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Early detection of low temperature plant stress: towards a tool for energy-efficient production

Final Report Summary - EARLYTOOL (Early detection of low temperature plant stress: towards a tool for energy-efficient production)

Greenhouse horticulture in Central/Northern Europe has very high productivity, thanks to a risk-averse temperature control, achieved at a high cost of heating (fossil) energy. On the other hand, the lack of temperature control in the Mediterranean region is a major cause of the huge “productivity gap” between both regions. This project has the final aim of improving resource use efficiency and productivity of European greenhouse crops by developing a tool for “lean” temperature management.
The objectives of EARLYTOOL have been (1) to increase our knowledge about the effect of incidental low night temperatures on the growth of horticultural plants and (2) to test the feasibility of a non-invasive “stress indicator” as tool for temperature management in greenhouses.
1. Effect of incidental low night temperatures on the growth of horticultural plants
The response of sweet pepper seedlings (Capsicum annuum L. cv. Fantasy) to different doses of low temperature (1, 4 and 7 consecutive nights at 6 °C; 18 °C being the control treatment) was evaluated by measuring changes in several plant physiological parameters, selected on the basis of previous literature. This has resulted in a model that explains which plant processes are affected by low night temperature (Fig. 1).
Low night temperature may decrease the hydraulic conductivity of the plant, which reduces water uptake and relative water content (RWC) of plant tissues. In contrast, the reduced RWC was not the result of water loss because transpiration rate and stomata conductance were not affected. The reduced RWC results in loss of turgor, which reduces stem elongation and leaf expansion, and therefore light interception: plants were shorter and had less (projected and actual) leaf area at 6 °C
Low night temperature decreases enzymes activity in general and, specifically, the activity of enzymes related to the degradation of sugars. One of the consequences is the accumulation of sucrose and starch in the leaves during the night. As sucrose breakdown is reduced, it cannot be used for ATP production or for increasing cell osmotic potential, key for cell elongation. Starch accumulation leads to a decrease of specific leaf area (SLA) and it may result in a decrease of photosynthesis due to feedback inhibition. CO2 uptake was not affected in our experiments, but low night temperature induced photo-inhibition of photosystem II.
As photosynthetic rate was not affected, so was total plant dry weight after 7 cold nights. However, the increasing reduction of light interception may affect plant biomass if stress doses are extended. The combination of reduced actual leaf area without any change in total dry weight can be explained by the decrease in SLA: leaves were thicker at low temperature.
2. Feasibility of a non-invasive “stress indicator” as tool for temperature management in greenhouses
To implement energy saving strategies in Northern countries or to avoid too strong stress in the South of Europe, growers need an early warning system of plant stress that gives them complete security when keeping climate settings outside of the safe range. To be reliable, such a system must detect plant signals of stress in an early stage (i.e. before they are obvious).
In EARLYTOOL, we have worked with two stress detectors having different methodologies: CropReporter (Phenovation B.V.) which captures images of chlorophyll fluorescence of the whole plant, and PlantEye (Phensopex B.V.) which records the hourly dynamics of plant growth. Both equipment are non-invasive and were able to detect stress in an early stage. Two cultivars of sweet pepper seedlings (Fantasy and DRP 2571; Monsanto) were subjected to 1, 4 and 7 days of low temperature during the night (6 and 12 °C; 18 °C being again the control treatment).
Stress was detected by the CropReporter only 2 hours after the onset of cold, both at 12 and 6 °C but with larger differences with respect to the control in the last case. However, increasing the doses of low temperature in terms of duration did not increase plant stress linearly. Stress was stronger after 4 days, but plants were similarly affected or even slightly recovered after 7 days.
Both cultivars were equally affected by low temperature, in spite of the fact that DRP 2571 is considered cold-tolerant. The reason is that plant processes affected by cold in the light are not the same as the ones affected in the dark. In both cases, the target tissue for stress seemed to be the apical meristem, as it can be seen at the images of Fv/Fm (maximum photochemical efficiency of PSII) (Fig. 2). However, after analysing the data derived from the images, we found that all tissues were significantly affected by low temperature in an early stage, but the apical meristem had lower values.
Stress was also detected by PlantEye at an early stage. This equipment measured plant height and projected leaf area every hour. The data made it possible to compute the growth of the plant and to estimate the circadian movement of the leaves. Plants grew taller as night temperature increased, and leaf movement varied depending on the treatment (the analysis of these data is still in progress).
Therefore we confirmed that there is an averse effect of low night-time temperatures on growth of sweet pepper. However, we concluded that smarter application of night-time heating could save a significant amount of energy in Dutch greenhouses and increase productivity in Mediterranean ones. This research has given the proof of principle that "lean" heating management relying on early warning tools such as the ones developed and applied here is possible.