Ok well It certainly isnt an evironmental factor, and its doenst sound like sun burn.
Im leaning more towards a Calcium and or Magnesium deffiency it could also be Potassium
So here are some general rules.
Calcium;
Calcium plays an important role in maintaining cell integrity and membrane permeability.
Calcium Deficiency
Young leaves are affected first and become small and distorted or chlorotic with irregular margins, spotting or necrotic areas. Flower development is inhibited, blossom end rot and internal decay may also occur and root may be under developed or die back. Deficiency will cause root tip die-back, leaf tip curl and marginal necrosis and chlorosis primarily in younger leaves. Symptoms: young leaves develop chlorosis and distortion such as crinkling, dwarfing, developing a strap-like shape, shoots stop growing and thicken.
Calcium Toxicity
Difficult to distinguish visually. May precipitate with sulfur in solution and cause clouding or residue in tank. Excess calcium may produce deficiencies in magnesium and potassium.
Magnesium;
Magnesium is a component of the chlorophyll molecule and serves as a cofactor in most enzymes.
Magnesium (Mg) deficiency.
Magnesium deficiency will exhibit a yellowing (which may turn brown) and interveinal chlorosis beginning in the older leaves. The older leaves will be the first to develop interveinal chlorosis. Starting at leaf margin or tip and progressing inward between the veins. Younger leaves will exhibit upward curling of leaf margins in some cases.
This can be quickly resolved by watering with 1 teaspoon Epsom salts/gallon of water. Until you can correct nutrient lockout, try foliar feeding. That way the plants get all the nitrogen and Mg they need. The plants can be foliar feed at 1/8 teaspoon/quart of Epsom salts (first powdered and dissolved in some hot water). When mixing up soil, use 2 teaspoon dolomite lime per gallon of soil.
If the starting water is above 200 ppm, that is fairly hard water, that will lock out mg with all of the calcium in the water. Either add a 1/4 teaspoon per gallon of epsom salts or lime (both will effectively reduce the lockout or invest into a reverse osmosis water filter.
Mg can get locked-up by too much Ca, Cl or ammonium nitrogen. Don't overdo Mg or you'll lock up other nutrients.
Potassium;
Potassium is involved in maintaining the water status of the plant and the tugor pressure of it's cells and the opening and closing of the stomata. Potassium is required in the accumulation and translocation of carbohydrates. Lack of potassium will reduce flower and fruit production.
Potassium deficiency (K).
Older leaves are initially chlorotic but soon develop dark necrotic lesions (dead tissue). First apparent on the tips and margins of the leaves. Stem and branches may become weak and easily broken, the plant may also stretch. The plant will become susceptible to disease and toxicity. In addition to appearing to look like iron deficiency, the tips of the leaves curl and the edges burn and die.
Potassium - Too much sodium (Na) displaces K, causing a K deficiency. Sources of high salinity are: baking soda (sodium bicarbonate "pH-up"), too much manure(a source of N), and the use of water-softening filters (which should not be used). If the problem is Na, flush the soil. K can get locked up from too much Ca or ammonium nitrogen, and possibly cold weather.
Potassium (K) Toxicity
Usually not absorbed excessively by plants. Excess potassium can aggravate the uptake of magnesium, manganese, zinc and iron and effect the availability of calcium.
I hope these help. I think you may have a K defficiency but I couldnt say for sure with out seeing the plant. It would also be helpfull to know what your ph is as this can sometimes lead to nutrient lockout as well.
For now you may want to pick up Growmores 20-6-16,
Theyve added Ca and Mg and I believe a few other micro-nutrients as well.
FWIW
Peace
