Water makes up 98.5 percent of the lettuce plant. The Irish potato has 70 percent water. If water is lacking, all other things tend to back up. All plant processes — use and movement of growth materials, and even structure — fail without water. (Plants wilt because water helps hold them up.) Water is also the plant’s main cooling mechanism on these hot summer days. Pests Low Fertility The garden is a great place to relieve the human stress that comes from work, traffic, bills. But garden plants can succumb to stresses, too. And as with humans, plants’ stress can come from many sources. This may be the No. 1 thing people think about when plant stress is mentioned. Sure, insects eat part of the leaf, and the plant must reroute its functions around that spot. The same is true of diseases. All of these cause the plant stress. But the pests most often overlooked are weeds and grass. They have probably been around longer than we have and are astute at devising ways to get a share of your plants’ nutrients and water. Grass and other weeds’ root systems are more aggressive than your tomato plants’ root systems. They will cause great stress in your garden. How many times have you seen your tomato wilted and the grass and weeds doing fine? Know the signs of plant stress. But better still, use commonsense gardening practices and avoid stressing your plants. Drought Heat Plant nutrients are required to provide the essential 17 elements for plant growth. Without these essential elements, plant-growth compounds can’t form, and the resulting nutrient deficiencies shut down plant-growth functions. The plant doesn’t care whether you give it organic or inorganic nutrients — it will call on either to furnish its requirements for growth and fruiting. Heat causes stress by itself in addition to causing water loss. In high heat, for instance, cucumbers may have a bitter taste. I just saw squash, too, that had spongy areas and wasn’t very tasty. This is caused by extreme water loss due to heat and by the plant’s being under heat stress, which makes it unable to use water properly.