Mostly_Harmless said:

---

Quote:

March 2010 issue of Cactus World (BCSS Journal)

Author: Roy Mottram except where otherwise stated

Too much sun?

In common with many other flowering plants, the cells of all cacti contain crystals of calcium oxalate, or more rarely silicon oxide, but their function is poorly understood. They might, for instance, be a defence against herbivores by rendering them gritty and unpalatable, or they may even cause them to suffer kidney stones (human kidney stones are also usually calcium oxalate), or they might be present as storage vessels for calcium.

The crystals tend to be more concentrated in the epidermal (skin) cells, where they might also play an important part in the reflection and scattering of light. They are able to increase in size in proportion to the light intensity and are therefore thought to be a moderator of the amount of light reaching the sensitive chlorophyll layers below, and consequently one of the defences against sun scorch. Pigments in the surface cells (chromoplasts) also play the same role, and are the reason why plants may turn reddish in bright light. This is not necessarily a sign of any stress problems, as is often supposed, although root loss will produce the same kind of coloration. The plant will tell you if it is the latter, because then shrinkage will occur as well, while neighbouring because then shrinkage will occur as well, while neighbouring plants will remain turgid and less coloured.

In poor light conditions, anything which blocks, reflects or disperses light away from the chlorophyll layers is clearly disadvantageous, so the crystals and chromoplasts in the surface layers tend to shrink during periods of poor light, and increase again when the light strength increases in order to prevent the chlorophyll from bleaching. They have the ability to change quite quickly, within hours or days. In practice this means that when plants have experienced a long period of dull, cloudy weather, they become much more vulnerable to damage from a sudden burst of bright sunshine, and that is when sun scorch is most likely to occur. On the other hand, plants which experience bright sunshine every day are well protected, and therefore do not burn easily.

Under glass, sun scorch most commonly shows up as a localised yellow discoloration if mild, or a complete break-down if severe (literally cooking) of the side facing the sun at its height, especially near the more sensitive apex of the plant. If there is good ventilation it is less likely to occur, but even then a bright burst of sunshine in early spring can sometimes result in bleaching some areas of the skin of the plant white. This is especially frequent with lithops, which can suddenly turn into blobs of white jelly. In certain of the less spiny trichocerei, it may result in unsightly black swollen wheals which can burst and may then become secondarily but only locally infected by opportunist bacteria oozing a black tar-like fluid, the usual symptom of bacterial infection. The trichocereus problem can be exaggerated if an insecticidal soap solution has been used on them, because that seems to break down the protective surface waxes and exposes the surface of the plant to potential sun damage, although it is only trichocerei and harrisias which appear to suffer from this particular problem.

All these conditions of damage are of course physiological, not caused by any pathogens, so the remaining sound part of the plant will continue to grow. However, the unsightly scars are there forever, until the plant has outgrown its old parts, usually very many years later.

Prevention is clearly the only solution to the sun scorch problem, so attention to opening doors and vents to keep the greenhouse cool in summer and the use of a 50% shade cloth fastened to the inside of the glass on the sunny south side of the greenhouse are often good husbandry. If you go away for even a day, make sure that all doors and vents are open before you leave, no matter if sun has been forecast or not. It does no harm unless you have a secondary problem of unwanted creatures that may wander in.

---