Response of the components of sugar beet leaf water potential to a drying soil profile

Abstract

SummaryFor many field-grown crops, including sugar beet, there is little information on the seasonal changes in leaf water potential and its components as the soil dries. Therefore, seasonal changes in leaf water, osmotic and turgor potentials of sugar beet were measured in two seasons, in crops that experienced differing degrees of soil moisture stress. In 1983 potentials of crops exposed to early and late droughts were compared with those of irrigated crops, and in 1984 measurements were made in a non-irrigated crop. In the irrigated crop the midday leaf water potential changed little during the season, except in response to fluctuating evaporative demand. In the drought and non-irrigated treatments there was a sharp fall in leaf water potential as soon as the soil water potential decreased. The size of the midday leaf water potential was primarily determined by soil dryness. However, the leaf water potential did not decrease below about — 1·5 MPa in either year. The leaf osmotic potential declined at the same time as the leaf water potential, but the extent to which this happened differed in the two years. Only in the 1984 non-irrigated crop did the osmotic potential continue to decrease as the soil dried, suggesting that osmotic adjustment had taken place in 1984 but not in 1983. Thus higher turgor was maintained in the 1984 crop than in the 1983 drought-affected crops. Some turgors were recorded as apparently negative in 1983.Since the leaf water potential declined to a minimum of about — 1·5 MPa, the soil water potential minima were also about — 1·5 MPa. However, deeper soil was not dried to this extent, suggesting that the extra resistance for water uptake from deep soil was limiting or the rooting density was too low.The pattern of recovery of leaf water potential overnight suggested that the rhizosphere resistance to water movement was small, even as the soil dried. However, measurement of stem water potentials in 1984 indicated that a significant resistance to water flow existed within the aerial part of sugar beet plants. This shows that the use of the water potential in leaves as an estimate of that in stems or roots can be misleading.