The sprouted stem fragments rested on a floating plastic mesh supported by a ring of plastic pipe, on the surface of each trash can’s nutrient solution. A sheet of opaque white plastic was wrapped around and over each trash can to block out sunlight preventing algae growth and high temperatures in the nutrient solution. The nutrient solutions in the containers were monitored two times per week during a 48-week growth period. Each check consisted of the following: addition of enough deionized water to bring the can’s nutrient solution level up to a 100-L mark, determination of the can’s electrical conductivity in full volume. A concentrated Hoagland solution was added to re-establish the conductivity of the nutrient solution to its original value. The pH was adjusted to 5.7. The harvest dates were partially determined by the growth of the plants as the experiment progressed. Harvested plants were separated into apical meristems, green leaf blades, brown leaf blades , green leaf sheaths, brown leaf sheaths , stems, rhizomes, and roots. Plant parts were dried to a constant weight at 60 C. Biomass of the tissue was determined and sub-samples were ground in a Wiley mill to pass a 0.5 mm mesh screen . The nitrogen and carbon contents of the tissues were determined using an organic elemental analyzer . Stem fragments with meristems can root and regenerate new Arundo plants , as has been reported earlier by . There were significant patterns in rooting success of meristems on Arundo stems throughout the growing season. In the winter months of November through January, rooting is low, and success percentage lies below 20%, with the exception of 28 ± 12% rooting for meristems from hanging stems in January . Nearly all meristems rooted from March through September. The speed with which meristems rooted showed a related pattern through the growing season . In the period with the lowest rooting success, t50 had the highest values,dutch buckets system indicating the slowest rooting.
Rooting was most rapid in the months of May through July, a time window that was more narrow that the period in which rooting is most successful.There are no significant differences between the results in plain water and half-Hoagland nutrient solution . The rooting success and speed pattern is similar in soil, but the single replicate meristems do not allow for inclusion in the two-way ANOVA. When compared within each sampling, the rooting success of meristems from hanging stems was significantly higher than that of meristems from upright stems . When split for rooting medium, there was no significant different in rooting success between hanging and upright meristems over time in plain water , but the differences remain significant in the nutrient solution and in soil . Like with rooting success, rooting speed of hanging meristems was significantly faster when compared to that of the upright meristems of the same sampling date . When separated among rooting media, the difference in the speed with which rooting occurs was most pronounced in plain water , but still exists in the nutrient solution and soil . Though these differences in rooting success and the speed of rooting may be statistically significant, they generally are too small to be ecologically significant. Stem diameter at the node where the meristem is placed is an indicator of relative height on the stem, and the age of the meristem. Within stems, there was no relation of rooting success or speed with the diameter of the stem at the point of the meristem, so the older meristems on an Arundo stem do not root better or faster than the younger meristems on Arundo stems.When the temperature of the rooting environment was controlled at 28/15 °C for 14/10 h during the entire growing season the seasonal patterns of rooting success and speed remained, and differences between the seasonal rooting patterns of the fragments from hanging and upright stems emerged . The overall patterns differed slightly from those in the greenhouse experiment, with the lowest rooting by both stem type fragments in February through March. The rooting percentages increased in April, and the highest rooting rates occurred from July through September at 80 – 92% for both stem types.
In October the rooting of the upright stem fragments decreased more than that of those from hanging stems. The lowest rooting rates of the stem fragments from upright stems were 0 – 10% in February and March, while the rooting rates of the hanging stem fragments only decreased to 30% . The positive influence of the seasonal effect in the months of July – September on the rooting rates of both stem types masked the difference that emerged in the Winter and Spring months. The rooting by meristems from upright stems benefits more from this seasonal effect that of meristems from hanging stems. The seasonal effect on the rooting rates of the stem types could be related to a number of environmental factors that change during the growing season, such as temperature, light intensity, and daylength. determined that stem fragments sampled at the same time, but stored at different temperatures, displayed different sprouting percentages when potted and regenerated at the same temperature in a single greenhouse. We hypothesize that the different ambient temperatures prior to sampling in our experiment was an ecophysiological equivalent of the experimental factor “storage temperature” in the Boose and Holt study, and one of the factors involved in the seasonal pattern of A. donax stem fragment rooting.The seasonal differences in the rooting percentages and speed between meristems from upright and hanging stems that was striking under controlled temperature conditions had been much less pronounced in the greenhouse rooting experiment. The results of the greenhouse rooting experiment show that the temperature at the time of rooting influence the effects of the seasonal factor. Environmental effects, such as temperature and inundation, are known to affect the success of invasive plants with either negatively or positively In the greenhouse experiment, the temperatures of the rooting medium varied with the ambient temperatures and solar irradiation, while the temperature of the rooting environment in the growth chamber experiment was the same throughout the growing season. The greenhouse, the temperature of the rooting media in the winter ranged from 0.5 – 2 °C at night to 19 –21 °C during the day. In the spring and summer, solar irradiation increased these temperatures to 16 -18 and 28 – 34 °C, respectively. To test the effect of the temperature at rooting, we tested the rooting of fragments of hanging A. donax stems at different temperatures in April and May, a period that in the year-round temperature controlled experiment the success rates were 45 ± 10% in 1998, and 45 ± 21% in 1999. In this test using constant temperatures, no rooting occurred at 10 °C during the 40 days of the experiment .
