Did you know that tomato plants have a special way of breathing and circulating nutrients all at the same time? Tomato plants have a vascular system that functions much like a siphon or a wick. Water and nutrients are soaked in through the roots, up into the plant where they are then met with sugars and other products of photosynthesis to feed the plant. When the water pressure reaches the leaves, there are tiny pores on the underside of the leaves called stomata, that open and express water vapor at the same time as they take in carbon dioxide. This process is called transpiration and happens mostly in low light or on cool, cloudy days.

Around the aperture (opening of the pore), there are two kidney-shaped guard cells that act as doors to open and close the stomata. When water is drawn up through the plant, the guard cells fill with fluid which creates pressure and forces them to open. After water vapor and oxygen have been expelled, the pressure decreases causing the guard cells to close. Higher temperatures and sun exposure cause the stomata to remain closed so that the plant does not lose excessive amounts of water and become dehydrated. Cooler temperatures and low sunlight allow the stomata to open, release water vapor and take in carbon dioxide. The plant works through the process of photosynthesis during sun exposure and then opens to “breathe” and consume nutrients during lower light and cooler temperatures.

So why is this information important to know? If you time your applications with the transpiration cycle, foliar feeding can be a very effective way to augment your plant’s nutritional needs. Spraying the underside of the plant’s leaves with a very dilute solution of liquid fertilizer just before sundown when the sun is not so strong and the temperatures are cooling off, will allow the nutrients to be readily available when the guard cells open.