Diurnal patterns of shoot elongation, obtained by mechanical growth recorders, were compared with those of leaf elongation and stem radius; the relationships of these to environmental factors were studied in the field and in growth chambers.
In sunny weather, minimal elongation (or shrinkage) occurred during the mornings, and maximal elongation rates during late afternoons. On rainy days, elongation patterns tended to be undefined.
The observed patterns appeared related to diurnal changes in water balance, air temperature and light. There was no evidence of an endogenous rhythm. Rising evaporative demand during sunny mornings resulted in contractions in shoot and leaf lengths and in the thickness of the stem live bark. Re-expansion occurred in the evening and at night, or during the day when evaporative demand was reduced by cloud cover and rain. Shoots also contracted on frosty nights, but expanded rapidly following thawing early in the mornings. Temperature also positively influenced growth (i.e. irreversible increase in size). Light appeared to inhibit shoot growth in the mornings, but this effect was confounded with the tendency of shoots to shrink in length due to dehydration.
Since growth and reversible changes in shoot length often occurred simultaneously in different portions of the same shoot and since the zones of reversible change and growth overlapped, the measurement and analysis of hourly growth responses can be complicated; a distinction must therefore be made between growth (which is irreversible) and elongation (which may include both growth and reversible changes in shoot length).
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