Empower plants through well-balanced irrigation

The goal of every greenhouse irrigation strategy should be to maintain water balance in the crop.
April 08, 2019
Written by Jan Voogt
Figure 1. The leaf temperature sensor can be an important tool.
Figure 1. The leaf temperature sensor can be an important tool.
What is plant empowerment? Typically, traditional plant production methods are based on a mixture of blueprints, best practices, common knowledge of plant physiology, as well as the ‘green fingers’ and ‘emotional perception’ of growers. This approach has been successful, but also has several limitations.

The concept of ‘Growing by Plant Empowerment’ (GPE) brings experience and knowledge together in an integrated approach. Its starting point is the natural behaviour of plants related to the greenhouse environment, as described by six balances concerning energy, water, CO2 and assimilates. Monitoring these balances with sensors, combined with crop measurements in a coherent framework based on physical and plant physiological knowledge and insights, provides hard facts required to control and improve the cultivation process.

In this first article of a multi-part series, we’ll explore the importance of GPE in a greenhouse’s irrigation strategy.

Well-balanced irrigation
Typically, an irrigation strategy consists of a mixture of different methods, based on clock times (such as start time, stop time and time intervals), radiation intensity (W/m2) and light sum (J/cm2), the measured percentage of drainage water, water content or weight of the slab, and so forth.

The goal of the irrigation strategy, however, should be to support the plant’s water balance and to ensure CO2 uptake for photosynthesis by keeping the stomata open under high radiation conditions. This maximizes the light use efficiency (LUE) and production of assimilates for growth and development. Therefore, irrigation should be closely aligned to evaporation rates at all times.

Since energy input is the driving force behind evaporation, it is possible to estimate actual evaporation rates by measuring the incoming radiation energy through a pyranometer outside or, preferably, a PAR sensor inside at crop level. The humidity deficit of the air, the heating pipe temperature, as well as other factors, can also be used to refine this estimation.

Of course, a method to check the applied irrigation strategy is required. That can be done by monitoring the water supply and the water content of the slab or the pot, but an even better way is by observing stomatal behaviour by measuring plant leaf temperature and monitoring the vapour pressure difference (VPD) between the plant and the air – we will revisit this later.

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The energy balance of the plant
Energy balance is the balance between the energy flows towards (input) and from (output) the plant. We can distinguish between four different types of flows:
  • Shortwave radiation – this is light from the sun, lamps or LEDs;
  • Longwave radiation – also called heat radiation or heat emission;
  • Convective energy – transferred by moving air around the leaves;
  • Evaporation energy – energy absorbed by the evaporation of water.
According to the physical Law of Conservation of Energy, energy cannot be created or destroyed but can be transferred into different forms. If energy balance is the sum of all the different energy flows, the net result must be zero as the total input energy must equal total output energy.

Typically, shortwave radiation only moves towards the plant and thus is always on the input side. Longwave radiation can be towards the plant, but also emitted from the plant depending on the temperature difference between the crop and the surrounding objects, such as the soil and greenhouse roof.

Convective heat transfer plays a decisive role in the balance and underlines the importance of air movement around the plant. The direction of the transfer depends on the temperature difference between the plant and the air, so it can contribute either positively (input) or negatively (output) to the balance.

Evaporation is only possible when the balance has a surplus of energy on the input side. It is also the link between energy balance and water balance because evaporation is essential for cooling the plant under high radiation conditions.

Recent research has revealed at least five crucial new insights related to the energy and water balance of plants:
  • The driving force for evaporation is energy absorption by the plant leaves and not the relative humidity (RH), humidity deficit (HD) or vapour pressure deficit (VPD).
  • By identifying two different sources of energy that contribute to evaporation energy (radiative and convective heat transfer), stomatal behaviour can be explained by the plant’s energy and water balance.
  • Air movement can stimulate plant activity or evaporation, as moving air supplies convective heat input. On the other hand, air movement can help the plant stay cool if not enough water is available for evaporation (plant temperature is higher than air temperature).
  • Low air humidity or a high humidity deficit can cause extra unnecessary evaporation under high radiation conditions, thus closing stomata and limiting photosynthesis.
  • Heat emission (outgoing long wave  radiation, OLWR) has a significant negative impact on the plant’s energy balance during nighttime, but also at the beginning and end of the day when the roof of the greenhouse is cold compared to plant leaf temperature. Evaporation and thus uptake of vital nutrients, especially calcium, may drop below the critical minimum value. That causes many different problems for the plant’s growth and development.


