THE RESPONSE OF S24 PERENNIAL RYEGRASS SWARDS TO IRRIGATION

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Abstract
Variations in soil‐water and plant‐water status were examined in swards of perennial ryegrass over a period of 2 years. Unirrigatcd swards were compared with swards partially irrigated after cutting and with swards fully irrigated after cutting and then again whenever a 25 mm potential soil water deficit (SWD) arose. Two levels of N were applied to the swards (Nj= 250 kg N/ha, NT= 500 kg N/ha in each year). Frequent measurement of the water in the soil profiles beneath these swards with a neutron soil‐moisture probe, enabled comparisons to be made of the actual SWD with the potential SWD on which the irrigation regimes were based. In the unirrigated swards the actual rate of water use by the swards fell below the potential rate of use at a potential SWD of approximately 50 mm. On the other hand, frequent irrigation at a potential SWD of 25 mm often failed to maintain the actual deficit within 25 mm of field capacity. Under both the unirrigated and the partially irrigated swards, greater use was made of the soil water below 30 cm depth in the N2 treatment than in the N1.The pressure‐chamber technique proved particu larly suitable for measuring the variation in leaf‐water potential (LWP) of grasses in the field. LWP was highest (least negative) at dawn and, provided that no dew was present on the leaves, was related to the actual SWD. There was no direct relationship between the daily minimum value of LWP and SWD, the former being determined largely by evaporative demand. However, LWP in frequently irrigated swards rarely fell below –15 atm, whereas much lower values were recorded in swards on drier soils. It is suggested that the effect of irrigation on plant‐water stress may be most readily assessed from the LWP at dawn.