Plant response to the soil water reserve: Consequences of the root system environment

Summary The ability of water balance models based on the concept of Transpirable Soil Water to predict the occurrence of water stress and the need for irrigation was tested for several environmental conditions of the root system, to determine in which conditions errors are likely to be appreciable. The response of evapotranspiration, stomatal conductance and leaf water potential to soil water reserve was studied under three conditions: (i) in pots with maize plants, (ii) in the field with deep soil and the root system placed in favourable conditions, with wheat during a dry year and with maize during four years with contrasting climate, (iii) in the field, with soil compaction which disturbed the maize root system, decreasing its efficiency for water uptake, during four year. (i) In the pot experiment, where the volume of the Transpirable Soil Water (TSW) is well defined, the responses followed the hypothesis of water balance models. (ii) The soil depletion was higher than the calculated TSW during two dry years in the field, because of an appreciable contribution of the non-rooted soil layers to the water balance. As a consequence, evapotranspiration, stomatal conductance and predawn water potential did not decrease over the whole range of soil water reserve. Grain yield was no lower in those years than in the wet years, in spite of the fact that the soil water reserve was depleted. Thus, a water balance based on the TSW would have underestimated in these conditions the ability of plants to withdraw soil water, overestimating the necessity of irrigation. Predawn water potential gave, on the contrary, indications consistent with the responses of the stomatal conductance and the net CO₂ assimilation. (iii) The water uptake by plants would have been overestimated in the case of compacted soil. Stomatal conductance was low even for high levels of the soil water reserve, except if the densely rooted top 0.1 m layer of soil was rewatered by irrigation. Water stress could not have been diagnosed in this case from indications of soil water potential or of pre-dawn water potential. These data confirm that some knowledge of the environmental conditions of the root system is necessary to determine if errors made using water balance models are likely to be appreciable, and to know if they lead to an underestimation or overestimation of the risk of water stress.