Journal of the Japanese Society for Horticultural Science
Online ISSN : 1880-358X
Print ISSN : 0013-7626
ISSN-L : 0013-7626
Leaf Water Potential as an Indicator of Irrigation Timing for Satsuma Mandarin Trees in Summer
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1980 Volume 49 Issue 1 Pages 41-48


The purpose of this paper is to compare the results obtained from previous experiments concerning effects of water stress on fruit quality in potted satsuma mandarin (Citrus unshiu Marc.) trees with those obtained from field experiments, using leaf water potential (ψmax) before sunrise as an index of water stress.
Tests conducted on individual trees in an experimental orchard on Sept. 1 indicated a wide range of ψmax′s according to soil conditions at the site (e.g. soil depth, and chemical and physical properties of the soil).
1. ψmax on Sept. 1 was closely related to refractometer index, titratable acidity and fruit weight or diameter in fruits harvested on both Sept. 7 and Dec. 6. A closely linear relationship was recognized between climax and refractometer index, and the very interesting fact was observed that juice of fruits harvested on Dec. 6 from trees exposed to severe drought (below a ψmax of -14 bars) decreased in refractometer index as compared with those harvested on Sept. 7. In the case of titratable acidity the relationships between curves from Sept. 7 and Dec. 6 harvestings were different from those in the case of refractometer index; the Dec. 6 curve looked like the result of parallel shifting of the Sept. 7 curve.
2. The relationship of the specific gravity of fruits harvested on Dec. 6 to ψmax on Sept. 1 was not simple. Until ψmax reached about -11 bars, specific gravity decreased linearly as ψmax reduced, but beyond -11 bars of ψmax specific gravity began to increase non-linearly.
3. Linear least squares analysis of the relationship between ψmax on Sept. 1 and peel color gave a correlation coefficient of -0.741. However, it is not clear that a linear relationship does in fact exist.
4. The critical lower limit of ψmax at irrigation time for producing fruits suitable for marketing seemed to be about -7 bars, judging from fruit weight and peel color. However, it appears to be more desirable to irrigate when ψmax reaches-5.5 bars if circumstances permit.
5. After ψmax decreased to about -5.5 bars, ψmax could be kept at about -5 bars by an irrigation of 3mm per day. With this treatment the fruit quality was similar to that of well-watered controls.

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