ABSTRACT
Water is essential for all life forms, including plants� Plants use more water than animals� Plants are 90% water, whereas animals may be 75% by weight� As mentioned in the previous chapter, water is part of the photosynthesis process and is present in cells� From the very beginning of seed germination, water is essential� Water is the medium by which plants take up minerals from the soil solution or the nutrient solution in the case of hydroponics� Plant roots actively absorb the nutrients from the water and transport all chemicals in and out of cells by water� The water is absorbed into the plant near the tips of the root by specialized root hair cells (Figure 6�1)� From the root hairs the water must enter the vascular tissue (xylem) that transports the water throughout the plant (Figure 6�2)� This is done through a selectively permeable membrane, a single layer of cells called the endodermis� This movement of water into the endodermis is the water going from a region where it is at a higher concentration to one where its concentration is lower (in the cell)� This process is termed “osmosis�”
Water moves upward in plants through the xylem cells, which are long, narrow, tubes containing no living matter (Figure 6�2)� They are joined end-to-end to create long tubular pathways from the roots through the stem to the leaves� The water moves up not by just capillary force, but by the cohesion force of water molecules� Water is lost from leaves by evaporation through the leaf stomata (Figure 6�3)� This is transpiration, also termed evapotranspiration, whereby the water moves out of the leaves and is the driving force to pull the water through the plant in its xylem vessels� In most plants, more than 95% of the water taken in by the roots is lost through evapotranspiration in the leaves� This has a cooling effect on the plant tissues� Higher temperatures and increased wind speeds increase the transpiration rate� As the transpiration increases, the uptake of water by the plant roots must increase to keep the plant turgid� If water uptake is less than water loss, the deficit in the plant will cause the stomata to close and the plant will lose turgidity causing wilting of leaves and then stems� This kind of stress will slow growth and production as when the stomata close, carbon dioxide cannot enter either, so the whole process of photosynthesis slows down or stops if water is not made available to the plant� This occurs in soil when it dries out to a level that the plants cannot take up sufficient water� It can also happen in hydroponic culture if there are large intervals between irrigation cycles and the substrate has insufficient water available to the plant�
In many fruiting crops, such as tomatoes and peppers, a water deficit in the plant will result in blossom-end rot of the fruit� This is caused by insufficient water uptake
and resultant loss of calcium uptake� The symptom is a dry, leathery-like, black tissue at the blossom end of the fruit� High humidity reduces transpiration rates, low humidity accelerates transpiration� An example of the effect of relative humidity on production is given by lettuce� Under high relative humidity, the plant does not release adequate water, so this slowdown of water movement from the root to the leaves causes a lack in calcium uptake resulting in “tip burn” (blackening of leaf margins) of lettuce� If you understand these functions of water within the plant and
how its lack can cause stress in the plant leading to symptoms and reductions in yields, you will know what signs to watch for and know when you must add water or reduce it through the irrigation cycles�
The nutrient transport system is the function of the phloem tissue (Figure 6�2)� It, like the xylem, requires water as a medium to transport the photosynthetic products (photosynthates) throughout the plant from its source to the areas of utilization (sinks) (Figure 6�4)� The sinks include all areas of the plant-roots, stems, and fruit-to where these food substances are transported� If you permit fruit such as tomatoes or peppers to ripen completely on the plant, you will get better flavor and higher nutrition in the fruit harvested “vine ripened” than picking the fruit before it is fully ripened� This is the outcome of allowing the fruit (“sink”) to accumulate more of the food substances as it matures�
Water management is controlling the amount of water supplied to the soil or hydroponic substrate to get optimum growth by avoiding any stresses to the plant� With hydroponics you will generally use an automated system� Irrigation controllers enable the gardener to set irrigation cycle frequency according to the plant stage of growth and weather conditions� You will irrigate more frequently and with longer duration of any cycle determined by light, temperature, relative humidity, and day length� The nature of the growing substrate influences the irrigation practices� More coarse particles will require more frequent irrigation cycles than finer particles� For example, perlite substrate may need five to six cycles per day, whereas coco coir or a peatlite medium that has higher water retention, two to three daily cycles would be adequate� The principle is to keep the levels of nutrients sufficient to be readily available to the plant roots at all times� However, excessive cycles can cause lack of oxygen due to too much free water in the void spaces of the substrate� Oxygen is critical to the plant roots to allow active transport of elements into the plant�
When irrigating plants to keep the nutrient solution from concentrating in the medium by evaporation, it is also important to have a percentage of leachate to occur during irrigation cycles� This will be a function of the length of time the irrigation is activated during any cycle� The percentage of leachate varies with the substrate�
In general, with rockwool and perlite cultures we want approximately 25% leachate, whereas with coco coir and a peatlite medium it should be approximately 10%–15%� When growing in soil, gardeners can use a tensiometer that tests the moisture level in the soil� With hydroponics, moisture sensors may be placed in the medium that sense the moisture level and will activate an irrigation cycle automatically based on a preset limit� This kind of feedback system provides the crop with more uniform irrigation than a simple time-clock type of controller�
In summary, recognizing the factors that determine the water usage by plants assist you in managing the irrigation practices to keep plants most productive� Water quality, mineral content, and plant consumption under variations of weather and plant growth, plant appearance, and symptoms all assist in recognizing any imbalances with irrigation of the plants�
