Water and Nutrient Uptake in Plants
Plant growth is dependent on two vital processes: the uptake and movement of water and nutrients from roots to shoots and the manufacture of sugars via photosynthesis in the foliage. However, it’s the plants internal pipes and plumbing that get these essential raw materials to where they are needed for growth. While we have a metabolically active pump in the form of a heart to pump liquid around a circulatory system, plants require a different system to conduct water and the compounds it carries around a vascular system.
Hydroponic systems cater to a plant’s thirst for water more effectively than most soils, provided the physical structure of the growing substrate is optimal and irrigation well managed. This is one of the main advantages of hydroponics, as a plentiful water supply that can meet plant requirements under stressful conditions of warm temperatures and low humidity significantly improves growth and yields. Much of the water uptake by hydroponic plants is lost through stomata in the process of transpiration, with only around five percent of all water ending up incorporated into plant tissue. This can mean a large, mature plant under conditions of high evaporative demand can require in excess of a gallon of water per day, and in some cases transport this through many feet of stem up to the growing points and canopy.
(Read also: Understanding Iron in Hydroponics)
The driving mechanism of water transport inside plant vessels is called the Cohesion-Tension mechanism and this relies on the forces generated by hydrogen bonding that makes water stick to itself. Hydrogen bonds and the cohesion created between water molecules allow water columns from leaves to roots to exist under substantial tension and this is what permits water movement inside the plant without any active pumping mechanism. The tension in this column is created by a negative pressure generated by evaporation of water from the foliage (i.e. transpiration). As water is lost from the leaves, surface tension pulls up water molecules to replace those lost and the force is transmitted along the continuous water column down to the roots where water is drawn in from the growing substrate. Since water is required for photosynthesis, cell growth and expansion, and a whole range of biochemical reactions, maintenance, and movement of this water column inside the plant is a vital process. While this may seem like an automatic mechanism, factors that drive or restrict transpiration directly influence water uptake and the delivery of the compounds and nutrients it carries. Transpiration is essential for water uptake and plant growth and this is a process we have some control over.
Tipburn is the result of a problem with the transportation of calcium which moves within the transpirational flow in the xylem from roots to leaf tips.
Growers have long known the beneficial effects of transpiration in allowing the plant to cool itself, thus keeping stomata open for CO2 absorption to keep photosynthesis ticking over. However, transpiration is also essential for several other factors like maintaining plant turgor and allowing calcium, which moves in the transpirational stream, to be deposited in new cells thus preventing certain physiological disorders such as tip burn and blossom end rot. By manipulating the plant’s growing environment via heating, cooling, humidity control, and the rate of air flow, we directly drive transpiration and the movement of water up from roots to leaves inside the xylem vessels. When transpiration is not occurring or is very limited, osmotic pressure can regulate some movement of water into root cells; this is commonly seen as root pressure and may cause guttation (formation of water droplets on the leaf margins) under certain conditions. Root pressure is usually not seen in rapidly transpiring leaves. Factors that can reduce water uptake and movement within plants include those that may damage the xylem vessels or root cells through which water travels. These are typically pathogens such as Pythium, Fusarium, bacterial wilt, and many others that not only damage and destroy root cells, but also the vascular tissue within the plant which forms the transport system. Other conditions such as drought and freezing can cause a break or gas bubble in the water column; in severe cases this limits photosynthesis and if prolonged can result in permanent wilting and death.
Nutrient Uptake and Movement
Nutrient movement through plants begins with ions being dissolved in the water surrounding the root system. From there root hairs take in these solutes through the trachieds and vessel elements of the vascular system (xylem vessels) and transport these long-distance upwards to where they are released inside the leaf or fruit tissue. Nutrient uptake can be carried out in two different ways, those being active and passive absorption. Active absorption of nutrient ions requires an energy source, ions move from the outer space of root cells into the inner space against a concentration gradient. The energy required for this is obtained from cell metabolism and stored as ATP. Passive absorption is where nutrient ions flow without any direct energy requirements. This is diffusion or movement across a membrane from a side of higher concentration (the soil water solution) to a side of lower concentration (inside the root cells).
