The Physiology of Flowering Plants - KHAM PHA MOI

GROWTH RATES 17524-hour cycle. The growth of tomato plants (Lycopersicon esculentum)is inhibited if the plants are subjected to light : dark cycles of 6 : 6 or24 : 24 hours; the endogenous rhythm of the plants apparentlycannot adjust to cycles so far removed from the natural. A diurnalgrowth cycle may show more than one maximum in 24 hours.Short-term rhythmsGrowth rhythms not related to the 24-hour period are also known.Growing shoot tips bend so as to rotate in a circle as viewed fromabove. This movement is known as circumnutation or nutationand is most pronounced in climbers, in which it aids the plant to findand to twine round a support. Tendrils also nutate till they contact asupport. Nutation involves a wave of cell extension growth movinground the axis tip; at any instant growth is most intense in onelimited region of the tip and the wave of activity completes a cycleround the tip in anything from 1.25 to 24 hours according to speciesand environmental conditions. Changes in cell turgor, however, alsocontribute to the bending. The nutational rhythm is not dependenton any rhythm in the external conditions, though the speed of rotationis affected by environmental factors such as temperature. Thedirection of nutation is fixed and in most species is anticlockwise asviewed from the top.Annual and other long-term rhythmsThe grand period of growth represents the entire life history ofephemerals and annuals; in perennials it corresponds to the courseof growth during one growing season. In most temperate-zone perennialsgrowth has a regular annual rhythm, with shoot growthcompleted within a short period in the spring and early summer;some trees for example complete 90% of a year’s growth in a 30-dayperiod, starting 7–14 days after the commencement of growth. Thefirst days represent the ‘slow phase’ of the grand period (Fig. 6.4).Such an early cessation of growth during prime climatic conditionsmust be the result of an internal control mechanism. Sometimes asecond flush of growth occurs later in the season. Root growth continueslonger into the summer: plants must tap fresh areas of the soilfor water and minerals even when the shoots are not growing.The annual growth cycle of plants living in a climate with distinctseasonal changes is synchronized with the climatic cycle so that thegrowth period coincides with the favourable season, environmentalsignals such as changes in daylength and temperature serving assynchronizing agents (Chapter 11). Nevertheless the endogenousrhythm also plays a part. Trees of the same species and age, growingside by side, may show slight differences in their times of e.g. leaf fall,which are consistent from year to year, with the same individualsbeing the earliest and the latest in each season.In tropical habitats, growth of a plant community as a wholecontinues all the year round with almost equal intensity, but in