|N.E. Andersson, K. Koefoed Petersen
|deficit irrigation, Hibiscus rosa-sinensis, Rosa hybrida, transpiration, weighing bench
Since climatic computers are widely used in greenhouse operation, the possibility to detect plant stress is open and can give advantages in growth control.
Chemical growth regulation is used to control plant height in pot plant production.
Application is time consuming and not always environmentally friendly.
Water stress introduced by deficit irrigation in combination with low nutrient availability, especially low P, is an alternative to chemical growth regulation.
However, reliable methods to monitor and avoid damaging stress levels are required.
If the available water in the peat soil is near the wilting point, the stomata will close leading to an increased leaf temperature which can reach a lethal level.
If stress situations can be detected, the greenhouse climate can be adjusted in order to reduce the stress. Hibiscus rosa-sinensis and Rosa hybrida (pot roses) were subjected to constant low or fluctuating water vapour pressure deficit (VPD) in combination with frequent irrigation or deficit irrigation.
The average VPD for the constant humidity treatment was 0.8 kPa and for the fluctuating 1.3 kPa.
The stomatal conductance was calculated from weight loss and climatic parameters (VPD, leaf temperature). The highest stomatal conductance was found for Hibiscus rosa-sinensis and in both plant species stomatal conductance increased with increasing irradiance or net radiation.
In pot roses stomatal conductance was the highest in plants frequently irrigated and only slightly influenced by the two humidity regimes.
When Hibiscus was grown at fluctuating humidity and deficit irrigation, the highest stomatal conductance was found, but also the variation in stomatal conductance was large.
Hibiscus grown at constant low VPD and frequent irrigation had similar stomatal conductance as plants grown at deficit irrigation, regardless of humidity regime.
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