Roots generate the plant hormone ethylene in response to stresses such as drought or poor aeration. Ethylene can reduce growth or cause leaf senescence or flower drop. Understanding how the amount of ethylene produced relates to crop stress caused by low substrate water content could lead to ways of monitoring ethylene to aid irrigation.
The technology is now available to measure and monitor root-zone ethylene fluctuations in response to changes in substrate moisture. In turn, this could be linked to management techniques such as automatic irrigation scheduling or root-zone bacterial treatments that mitigate the effects of ethylene on the crop.
Using tomato as the model species, AHDB Horticulture student Antje Fiebig varied irrigation schedules according to substrate moisture thresholds and recorded the plants' physiological responses to these regimes.
After four weeks of overwatering, plant growth (fresh weight) had fallen behind by 62 per cent and total leaf area by 70 per cent compared with plants in a regime where irrigation matched their need for water. Short-term flooding of otherwise "normally watered" plants induced more pronounced changes in soil oxygen concentration than did long-term continuous over-irrigation.
Overwatering had no effect on leaf concentrations of the stress hormone abscisic acid. But it did increase ethylene levels in shoots and leaves more than threefold, although levels in the root zone were the same as in plants watered according to demand.
Fiebig also found that overwatering reduced leaf nitrogen content by a third. Daily additions of small amounts of calcium nitrate to the substrate of overwatered plants restored foliar nitrogen levels, ethylene concentrations and growth to normal.
For details on all AHDB Horticulture activity, see horticulture.ahdb.org.uk