The Physiology of Flowering Plants - KHAM PHA MOI

168 GROWTH AS A QUANTITATIVE PROCESStheir absolute growth rates, i.e. the total growth per unit time; ortheir relative growth rates (RGR), the growth of each per unit timeexpressed on a common basis, e.g. per unit mass. To estimate plantyield the absolute amount of growth may be appropriate, but forcomparing the growth activities of two different systems values ofRGR are more meaningful. If two leaves with respective leaf areas of5cm 2 and 50 cm 2 both expand by a further 2 cm 2 in a day, the absolutegrowth rates are the same, but the smaller leaf has a ten times higherRGR and generally would be considered as growing faster.The relative rates of linear elongation growth of a number offlowering plant organs, and two fungi, are compared in Table 6.2.The table is compiled from results obtained under varied experimentalconditions, but the differences in growth rates are far greater than canbe accounted for by differences in external conditions. Under noconditions will the growing zone of a Vicia root double its length perminute, as does that of the fungus Botrytis cinerea. Pollen tubes andstaminal filaments show rates equalling or approaching those offungal growth, but these rates are short-lived; staminal filamentsexpand over a period of minutes, whereas the fungal growth is maintainedsteadily over long periods.Bacteria have the highest growth rates of all terrestrial livingorganisms; bacterial cells can double their mass and divide to formdaughter cells in 20–30 minutes. The duration of a cell cycle in plantsis much longer than this; in pea root tip meristems, mitosis takes onehour at 30 8C and the interval between divisions is much longer (seeChapter 8). The high growth rates of microorganisms reflect theirTable 6.2 Relative growth rates (RGR), using linear elongation as the growthmeasure, of some higher plant organs and, for comparison, hyphae of twospecies of fungi. Sources of data as for Table 6.1.Organ Species RGR aPollen tubesImpatiens hawkeri220(New Guinea balsam)Impatiens balsamina100(garden balsam)Staminal filamentsTriticum sp. (wheat);37.5Secale sp. (rye)Shoot growing zones Bambusa sp. (bamboo) 1.27Bryonia sp. (white bryony) 0.58Root, fastest growing zone Vicia faba (broad bean) 0.36Fungal hyphae Botrytis cinerea 83–200Mucor (= Rhizopus) stolonifer 118a RGR is expresed as percentage increase per unit length of growing zone per minute,i.e.length increase per min ðmmÞlength of the growing zone ðmmÞ 100