Estimating sap flux densities in date palm trees using the heat dissipation method and weighing lysimeters

Abstract

In a world of diminishing water reservoirs and a rising demand for food, the practice and development of water stress indicators and sensors are in rapid progress. The heat dissipation method, originally established by Granier, is herein applied and modified to enable sap flow measurements in date palm trees in the southern Arava desert of Israel. A long and tough sensor was constructed to withstand insertion into the date palm's hard exterior stem. This stem is wide and fibrous, surrounded by an even tougher external non-conducting layer of dead leaf bases. Furthermore, being a monocot species, water flow does not necessarily occur through the outer part of the palm's stem, as in most trees. Therefore, it is highly important to investigate the variations of the sap flux densities and determine the preferable location for sap flow sensing within the stem. Once installed into fully grown date palm trees stationed on weighing lysimeters, sap flow as measured by the modified sensors was compared with the actual transpiration. Sap flow was found to be well correlated with transpiration, especially when using a recent calibration equation rather than the original Granier equation. Furthermore, inducing the axial variability of the sap flux densities was found to be highly important for accurate assessments of transpiration by sap flow measurements. The sensors indicated no transpiration at night, a high increase of transpiration from 06:00 to 09:00, maximum transpiration at 12:00, followed by a moderate reduction until 08:00; when transpiration ceased. These results were reinforced by the lysimeters' output. Reduced sap flux densities were detected at the stem's mantle when compared with its center. These results were reinforced by mechanistic measurements of the stem's specific hydraulic conductivity. Variance on the vertical axis was also observed, indicating an accelerated flow towards the upper parts of the tree and raising a hypothesis concerning dehydrating mechanisms of the date palm tree. Finally, the sensors indicated reduction in flow almost immediately after irrigation of field-grown trees was withheld, at a time when no climatic or phenological conditions could have led to reduction in transpiration.

Is it true that date-palm consumes a lot of water?

“…This appreciation has been performed in other similar works on palm stems (Dufrêne et al, 1992;Ringersma et al, 1996;Sellami and Siffawi, 2003;Roupsard et al, 2006). For more accuracy, some recent studies (Sperling, 2012;Fathi, 2014) tried to differentiate a radial conduction variation inside the date-palm stem but the results are various. For our case, visual observations lead us to consider, in a simplification trend, that the whole whitecolored sapwood section is uniformly sap conductive.…”

Direct in vivo evidence of immense stem water exploitation in irrigated date palms

“…The probes met the requirements of measuring sap flow ( SF , l h –1 tree –1 ) in mature date palm trees as they were long (33cm) and tough (1cm diameter tubes filled with hardened epoxy glue). The sensors were able to penetrate outer fibrous tissues into the inner stem and measure the surroundings of the thermally conductive tip (2cm long and made of stainless steel, Sperling et al , 2012 ). The heat dissipation factor, k (–), was derived from the maximum ( ∆T max , °C) and momentary (∆ T , °C) temperature differences:…”

“…SF was calculated according to Renninger’s calibration ( Renninger et al , 2010 ), recently reconfirmed for date palms by Sperling et al (2012) :…”

“…Thus, the parenchyma cells can deliver water to xylem vessels and maintain high water potentials ( Parthsarathy and Klotz, 1976 ) with no mechanical stresses ( Holbrook et al , 1992 ; Holbrook and Sinclair, 1992b ). Parenchyma cells in palms are more abundant in the stem centre and lower compartments ( Holbrook and Sinclair, 1992b ) and can support the elevated sap flux densities found in the inner stem ( Sellami and Sifaoui, 2003 ; Sperling et al , 2012 ).…”

Horizontal soil water potential heterogeneity: simplifying approaches for crop water dynamics models

“…Yet, the simplifying assumptions of this approach may be constraining. In reality, local uptake is not at constant rate, but highly variable on a daily basis, notably due to variations of plant transpiration (Jolliet and Bailey, 1992;Sperling et al, 2012). In addition, differences in root hydraulic properties between different root types and horizontal heterogeneity of root density may lead to biased predictions of RWU when homogeneously distributed roots with similar hydraulic properties are assumed (Schneider et al, 2010;Durigon et al, 2012).…”