Physiological studies of rooting in populus cuttings

Some physiological changes occurring when attempting to root leafless hardwood and leafy softwood cuttings of an easy and a difficult-to-root species of Populus were investigated. P. euramericana was selected ass an easy rooter, and P. tremula as a difficult tooter. Cuttings were cultured in a controlled environment room at 15oC and 20oC air and soil temperature respectively with 18 hr light/6 hr dark and mist. Morphological changes were noted and photosynthesis and respiration measured at intervals during the rooting period. Both leafless hardwood (100% success) and leafy softwood (83% success) cuttings of P. euramericana rooted whereas only the softwood cuttings (10% success) of P. tremula rooted. When cuttings rooted, four distinct morphological changes occurred as follows:- (a) protuberances formed on the periders (b) callus formed at the lower cut surfaces (c) roots emerged from the protuberances (d) roots emerged from the callus (wound-roots) Cuttings were sampled for physiological studies, measuring rates of not photosynthesis and respiration, and levels of carbohydrates, cytokinin and auxin. These levels were measured for the top and bottom halves of the cuttings separately. Positive net photosynthesis did not occur in hardwood cuttings until foliation, by which time roots had formed in the easy species exhibit positive net photosynthesis from the time of planting. Consequently in P. euramericana net photosynthesis increased in the leafless hardwood cutting but decreased initially in the softwood with subsequent increase when roots formed. In P. tremula leadless hardwood cuttings, net photosynthesis increased when the leaves expanded, then decreased as shoots deteriorated initially in the leafy softwood cuttings with subsequent increase when roots formed. The carbohydrate content accordingly decreased in the hardwood cuttings but increased in the leafy softwood cuttings during the rooting period. The auxin and cytokinin content of the top halves of both species decreased independently throughout the rooting period with a slight increase of the latter in the end. The carbohydrate content did not change appreciably at the time of callus formation but thereafter decreased. In contrast, the cytokinin and carbohydrate content of the lower halves of both species decreased while the auxin content increase. This auxin increase was transient in P. euramericana, disappearing before the roots emerged from the protuberances, but persistent in P. tremula. The patterns of carbohydrate and cytokinin depletion were parallel over much of the rooting period in P. euramericana, but when wound-roots formed the patterns diverged, carbohydrate content becoming minimal while cytokinin content regained its original level. There was no such similarity in trends f carbohydrate and cytokinin levels in P. tremula, but like P. euramericana, cytokinin level increased when carbohydrate content was minimal. The influence of these changes ion the rooting process is discussed.