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 (ψ
max) 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 ψ
max′s according to soil conditions at the site (e.g. soil depth, and chemical and physical properties of the soil).
1. ψ
max 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 ψ
max 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 ψ
max on Sept. 1 was not simple. Until ψ
max reached about -11 bars, specific gravity decreased linearly as ψ
max reduced, but beyond -11 bars of ψ
max specific gravity began to increase non-linearly.
3. Linear least squares analysis of the relationship between ψ
max 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 ψ
max 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 ψ
max reaches-5.5 bars if circumstances permit.
5. After ψ
max decreased to about -5.5 bars, ψ
max 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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