Monitoring the simultaneous response of various water status indicators for use in the irrigation scheduling and drought stress detection of a greenhouse tomato crop.

Abstract

An experiment was conducted in a greenhouse at the Department of Biological Sciences, University of Zimbabwe to test the use of various plant water status indicators for use in the irrigation scheduling and water stress detection of a greenhouse tomato crop. These indicators included midday leaf water potential (LWPmidday), leaf temperature, stem diameter, fruit diameter, stomatal resistance and sap flow. Two replicated treatments were used for the experiment namely a well watered treatment in which plants were always given sufficient water (‘sufficient’ being based on calculations of water requirements made using the FAO-Penman-Monteith equation), and a drought stressed treatment in which plants were periodically subjected to drought stress by turning off their water supply valves for a number of days. The procedure was to monitor the variation of these indicators with time for the 60 days of the experiment in both treatments (and their replicates) and correlate these indicators to plant water requirements calculated using the FAO-Penman-Monteith equation. Also twice during the 60 day period of the experiment the drought stressed treatment (and its replicate) were subjected to the drought stress mentioned earlier and the physiological responses noted.

In terms of determining irrigation quantity, sap flow and daily mean leaf temperature showed the highest correlations with ETo (R2 = 0.64), these were followed by daily maximum leaf temperature and maximum daily stem shrinkage (MDS) with R2 values of 0.454 and 0.401, respectively. Daily fruit growth was a distant fifth (R2 = 0.221) with stomatal resistance and leaf water potential being found to have no value in determining irrigation quantity of greenhouse grown tomatoes in this case. In terms of stress detection and irrigation timing, LWPmidday showed the best and quickest response (≈ 2 days) with sap flow (≈ 3 days) and leaf to air temperature difference (≈ 4 days) being second and third, respectively. For LWPmidday it was possible to define a threshold of ≤-1600kPa while for leaf temperature a threshold for leaf-air temperature difference between the time of 3:00 pm and 4:45 pm of ≥-2°C was determined at which point plants require water urgently. Stem diameter fluctuations and fruit growth showed some value in terms of monitoring growth rates in a commercial setting. However, they had little value in drought stress detection mainly because of the difficulty in coming up with defined thresholds at which water stress is said to be occurring. Stomatal resistance showed good response to drought stress but suffers due to its reliance on other environmental parameters. However, it was found that stomatal resistance is higher in water stressed plants, and in severely stressed plants will increase with increasing solar radiation; possibly as a defense mechanism to reduce transpiration. LWPmidday showed promise for future research on irrigation scheduling and stress detection. However, the need for commercially operational and possibly automated systems points to the use of sap flow, leaf temperature and MDS in irrigation scheduling and stress detection of greenhouse crops. Future experiments or operational tests should include an experimental phase for data collection followed by a validation phase in which the indicators and the results obtained from them are used in operational automated systems and compared against each other.