Cucumber bitterness is a function of compounds that sometimes appear in high concentrations in the fruit skin. If plants are severely stressed, especially as a result of drought, these compounds may also develop in the fruit flesh. Fruits that are misshapen or growing on stressed plants, or on plants damaged by insects and disease, are more likely to be bitter. Fruit bitterness is also a function of the cultivar. Select those that are purported to be bitter-free (have low concentrations of bitter compounds).
Pepper pungency (hotness) increases with ambient temperatures, length of time from planting to harvest, and thinness of pod wall. The chemical compounds for pungency are located on the seeds and membranes surrounding the seeds. There can also be significant variation in pungency from plant to plant within a particular variety, and even between pods on the same plant. This is especially true for less domesticated, non-hybridized types of chiles like poblano, mulatto, pasilla, and numex types. Peppers do not increase in pungency as a result of being grown near more pungent types.