There were several legitimate reasons to be worried. A few days before departure, I discovered strangely discoloured spots on the leaves (see below). Then, most worryingly, I found out that the roots lost most of their finest hairs, and instead were covered in slime, seemingly dissolving into the water, in the process creating an ugly looking whitish film on the surface. In other words, it seemed the roots are slowly dying, and possibly are already beyond repair.
From a quick online search, it seemed the tomatoes suffer from a bacterial infection of the roots. However, I couldn’t exclude the possibility that somehow, the growing method itself is at fault, not providing the plants with enough oxygen (as it is a passive system with no water movement).
As first aid, I discarded one plant that was affected the most, changed the whole nutrient solution, and cut off all the diseased leaves. Even so, I was resigned to finding the plants dead (or close) upon my return two weeks later.
Fortunately, it turns out that the change of water and drastic pruning helped – at least for now. Yesterday I have found the plants not just alive, but healthier, and bigger than before, plus starting to flower. The roots have mysteriously recovered, and re-grown, and almost all the water in the container was consumed or evaporated during the heat-wave (only about 3cm were left at the bottom), requiring an immediate re-fill.
As is apparent from the photo taken a moment ago, the tomato plants are attached (using strings) to an improvised support system built from IVAR Ikea elements, and a few horizontal straight wooden sticks. I also acquired a small and cheap USB ventilator to keep the plants cooler during hot days and to make sure the flowers move in the breeze (without movement, they might not pollinate and bear fruit).
So it seems that so far so good, but I don’t think I’m out of the woods completely. A few leaves are still sick, the infection can return at any time, and I don’t know how the plants will react to the nutrient solution re-fill, due to which roots previously out of the water will be immersed again. Since I’m not sure what caused the problem in the first place, I’m equally in the dark about preventing its return in the future.
So while I’m prepared for the worst, I’m still hoping for the best.