When is a plant actually dead?
ICT International, 2013.
There has always been a strong interest in how adverse environmental conditions, particularly water deficit, affect plant health and mortality. In recent years research has been expanded to include the phenomenon of climate extremes (Pfaustch and Adams 2012) where heatwaves, compounded by lengthy droughts, can have a significant effect on plants. Modelling and predicting the physiology of plant mortality is critical in the management of crops and forests. However, scientists currently face an intriguing problem – at the moment there is no definition, or consensus, for when a plant is actually dead!
Anderegg et al, writing a review article in Trends in Plant Science, sought to address this significant shortcoming in plant physiology. Although it should be obvious when a plant is dead, in reality it is more complicated. For example, resprouting species offer a good example of the complexity behind plant mortality. A fire can ravage a plant community leading to a seemingly barren landscape with a series of protruding sticks.
Resprouting species have a number of adaptations, such as epicormic buds and lignotubers, which can lead to vigorous plant growth shortly following fire. A plant which once appeared dead can, within a season of growth, have a greater amount of growth and biomass. The plant, Anderegg et al argued, should be considered as a series of highly complex interactions between organ subsystems which make the entire organism. They offer the analogy of a human being which is a complex integration of interactions between organs mediated by the nervous and circulatory systems. In the medical literature, death is defined as the point beyond which the subsystems can no longer maintain the organism as a whole, even if individual cells, tissues and organs are still “alive” (a good thing if you are in need of that kidney or heart transplant!)
Similarly, Anderegg et al defined plant mortality as the “cessation of the highly complex interaction of its organs subsystems”. This definition is clarified further as the “irreparable damage to the interaction and co-ordination of tree organ subsystems in xylem and/or phloem transport within the tree”. This definition emphasises the importance of the xylem/phloem transport systems in leading to whole-system failure.
The xylem and phloem interconnect the roots and leaves. The xylem and phloem systems need to be within a point of recovery for a plant to avoid whole-system failure. Farmers have long known this with the phloem as they “ringbark” trees to clear land – the phloem is severed and sugars can no longer be transported from leaves to roots.
In natural systems, particularly those experiencing drought or climate extremes, it is an accumulated water debt which can lead to whole-system failure. Plants continuously lose water to the environment one way or another. Plants can lessen the extent of water loss through leaves, branches, fruits and flowers, but plants can never completely cease water loss. During periods of water deficit, more water is lost than can be replenished through the roots. Eventually, the plant loses so much water that it can no longer recover. This transport failure not only affects the xylem, but also affects the phloem, xylem-phloem interaction, and short-distance signalling.
Now that a definition has been established, it is essential to be able to quantify the accumulation of water debt, or water loss, until a plant is beyond the point of recovery. An essential part of the definition of plant mortality is whole-system failure. Therefore any technique which measures a plant organ or cell will be inadequate.
Instruments which measure photosynthesis, stomatal conductance, or reflectance indices (e.g. NDVI) are therefore not appropriate. Additionally, Anderegg et al argued that continuous monitoring of the plant is critical, just as it is critical to continuously monitor the heart rate of a hospital patient with an electrocardiogram (ECG).
Two techniques are available for the continuous monitoring of plant water relations. The first is sap flow via the SFM1 Sap Flow Meter (Figure 1A) or HFD Heat Field Deformation Meter (Figure 1B). Sap flow can be monitored over a period of time to establish a healthy plant, a stressed plant, and a plant tending towards death. Figure 2A shows the sap flow of a maple tree during a period when it is healthy, experiencing moisture deficit and recovery following an irrigation event. In this instance the tree was not tipped beyond a point beyond repair.
Figure 2B, in contrast, shows another maple tree as it is tending towards mortality. Here the sap flow declines until it reaches a flat line and there is no water activity.
Figure 2. (A) An example of a decline in sap flow due to an increase in water stress or water debt. Irrigation on the 6th day led to an instant recovery in this plant. (B) An example of water activity, decline and eventual death on a plant stem.
The second technique is stem psychrometry and is the technique which is advocated by Anderegg et al. Stem psychrometry measures plant water potential which is a direct measure of the water debt of a plant. Plant water potential is essentially how hydrated a plant is with values close to zero megapascals (MPa) indicating a well hydrated plant and values which are more negative indicate a stressed plant. Although there is no one absolute number which universally indicates when a plant is dead, an examination of the data will clearly show a stressed plant, a plant which can recover, or a plant which is beyond the point of recovery.
Figure 3. The PSY1 Stem Psychrometer manufactured by ICT International.
A stem psychrometer is the PSY1 Stem Psychrometer manufactured by ICT International (Figure 3). The PSY1 Stem Psychrometer can continuously measure the water potential of a plant. For example, Figure 4A shows stem water potential data on a coffee plant, growing in Costa Rica, over a 7 day period (Downey and Arias 2010). Over the first 5 days the plant is experiencing increasing water debt (as indicated by more negative values). The plant was then irrigated and recovered to a well-hydrated state.
Figure 4B shows data from a cotton plant grown in New South Wales, Australia. Following installation, the plant had a water potential of -1.2 MPa. At night, the plant became more hydrated to have a value of -0.2 MPa. The following day, plant water potential peaked at -1.4 MPa at 2.30pm before recovering to -0.6 MPa at 5.00pm. At this point, the plant was uprooted from the soil. Immediately water potential became more negative until it plateaued at -2.5 MPa – this is the point of plant death (Downey 2010).
Figure 4. (A) An example of a plant experiencing increasing water stress (water debt) before an irrigation event on Day 6. (B) A cotton plant has reached a level of water debt which it can no longer recover.
Anderegg et al have put forth a convincing framework for considering tree death as a failure in the tree’s transport system. Measuring the xylem system is achievable with instruments such as SFM1 Sap Flow Meter, HFD Heat Field Deformation Meter, or PSY1 Stem Psychrometer.
For more information on these instruments and techniques contact ICT International.
Anderegg WRL, Berry JA and Field CB (2012). Trends in Plant Science. 17: 693-700.
Pfautsch S and Adams MA (2012 –published online). Oecologia. DOI:10.1007/s00442-012-2494-6