At 15 °C, rooting was better than at 10 °C , but significantly less than at 17.5, 20, and 22.5 °C . In the greenhouse experiment, the seasonal pattern of rooting success was present, but the inherent advantage of fragments of the hanging stems in the winter months was masked by the negative effect of the lower temperatures of the rooting media. The temperatures chosen for the year-round temperature controlled experiment were selected to reflect the temperature conditions in the habitats invaded by A. donax in Southern California in the months of April and May. From the results of this April constant temperature experiment, it appears that the lower night temperature in the 28/15 °C for 14/10 h experiment led to a reduction in rooting success from the maximum possible in that month. This reemphasizes the effect of in situ temperature on the success of stem fragment meristem rooting, and the ecological danger of the floating stem fragments in shallow waters. The inherent seasonal pattern observed in the year round temperature controlled experiment may have resulted from cycles in the concentrations of the plant growth regulators that play a role in the growth of the side shoots, and the apical dominance of the top of the main stems. One of the growth regulators that plays a major role in the regulation of apical dominance is indole-3-acetic acid. The effect of IAA on the rooting of axillary bud on A. donax stem fragments throughout the growing season was tested through the use of exogenous IAA in the rooting medium,dutch buckets and the determination of endogenous IAA levels in the shoots that grew from the axillary buds. When the stem fragments and their axillary buds were exposed to 5 and 10 µM IAA in the rooting medium, the difference between the hanging and the upright stems disappeared. The main effect of the exogenous IAA was a significant improvement of the rooting percentage and speed of the upright stem fragments in the winter and spring periods, so that the difference between the two stem types was minimized. The exogenous IAA had little effect on the rooting success and speed of the hanging stem fragments . At 20 µM exogenous IAA, the highest concentration applied, the success rate and the speed of upright stem fragment rooting decreased from the optimum observed at 5 and 10 µM, almost down to the percentages and t50 observed in the absence of the hormone . The IAA in the rooting medium may have reached the axillary bud through the vascular bundles of the main stem piece, directly through the cuticle of the bud itself, which was positioned immediately below the rooting medium surface, or both. In early studies into the effect of IAA onplant growth, the growth regulator was sometimes applied to the leaf tissues, and the position of the axillary bud in the rooting medium could have resulted in a similar situation. There is a striking similarity between the seasonal pattern of the endogenous IAA levels in the shoots that grow from the hanging and upright stem fragments and their seasonal rooting patterns. The lowest IAA levels for both stem types occurred in the spring, and the highest levels in late summer . At the time that the endogenous IAA levels are low, levels in the shoots from the upright stem fragments are significantly lower than in those from the hanging stems. As the IAA levels in the shoots increase with the progression of the growing season, the levels in the shoots from the upright stem fragments increase more than in those from the hanging stems, and the difference between the shoots of the two stem types disappears . The role of this IAA is unclear. Although studies have also shown more complicated mechanisms , IAA produced in the main stem apex plays an important role in the apical dominance, and the growth of side shoots. A difference between the rooting and the endogenous IAA patterns is that the seasonal pattern of endogenous IAA concentrations in the new shoots runs approximately one month behind that of the rooting success. This makes it less likely that the IAA concentrations in these new shoots, of which the growth is initiated prior to root growth, is the direct cause of the rooting of the stem fragments. It is possible that both the rooting pattern and the endogenous IAA pattern result from seasonal variation in another factor, possibly the concentrations of another plant growth regulator. Preliminary enzyme kinetics results have shown higher NADP-dependent indole- 3-acetatealdehyde oxidase activity in the new shoots that developed from the hanging stems in April than in those from the upright stems . This indicates that the IAA measured in the new shoots may be the product of de novo synthesis in these shoots, rather than a trace of the IAA that may have been stored in the main stem fragment that was used in the experiment. IAA plays an important role in tracheary cell differentiation and xylem regeneration . The support for the formation of the vascular system by IAA and its promotion of the formation of adventitious roots may be related, but no causal relationship was determined in this study.