François-Alain Daudet

Significance and limits in the use of predawn leaf water potential for tree irrigation

Research in estimating the water status of crops is increasingly based on plant responses to water stress. Several indicators can now be used to estimate this response, the most widely available of which is leaf water potential (ΨLWP) as measured with a pressure chamber. For many annual crops, the predawn leaf water potential (ΨPLWP), assumed to represent the mean soil water potential next to the roots, is closely correlated to the relative transpiration rate, RT. A similar correlation also holds for young fruit trees grown in containers. However, exceptions to this rule are common when soil water content is markedly heterogeneous.Two experimental conditions were chosen to assess the validity of this correlation for heterogeneous soil water content: 1) young walnut trees in split-root containers. The heterogeneity was created by two unequal compartments (20% and 80% of total volume), of which only the smaller was irrigated and kept at a moisture content higher than field capacity (permanent drainage). 2) adult walnut trees in an orchard. In this case, soil water heterogeneity was achieved by limiting the amount of localised irrigation (20% of the irrigated control)which was applied every evening.Values of sap flux and of minimum and predawn leaf water potentials with homogeneous and heterogeneous soil water content were compared for trees grown in the orchard and in containers. In spite of intense drought reflected by very low RT or stem water potential, ΨPLWP of trees under heterogeneous moisture conditions remained high (between -0.2 and -0.4 MPa) both in the orchard and in containers. These values were higher than those usually considered critical under homogeneous soil conditions. A semi-quantitative model, based on the application of Ohms analogy to split-root conditions, is proposed to explain the apparently conflicting results in the literature on the relation between ΨPLWP and soil water potential.

Seasonal variation of photosynthetic carbon flow rate into young walnut and its partitioning among the plant organs and functions

Summary Daily fluxes of photoassimilates into young, branched walnut trees were investigated over the whole season through gas exchange measurements. Their partitioning among the different plant organs and functions was assessed by quantitative 14 C labelling in August and October. At harvest, 7 hours or 5 days after labelling, the labelled compounds were partitioned among 4 fractions: starch, sugars, lipids, and the «residue» fraction containing all other compounds, mainly cell walls and proteins. It took ca. five days for the bulk of exportable photoassimilates to be actually exported and metabolized in sink areas. Daily fluxes of recent photoassimilates were derived from applying the 5-day partitioning pattern of the 14 C recovered in the evening of the labelling day to the total C gained over the whole day by net assimilation (daily 43g CO 2 in August, and 10g in October, for a leaf surface area of 3.8 m 2 ). The translocation pattern, which was already highly basipetal in August, became even more so in October; in the same time, the daily fluxes of recent photoassimilates incorporated into all biochemical fractions decreased significantly at the whole plant level. Furthermore, the relative part of sugars in the total exported C doubled, which indicated a slower metabolic rate. However, whereas some organs like the tap-root apparently just slowed down but did not qualitatively change their activity — growth vs. reserve storage, as indicated by the starch : residue labelling ratio —, others like the fine roots exhibited a considerable shift from quasi-exclusive growth to a much more balanced activity. In contrast to the fluxes of recent assimilates incorporated into the plant dry matter, the fluxes allocated to respiration in October were only slightly lower than in August; expressed relative to the total incoming C, they appeared much greater. However, in both situations the part of recent (i.e., up to 5 days old) photoassimilates in the total plant respiration did not exceed 25 %, which stressed the importance of older carbon remobilization.