Heat Stress :
Look closely below, and you'll see the brown leaf edges that are indicative of heat stress. This damage looks alot like nutrient burn, except it occurs only at the tops of the plants closest to the lamps. There's only one cure for this...get the heat away from the plants, either by moving the lamps or moving the plants.
Figure 1
Nutrient Solution Burn:
There's a good chance that this leaf was subjected to nutrient solution burn. These symptoms are seen when the EC concentration of hydroponic solutions is too high. These symptoms also appear when strong nutrient solution is splashed onto the leaves under hot HID lamps, causing the leaves to burn under the solution.
Figure 2
Many hydroponic gardeners see this problem. It's the beginning of nutrient burn. It indicates that the plants have all the nutrients they can possibly use, and there's a slight excess. Back off the concentration of the nutrient solution just a touch, and the problem should disappear. Note that if the plants never get any worse than this leaf (figure 3), then the plants are probably just fine. Figure 4 is definitely an over-fert problem. The high level of nutrients accumulates in the leaves and causes them to dry out and burn up as shown here. You must flush with clear, clean water immediately to allow the roots to recover, and prevent further damage. Now find the cause of the high nutrient levels.
Figure 3 (left) and Figure 4 (right)
Over Watering:
The plants in figure 5 were on a continous drip system, where nutrient solution is constantly being pumped into the medium. This tends to keep the entire root system completely saturated. A better way would be to periodically feed the plants, say for 1/2 hour every 2-3 hours. This would give the roots a chance to get needed air to them, and prevent root rot and other problems.
Don't be throw off by the fact that the plants in figure 5 are sitting in still water, this is actually an H2O2 solution used to try and correct the problem. Adding an airstone to the tub would also help add O2 to the solution.
Figure 5
pH Fluctuation:
Both of these leaves in figure 6 and figure 7 are from the same plant. It could be over fertilization, but more likely it is due to the pH being off. Too high or too low a pH can lock up nutrients in the form of undisolvable salts and compounds, some of which are actually toxic to the plants. What then happens is the grower then tries to supplement the plants diet by adding more fertilizers, throwing off the pH even more and locking up even more nutrients. This type of problem is seen more often in soil mixes, where inconsistent mixing of the medium's components leads to "hot" spots.
Figure 6 (left) and Figure 7 (right)
Ozone Damage:
Ozone damage typically found near the generator. Although a rare problem, symptoms generally appear as a Mg deficiency, but the symptoms are localized to immediately around the generator.
