The Physiology of Flowering Plants - KHAM PHA MOI

RESPIRATION AND PLANT ACTIVITY 53of ADH and pyruvate decarboxylase, giving the plants a greaterpotential for alcoholicfermentation. In maize, development of floodingtolerance involves increases in activity of several genes codingfor ADH and other enzymes of anaerobic respiration (Sachs et al.1996).2.11 Respiration and plant activity2.11.1 Correlation of respiration rate with physiologicalactivityRespiration proceeds unceasingly in all active (i.e. non-dormant) livingcells. Even when a cell is not performing any net metabolic workand is simply subsisting unchanged – say a mature pith parenchymacell – it still requires repair and resynthesis of protoplasmic components,which are labile and in a constant state of turnover. Membranepotentials are sustained only by continued pumping of ions acrossmembranes, requiring ATP. This aspect of cellular activity has led tothe concept of maintenance respiration, required for such processes.The proportion of respiration that supports net cellular workis then termed growth respiration (synthetic respiration). Moreprecise definitions of the terms have been attempted, while someplant physiologists have disputed the validity of any division ofrespiration into these components. The general concept of maintenancerespiration is, however, useful as a reminder that cells mustspend energy for their survival, without cessation. It is not impliedthat there is a biochemical distinction between growth respirationand maintenance respiration. Measurement of the proportion ofmaintenance respiration is even more controversial than its definition,but for plant cells, its magnitude has been estimated at up to50% of the total.Respiration rates are positively correlated with physiologicalactivity. On a unit mass basis (fresh or dry mass), high rates arefound in young, actively growing regions, such as growing apices,or in tissues performing metabolic work at a high rate, such as glands(Table 2.2). To some extent this is the result of the higher ratio ofliving protoplasm per unit mass in such tissues; mature and metabolicallymore inert tissues have larger proportions of cell wall and/orvacuoles and storage materials per unit mass, and these tissue compartmentsdo not contribute to respiratory activity. On a unitnitrogen basis, which reflects more truly the ‘living’ mass, differencesbetween tissues of varying maturity and metabolic activitybecome less marked. But in the same tissue, respiration rate can beshown to increase with increasing activity. For instance, when rootsare washed in distilled water and then transferred to a nutrientsolution from which they proceed to take up ions, their respirationrate increases concomitantly with ion absorption. In nectaries, theperiod of rapid sugar secretion coincides with a period of rapidrespiration.