Carbohydrate trends in Opuntia imbricata (Haw.) D.C. (Cholla)
Kunst, Carlos Roberto Guillermo
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Opuntia imbricata (Haw.) D. C. ("cholla") is featured as a "grassland species" because it occurs in these vegetation types of the southwestern U. S. Its status as a range plant is somewhat contradictory because of its desirable and undesirable traits. Its main effect as a weed is mechanical interference with livestock grazing and handling. This study was conducted at Lovington, Lea County, New Mexico, to (a) evaluate total nonstructural carbohydrates (TNC) trends in new and old shoots, flowers/fruits and roots; (b) relate changes in TNC concentration of an organ to variations in TNC concentration of other organs and/or phenological and environmental events to establish TNC source-sink reiationships among the piant parts; and (c) utilize that information to define key stages so that foliar applied herbicides might be more effectively timed. Samples from the selected plant organs where taken monthly from May 1986 to November 1987. Phenological status of each plant, soil temperature and moisture were recorded each sampling date. TNC concentrations were analyzed using the acid hydrolysis with 0.2N HCl and determined spectrophotometrically. The most remarkable environmental difference between the two growing seasons sampled, May-August 1986 and May-November 1987, was the higher soil water content during the latter, mainly due to early spring rainfall. This climatic event triggered the development of numerous cohorts of reproductive and vegetative organs, which had a significant effect on the TNC dynamics of the plant. The interaction between plant parts and environmental parameters upon the TNC concentration of the organs sampled was significant (P<0.05). TNC concentrations in organs of O. imbricata were low during the first growing season, gradually increased during the following fall-winter and reached their maximum values just before the second growing season. In that season, flowers/fruits produced the highest TNC concentrations, while TNC values of aboveground vegetative organs were lower than those of reproductive organs, but higher than their respective levels of the previous growing season. Throughout the study, roots usually had the lowest TNC concentrations and the least amount of variability. Analysis of correlation was used to determine the source-sink relationships between plant organs and to define the appropriate timing for herbicide applications. In the long-term (entire study data), TNC source-sink relationships were apparently nonexistent or negligible. Organ linkages, evaluated through the significance of the partial coefficient of determination among TNC trends of plant parts in the short-term (seasonal data), were a seasonal event, and capable of being heavily influenced by the past and current environmental and physiological conditions of the plant. Judging by the magnitude of these determination coefficients, changes in TNC concentration in a particular organ that could be attributed to TNC concentration variations in another organ(s) were small compared with other possible sources of variation like phenological variations of the organ itself, age, diameter, position in the plant, etc. Since they represent the highest proportion of plant biomass and support the new vegetative and reproductive growth, old shoots were selected as "target organ" of 0. imbricata. The TNC stream should be directed toward the roots to select an appropriate time for herbicide application. In both fall seasons sampled, linkages between new shoots and the target organ were high, while the behavior of the TNC trends suggested downward TNC translocation. Therefore, the fall should be the appropriate season for control of O.imbricata with broadcast sprayings of foliar-applied herbicides. The presence of flowers/fruits was an important factor that altered the seasonal TNC dynamics and flux. Although they constituted a small part of the total biomass of the plants sampled, they maintained relatively higher TNC concentration levels and upward TNC trends during the second growing season. This shift in the importance of old shoots and roots as TNC sinks, especially in relation with reproductive organs when good weather conditions are encountered, could decrease the efficiency of the application even if the herbicide is applied in the appropriate season.