Part of the SCRI project is to use remote sensing of ET (RSET) to determine tree stress, but over the period of the project, we have found that neither RSET nor measured ET drops during periods when we measure stress.
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This is an example of directly measured ET (expressed as a Kc), with only a hint of a decline when the stem water potential (SWP) declined to between ‐15 and ‐20 bars.
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In other almond studies, by the time SWP reached 20 bars, leaf conductance was reduced by about 70%. A 10 bar reduction was enough to reduce conductance by about 50%.
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In a peach study, whole tree ET was also reduced about 50% by a 10 bar reduction in SWP.
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Since all previous data was collected from sun exposed leaves, we hypothesized that the lack of a canopy response to SWP in almonds might be due to shaded leaves not closing stomata under stress. At the SCRI and other almond sites in 2011, we made detailed measurements of both conductance and photosynthesis in different parts of the canopy (shaded and exposed). Again, a 50% reduction was shown for about a 10 bar drop in SWP for sunlit leaves. Shaded leaves had an overall lower conductance, but also responded to SWP. Hence the lack of a canopy response to SWP was not explained by the behavior of shaded leaves. Conductance was more sensitive to SWP than it was to sun exposure.
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Photosynthesis was more sensitive to sun exposure than to SWP. For sunlit leaves, a 50% reduction in photosynthesis required about a 15 bar drop in SWP.
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Water use efficiency at the leaf level was much lower for shaded leaves than for sunlit leaves, but both increased with stress (as reported in many species). This is interesting, because at the canopy level, if ET is not reduced by stress, then even slight reductions in photosynthesis should lead to a decreased water use efficiency. Hence, observations at the leaf level appear to be inconsistent with observations at the canopy level.
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In 2011 a novel SWP sensor was tested in the SCRI and other almond plots, with very promising results. In most cases the SWP sensor agreed with SWP measured by the pressure chamber (“Bomb”), and following irrigation at this site, showed a clear recovery to baseline values. This sensor will be further developed in 2012.
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