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Jack B. Fisher, Anders Lindström, and Thomas E. Marler

We made drill holes in the stems of six Cycas species and used a standard microtechnique and microscopy methods to determine the structural responses of the soft-wooded, parenchymatous (manoxylic) stems after 2 and 12 months of recovery. We also injected an aqueous stain to one plant per species to determine the transverse hydraulic pathways among the discrete tissue categories. Expanding secondary tissue decay and insect larva infestations were evident in some wounds after 2 months. Wounds that lacked the secondary complications had recovered as a result of wound periderm after 2 months. Large areas of secondary rotting after 2 months were bounded by wound periderm after 12 months. The wound periderm formed a thick, leathery phellem (cork) that was continuous across cortex, vascular, and pith tissues. The six species represent a range in ease of horticultural management, yet the form of recovery from the wounds was similar for all species. Mucilage exudation from cut surfaces was copious, and species differences in volume of mucilage were also not related to extent or form of recovery from the wounds. Stain injected into the hole moved longitudinally and laterally within a vascular cylinder to adjacent vascular cylinders and to the cortex by way of persistent leaf traces. Results indicate a transverse hydraulic connection among the concentric vascular cylinders and leaf traces that extend into the cortex. Moreover, the succulent nature of this manoxylic stem is highly susceptible to secondary infection whenever the protective bark is removed or the internal tissues are injured. Therefore, use of prophylactic treatments to minimize the risk of secondary complications is warranted whenever pruning or amputation of adventitious shoots for propagation remove the protective bark from the succulent cortex tissue.

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Thomas E. Marler, Anders Lindström, and Jack B. Fisher

Dimensions of pith, vascular tissue, cortex, live leaf bases, and periderm layers comprising the diameter of the stems of six Cycas species were measured at the standardized stem height where two vascular cylinders existed. The six species represent a range in susceptibility to injuries that occur in routine horticultural operations. We assigned a subjective numeric ranking from 1 for difficult to 10 for easy and then determined if this ranking correlated with any of the dimension characteristics. Pith diameter and cortex width differed among the species with the highly sensitive C. macrocarpa Griff. exhibiting the widest pith and most narrow cortex. Width of tissues peripheral to the vascular tissue (cortex, leaf base, and periderm layers) also differed among the species as did the proportion of total stem diameter occupied by these peripheral tissues. The sensitive C. macrocarpa exhibited the smallest values for these two variables. Simple correlation and multiple regression analyses indicated cortex width, total stem diameter, absolute width of peripheral tissues, and the relative proportion of these peripheral tissues in relation to stem diameter were positively correlated with susceptibility ranking. Of these, the relative proportion of peripheral tissues emerged as the variable with the most significant association with susceptibility ranking. Among these six representative species, the species that tend to be least susceptible to injuries during horticultural operations protect the youngest vascular tissues within a relatively wide zone of peripheral tissue. In contrast, the sensitive species exhibit a narrow zone of protective peripheral tissues.

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Seema Sah, Stewart Reed, Krishnaswamy Jayachandran, Christopher Dunn, and Jack B. Fisher

Since arbuscular–mycorrhizal (AM) fungi are aerobic, symbiosis was not considered significant under flooded conditions. However, AM colonization of wetland plants is now believed more common than previously thought. In the humid tropics, storms that result in standing water for 24 hours or less are common. Short-term floods, especially on sandy soils, may leach banded fertilizer, reducing uptake efficiency. Crops planted in flood prone areas are not normally enhanced with mycorrhizal mixes. However, mycorrhizal associations tolerant to wet conditions may improve nutrient uptake as plants recover from short-term flooding. Greenhouse studies were initiated to determine the effects of frequent short-term floods (two to four events) on mycorrhizal colonization and subsequent development in snap bean (Phaseolus vulgaris

Phaseolus vulgaris L.) plants. Flooding produced no obvious long-term physical effects on plant shoots. In the first study, flooding did not affect survival of colonies established before the first flood event. Percent root colonization in flooded vs. nonflooded treatments was not significantly different at either 31 or 50 days after planting (DAP). As root length increased there was a concomitant increase in colonization so that percent colonization remained approximately the same in both flooded and nonflooded treatments. In the second study, three weekly floods beginning 13 DAP (cotyledon leaf open only) did not inhibit initial mycorrhizal colonization. Mycorrhizal associations should form with snap bean under conditions subject to short-term flooding. Additional research is needed to determine the efficacy of different mycorrhizal mixes under short-term flooded conditions in the field.