Metabolic regulation of leaf respiration and alternative pathway activity in response to phosphate supply

In this study the question whether the alternative respiratory pathway acts as an electron bypass for the cytochrome pathway under conditions of growt h on limited phosphorus in leaves of bean (Phaseolus vulgaris L.), tobacco (Nicotiana tabacum L.) and Gliricidia sepium Walp was investigated. The oxy gen isotope fractionation technique was used to assess the in vivo activiti es of the cytochrome and alternative respiratory pathways in the absence of added inhibitors. The response of respiration to low phosphorus supply var ied among species. Growth at low phosphorus reduced cytochrome pathway acti vity in bean and tobacco. Alternative pathway activity increased only in be an leaves in response to low phosphorus and not in tobacco. In the case of G. sepium, cytochrome pathway activity remained unchanged whereas the alter native pathway activity increased with low nutritional phosphorus. At low p hosphorus, alternative oxidase protein levels increased in the leaves of be an and G. sepium but not in tobacco, suggesting a dependence of alternative pathway activity on protein level. Alternative pathway activity was also n ot correlated with soluble carbohydrate concentration in bean or tobacco at any phosphorus level. These results show that the alternative pathway does not always act as an electron bypass in response to the downstream restric tion of the cytochrome pathway imposed by low phosphorus supply. These resu lts suggest that factors in addition to cellular carbohydrate level and ade nylate control can act to regulate alternative pathway activity.