[GpsFrontier]

The first thing that comes to mind is temperature, both air and water, but especially the water temp. Plants can wilt look like they are dying if the water temp is to high, even when the roots are getting plenty of water (especially if there in the sun at the same time). That would be heat stress. Think of it like wanting a nice glass of ice water after a hike in the desert on a hot day, but getting a bunch of of hot water to drink instead. As the plants transpire they suck up cool water from the roots to help regulate their internal temp (transpiring more the warmer the air temp gets). But if the water at the roots is already warm or hot, they cant do anything to help regulate their temp. If the buckets are in the sun, the water temp could easily be higher than the air temp, especially later in the day when the buckets have had time to absorb the suns rays/heat.

Also possibly the water level. The plants could be looking fine in the morning when it's cooler, but then when the air temperature warms up and the plants start to transpire more, if the water level is low they may not be getting the moisture/water uptake they need to support the increase in transpiring (water usage) later in the day when it's warmer. It could also be a combination of high water temp, and a low water levels, and the combined effects of both.