The physical structure of the growing substrate and control over irrigation are important aspects of both water and nutrient uptake.
Nutrient uptake via the root system and movement through plant pathways is also dependent on several other factors — oxygen is required for the high rates of root cell respiration that can occur under warm growing conditions, generating the energy for active absorption of nutrient ions. pH conditions in the root zone determine availability of certain ions for uptake and water stress or high EC can severely restrict the uptake of some elements. Temperature plays a role in nutrient uptake and this can differ considerably with plant species. Nutrient interactions can play a role in availability for uptake of certain ions and competition for uptake can occur to the point where deficiencies are induced. Another factor that can play a role in nutrient uptake and movement are symbiotic associations with microorganisms. While these have long been considered to be largely soil-based interactions, there is strong evidence that such beneficial partnerships are important for soilless crops as well. It is possible that up to 80 percent of all plants establish some type of mycorrhizal symbiosis which assists with nutrient uptake. Mycorrhiza are fungi of several different classes which have various methods of colonizing plant root cells and assisting with nutrient movement into roots. One of the most common of these are arbuscular mycorrhizal fungi (AMF), which penetrate the epidermis of the plant root and extend hyphae into the growing substrate thus increasing the roots absorptive surface area. Microorganisms of various other species are also likely to play a role in nutrient absorption, which is not to be overlooked in hydroponic systems despite the plentiful nature of elements supplied via a complete and balanced nutrient solution.
(Read also: 7 Ways to Disinfect Hydroponic Nutrient Solution)
Inside the plant, some nutrients may also be scavenged and remobilized from older tissues (such as the lower leaves) to other parts where they are needed for growth and development. Only some nutrient elements can be mobilized, these being N, P, K, Mg, Cl, Zn, Mo, and in situations where this is occurring deficiency symptoms will be seen on the older foliage first. Under hydroponic production with a plentiful supply of ions, nutrient mobilization from old to young tissue may not seem like it would be required, however, competition between some elements and factors that might restrict uptake in the root zone do occur. The most commonly seen of these are induced magnesium deficiency on older tomato leaves caused by high uptake rates of potassium in the root zone. Inside the plant, nutrients move both within the xylem and phloem vessels, with N, P and K being phloem mobile, while Ca only moves within the xylem vessels carried within the transpirational flow.
Summary for Growers
To maximize water and nutrient uptake and efficient transportation within a hydroponic plant there are two main zones that need to be controlled. Firstly, the root zone, where access to sufficient moisture and oxygen will, at all times, ensure a healthy root system with many fine root hairs and a high surface area for uptake. This means selection of a suitable hydroponic growing substrate that can hold sufficient moisture, but at the same time have sufficient porosity to provide oxygenation. In solution culture, maintaining dissolved oxygen rates in the root zone becomes even more essential, particularly as temperatures increase and less oxygen is available for root uptake. Secondly, the upward movement of water and solutes to the areas of active cell growth through the vascular system. This is largely driven by transpiration and maintaining good rates of air flow over the leaf surfaces is a vital component of keeping plants hydrated and actively growing. Use of horizontal air flow (HAF) fans and ventilation control in greenhouses and other growing areas are a vital part of this process in hydroponic crops. Humidity control to facilitate a good rate of transpiration is also important. Humidity or vapor pressure deficit should be kept within an ideal range that promotes a good rate of transpiration, but does not cause evaporative water loss so rapid that the plant can not replenish and transport water fast enough from roots to leaves.
More complex interactions involve control over pH, EC, and nutrient ratios and balance within the root zone to ensure these don’t restrict the uptake of certain elements and the potential use of mycorrhizal and other inoculants to assist with nutrient uptake.
