Ok,
I discovered this damage last week. The A. isthmensis and guadalajarana (first two) are from Poway, the titanota damage was noticed at Rancho Soledad. I know the plants in Poway had not been moved because they are mine. In addition, this happened in the outdoors. There was damage of the same age on other plants at both places... and a gang of usual suspects has emerged. As with a couple outdoor incidents last Fall/Winter, damage was most severe on guadalajarana, gypsophila, isthmensis, macroacantha, and many potatorum types. A pattern has also emerged with regards to the weather preceding obvious symptoms, in cases where the disorder has happened outdoors. Judging by the age of the latest symptoms, the damage coincided with an event spanning Thanksgiving day through the following weekend. In a period of three days, average daytime highs dropped between 20 and 30 degrees F throughout much of So Cal. Moreover, the San Diego region experienced a sudden onslaught of cloudy rainy weather,
during daytime hours. I've ruled out heat stress because other heat sensitive plants nearby showed no stress. The damage had to have occurred because of the change. In Van Nuys, we had the drop in temps, but we were spared rain and overcast that Saturday. No damage to my agaves occurred in van Nuys... and I have many hundreds. In my observations last year the symptoms appeared
after a sudden onslaught of cool cloudy weather. Therefore, I conclude that Climate Change is the likely culprit...
Seriously, I think that the sudden drop in temperature accompanied by lower light levels (clouds) is suddenly dropping the rate of photosynthesis...though I think light has more to do with it. With respects to light, a similar thing probably happens when a susceptible plant is suddenly moved into a dark enclosed space during the daytime. Again, I've had it happen under a wide range of temperatures. Why does the sudden drop in photosynthesis matter? For a CAM plant like an agave mild to warm temps should accompany the usual CAM cycle. I think a the amount of CO2 a plant takes in at night corresponds to how many "cylinders" it was firing on earlier in the day or the day before. In other words, there's a momentum. If the following day is much cloudier, or the inside of my van is much darker, the light energy required to run the Calvin Cycle in photosynthesis is reduced. The cycle slows, however, the release of CO2 gas inside the leaf from malic acid (CO2 concentrated in a solid/liquid?) occurs independently during the daytime. If this rate of release exceeds that which is being used in photosynthesis, excess CO2 gas could be building up within the leaf. Even though the weather is cooler and transpiration lower, I think the plant can't change its habit of closing stomata during the day quickly enough. The gas may just be trapped in the leaf because the stomata are shut, and the internal pressure may be enough to rupture the cells.
That's my current theory in a coconut shell. I think I'm going to take some agaves that had been used to mild sunny weather and stick them in dark refrigerator during daylight.
Matt