Measurement of Traits Regarding Plant Photosynthesis and Water Use

Plant physiology is a discipline of studying the variety of biological processes and functional activities occurring in plants. Plant physiological processes related to photosynthesis and water use provide organic compounds and energy for their own growth, as well as the food source for other organisms. Improving photosynthetic capacity and efficiency are essential for increasing the yield potential of crops.

Lifeasible, as a leading plant biotechnology company, provides services for the measurement of traits regarding plant photosynthesis and water use, with both high-throughput and conventional methods. We are devoted to offering our customers worldwide with highly accurate and repeatable data.

Photosynthesis and respiration rate

Photosynthesis is a process in which green plants use energy from the sun to transform water, carbon dioxide, and minerals into oxygen and sugar (Figure 1). The evaluation of photosynthesis rate can be based on uptake of carbon dioxide, production of oxygen, production of carbohydrates, or increase of dry biomass. Opposite to the photosynthesis process, the respiration process utilizes the sugar and oxygen to produce energy for plant growth (Figure 1). Therefore, respiration rates are usually reflected by volume of carbon dioxide produced, volume of oxygen consumed, or fresh product consumed within a certain period of time.

Figure1. The processes of photosynthesis and respiration.

Parameters reflecting the photosynthesis and respiration rate can be measured by multiple portable photosynthesis systems with non-dispersive infrared gas analyzers (IRGA), such as Li-6400, Li-6400XT, Li-6800, CI-340, LCi-SD, and so on. In addition, we offer carbon isotope analysis for photosynthesis pathway determination. This method relies on the fact that photosynthetic enzymes (ribulose bisphosphate carboxylase) discriminate against the heavier stable isotope ¹³C (relative to ¹²C) during photosynthesis, so that C in leaves is always depleted in ¹³C compared with that in the atmosphere. The isotope ratio mass spectrometer (δ¹³C) for C3, C4, facultative crassulacean acid metabolism (CAM), and obligate CAM photosynthesis is -21‰ to-35‰,-10‰ to -14‰, -15‰ to -20‰, and -10‰ to-15‰, respectively.

Leaf nitrogen content

Nitrogen plays a key role in the plant life cycle. It is the main mineral nutrient resource for the production of chlorophyll and other plant cell components (proteins, nucleic acids, amino acids). Therefore, the measurement of leaf nitrogen content is important for the evaluation of photosynthesis rate and crop yield.

We offer multiple methods for monitoring leaf nitrogen status (Figure 2), including:

• Tissue analysis. Methods based on tissue analysis such as Kjeldahl-digestion and Dumas-combustion can provide reliable organic nitrogen content.
• Leaf chlorophyll content. As nitrogen is an essential element in photosynthetic protein synthesis, leaf chlorophyll content can be used as an indicator of nitrogen levels. Soil-plant analyses development (SPAD) chlorophyll meter can estimate the chlorophyll content by the transmittance properties of leaves in a nondestructive manner. The Dualex is a device to measure polyphenolic compound content in leaves by means of chlorophyll fluorescence, and is able to detect nitrogen deficiency among other stresses.
• Canopy reflectance measurement systems. We also provide ground-based active-mounted systems (e.g., Yara N-Sensor, GreenSeeker, CropScan) and satellite-mounted sensors (e.g., QuickBird) for canopy reflectance measurement to estimate crop nitrogen levels.
• Nitrate sap content and electrical meters. Nitrate sap content can be measured by a hand-held reflectometer (e.g., the Nitrachek and the RQflex) according to the color change of nitrate test strips when exposed to the sample. Besides, nitrate content can also be reflected by electrical conductivity changes of plant sap, and can be measured by an ion selective electrode.

Figure 2. Methods for plant nitrogen sensing (Muñoz-Huerta et al., 2013).

Stomatal conductance

Stomatal conductance reflects the rate of gas exchange (i.e., uptake of carbon dioxide) and water loss (i.e., transpiration) through the leaf stomata. It can indicate the degree of stomatal opening and plant water status. Stomatal conductance is usually expressed in mmol m^-2 s^-1, and can be measured using different types of leaf porometers:

• Steady state leaf porometer (e.g., SC-1 or PMR-5). This porometer relies on the monitoring of relative humidity (RH) at two points along the flux path. The stomatal conductance is calculated when the flux gradient reaches a steady state. A leaf with a rapidly changing gradient indicates that the stomata are relatively open.
• Dynamic diffusion leaf porometer (e.g., AP4). This method measures the rate of RH increase in a chamber clamped to the leaf surface. A relatively rapid RH rise in a chamber shows that the stomata are relatively open.
• Viscous or mass flow leaf porometer (e.g., Thermoline). This porometer measures the time (in millisecond) to force a fixed volume of pressurized air through the leaf. The large stomatal conductance causes a relatively rapid drop in pressure or a fast flow rate.
• Null balance leaf porometer (e.g., LI-1600, Li-6400XT, Li-6800, or CI-340). This technique measures the rate of gas exchange needed to keep stable RH inside the chamber. A relatively high flow rate to maintain a null balance means the stomata are relatively open.

Transpiration rate

Transpiration is a process that leads to water loss through the stomata of plants via evaporation. Transpiration not only promotes uptake of water and mineral nutrients from the soil, but also help cools down the plants. The transpiration rate for a leaf with more opened stomata is potentially higher.

At Lifeasible, a number of techniques, including photometers (Figure 3), lysimeters, porometers (e.g., SC-1 or AP4), photosynthesis systems (e.g., Li-6400, Li-6400XT, Li-6800, CI-340, LCi-SD) and thermometric sap flow sensors, are available for the measurement of transpiration rate. The transpiration rate can also be measured by carbon isotope discrimination. Due to the different diffusivities of ¹³C and ¹²C across the stomata, and the fractionation by the photosynthetic enzyme, the ratio of ¹³C to ¹²C can be discriminated by the mass spectrometer, and the transpiration rate can be calculated accordingly.

Figure 3. A diagram of bubble photometer for transpiration rate measurement (from Wikipedia).

Water Use Efficiency

Water use efficiency (WUS) reflects carbon gain per unit water loss. The instantaneous WUS can be calculated as the ratio of photosynthesis rates to transpiration rates or stomatal conductance. On the other hand, long-term WUS is evaluated by transpiration rate via the carbon isotope approach. In general, higher values indicate higher WUS.

Leaf water potential

Leaf water potential is a simple indicator of leaf water levels. The more negative the value, the more dehydrated the leaf is. We provide three methods for leaf water potential measurement.

• Thermocouple psychrometry (e.g., WP4C). The psychrometer employs the principle that the water vapor pressure developed over a solution or piece of tissue is directly related to its water potential.
• Pressure chamber (e.g., Arimad-instrument). For this method, the leaf is placed in a chamber and the water potential is measured as the balance pressure to force the sap to the surface of the cut end of the petiole.
• Chardakov's method. The Chardakov’s method relies on the density change in a solution that occurs after a tissue has been immersed in it. If the density of the solution does not change, it means the tissue has the same water potential as this solution (no net movement of water).