arid terrain where the simplest of logic would argue for germination under all light conditions (assuming sufficient moisture etc.) as survival is stacked aginst survival.

It does beg the question as to how/why this has evolved in the classic example of positive photoblastic seed - the field poppy. Has agriculture positively selected for it over the millennia?

Would I want 80-90-100% germination as routine from packets of 20+ seeds - not really, even allowing for the pleasure in seeing them directly after they do germinate - just more hassles over space and getting rid in 99% of cases.

For the curious, in the past, I have always used a very few mm of fine granite grit as a top dressing, so the grit was/is totally opaque. The flint grit that I use now is also probably verging on opaque at the depth that I use as it is extremely dark, translucent brown as single pieces - easily checked by spreading the grit onto a sheet of glass and looking from underneath.

[SimonT]

Of course you can extend the study of the dependency of seed germination on light to compare different light intensities, light durations, spectra, different CCT levels or any other properties of light you can measure.

In habitat, cactus seeds might persist in soil for a number of years after they are dispersed. If these seed germinated well when buried deep in the soil there might not be enough resources in the seed to reach the soil surface. Seeds might be in other unsuitable positions where they do not receive light eg under a stone but on the soil surface. But if germination is strongly stimulated by light, germination would only happen under otherwise suitable conditions, when the seed is getting light close to the soil surface. This would allow plants seed to persist in soil for a time, as a so-called "soil seed bank", and wait for the right conditions for germination. This provides one possible selection pressure for cactus seeds to evolve positive photoblasic germination.

[Davey246]

I wouldn't agree about buried cactus seed etc.
How many substrates would naturally be disturbed? I'd argue rather few and to only minor degree. Plus, assuming regular flowering, seed production over the life of any plant should be absolutely vast and only a single one needs to survive to flowering to produce a stable replacement in the great majority of cases.
Positive (or negative) photoblastivity does not need to be advantageous for it to occur, it just needs not to be disadvantageous.

[SimonT]

Well I am happy to agree to disagree on this. I just gave one plausible selection mechanism for this trait as an example. There is quite a bit of literature on this sort of thing- I find it easy to access from search engines like scholar.google.com.