Water balance of the plant

Water balance describes the input and output of water from the plant. Output is mainly by evaporation. Only a little portion of total water uptake is stored in the plant and fruit. In practice, evaporation is mostly driven by radiation from the sun, supplemental lighting and heating pipes. If there is no radiation, the only source of energy is convective heat transfer by air movement. In that case, evaporation mainly depends on RH or HD.

Plants need to evaporate for two reasons: for uptake and transport of nutrients, and for cooling. To keep the water balance in equilibrium, uptake of water from the rootzone must be equal to at least the evaporation rate. For the short term, the plant can also extract water from the fruits, which thus work as a buffer. That explains why the water balance reacts differently in the morning compared to the afternoon. The same goes for sunny days versus dark days. However, to ensure sufficient water availability for the longer term, irrigation needs to be aligned with evaporation rates.

Detecting water stress
The degree of opening of the stomata is directly related to water availability in the leaves and reveals if the crop is properly irrigated. Because of their microscopic size, the openness of the stomata is hard to measure directly. However, the effect of stomatal closure, which causes a proportional rise in plant leaf temperature, can be monitored relatively easily with an infrared sensor.

The infrared plant leaf temperature sensor is (Fig. 1), without a doubt, the most essential plant sensor for practical applications. It shows how plant leaf temperature is influenced by different factors such as changing air temperature, RH, solar radiation, artificial lighting, opening, and closing of screens, vents, and so on, including irrigation strategy. Thermal behaviour reveals the status of the internal energy and water balances, and is, therefore, also directly related to stomatal behaviour.

Figure 2 illustrates how data measured by different sensors changes over two days. During the night, plant temperature (bright green line) is slightly below air temperature (red line) due to evaporation (also called the wet bulb effect). Varying PAR light intensity (yellow line) influences plant temperature in relation to air temperature. Typically, a steep increase in PAR light makes plant temperature rise temporarily above air temperature because the evaporation rate cannot be increased as quickly by the plant (see boxed area). If plant temperature is continuously above air temperature, however, this may indicate a lack of water availability and stomata are likely to be partially closed. Evaporation rate (light blue line) can be derived from the energy balance by calculating the surplus of energy on the input side. Combining this with the calculated VPD between the plant and the air (green line) reveals the behaviour of the stomata: the higher the VPD in relation to evaporation, the lower the stomatal conductance (yellow-green line).

VPD values above 1.5 to 2 kPa signal that stomata are closed and plants are suffering from water stress. This also hampers CO2 uptake for photosynthesis.

Rootzone management
Irrigation strategy is not the only factor that determines the water balance of the plant. Root development, which is directly related to assimilates balance, also plays a crucial role here. Furthermore, rootzone temperature, EC and pH in the substrate deserve attention. For instance, a too-low root temperature or a too-high EC can hamper water uptake under high radiation conditions.

Summary
Irrigation should allow the plant to benefit optimally from the available light for photosynthesis. In practice, different strategies can lead to reasonably good results. The best irrigation method, however, takes the energy and water balances of the plant as a starting point, while also making adjustments based on real-time data and the plant’s stomatal behaviour.

About Plant Empowerment
GPE is based on the same principles as ‘Next Generation Growing’ (NGG), developed in the Netherlands over the last 15 years. It has been shown that this approach can deliver significant advantages regarding production and quality, plant health and energy savings for many different crops. For the most part, this can be achieved without extra investments in greenhouse equipment.

The basic principles have been further developed, refined and extended, focusing on optimal growth in a sustainable way and going by the new name ‘Growing by Plant Empowerment’. For more on GPE and the book Plant Empowerment: The Basic Principles, visit plantempowerment.com. Copyrights of text and illustrations: LetsGrow.com, 2019.


Jan Voogt is a senior researcher at Hoogendoorn Growth Management and is one of the authors of Plant Empowerment: The Basic Principles.

Other articles in this series

Questions or comments? Email greenhouse@annexweb.com

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