The patterns of the IAA levels that developed in less than 10 days in the new shoots on the hanging and upright stem fragments may have resulted from at least two different factors. First, we hypothesize that the overall seasonal pattern resulted from the in situ temperature conditions at the time we sampled the main stem fragment. Secondly, the difference between IAA levels in the new shoots on the hanging and the upright stem fragments may have resulted from the effect of stem orientation on the inter- and intracellular distribution of plant growth regulators in the plant tissues. The near horizontal positioning of that part of the hanging stem where the side shoots were growing from the stem caused these side shoots to grow vertically, perpendicular to the direction of the original stem. This gravitropic response of the side shoots is the result of the different inter- and intracellular distribution of plant growth regulators that resulted from the horizontal orientation of the main stem. Rooting occurred for the intact meristems on both the intact stem fragments and those split lengthwise, with no significant difference in rooting percentages . Zero rooting or shoot growth occurred from meristems that had been cut. Therefore, only stem fragments without or with a damaged meristem will be totally harmless. Even stem fragments as small as a meristem can result in the establishment of a new plant, and through that, the establishment of a new stand/clone. Management personnel active in the removal of Arundo state that based on their observations,grow bucket regeneration from stem fragments is rare, compared to regeneration from rhizome fragments. However, new Arundo plants that had regenerated from stem fragments were observed in situ during this study .
Regeneration from Arundo stem fragments under a series of environmental conditions was also reported by . There may be two reasons for the limited observations of Arundo regeneration from stem fragments in the field. If most eradication efforts that introduce stem fragments into the environment occur in the months of October through February, regeneration from these stem fragments will be inherently low , and the ambient temperature will be low as well. There are limitations in place that prevent eradication activities during the breeding season of the endangered bird species that nest in the river basins that have been invaded by Arundo. The other reason is that after a growing season, it will be difficult to distinguish between Arundo plants that have grown from a stem fragment or from a rhizome fragment , because the new plant will have grown a substantial rhizome . When the a plant has regenerated from a rhizome fragment, not much of the original large rhizome fragment remains after it has supported the regeneration of a new Arundo plant. .Growth of the tissues with the highest CNC, the leaves, stopped earliest , but the leaves remained green. Their continued photosynthesis supplied the carbohydrates for the continued growth of the rhizomes , stems , and to a lesser extent that of the roots . Both below ground tissues, the rhizome and the roots, have a lower CNC level than the leaves. For those parts of the Arundo that is primarily responsible for growth, the leaves and the roots, their growth patterns mirrors their internal N:C ratio. As these tissues near their CNC, their sink strength for photosynthates, and therefore their growth is reduced. Both the leaves and roots have reached their CNC approximately since day 132, and their growth started tapering off since that time. If the experiment would have been continued longer, this would have shown better for the roots. After 60 days, the root masses of the individual A. donax plants could not be separated, and each plant was assigned a quarter of the root biomass, to show that overall root growth was tapering off near the end of the experiment.
The tissue with the lowest CNC, was not, as expected, the rhizome which had a CNC of 0.030 g N/g C, but the stem, for which the N:C ratio went as low as 0.013 . Unlike the leaves, roots, and rhizome, which reached their CNC after approximately 130 days of growth, the N:C ratio did not reach its lowest levels until day 245. The rhizome of Arundo donax act as a storage tissue for reserves. The reserves stored will support stem regrowth from meristems on the rhizome in the spring . In addition to the rhizomes, spring regrowth is also supported by the stem tissues. Unlike the common reed, Phragmites australis, new side shoots grow from the upper section of Arundo stems in the spring . Both tissues are originally stem tissues, and both play a role in the spring regrowth of the Arundo plant. Both the Arundo tissues that support spring regrowth, stems and rhizomes, have CNC levels below that of the leaves of Arundo.This resulted in significantly more stem growth than rhizome growth, with a final stem biomass of 1190 ± 95 g, and a final rhizome biomass of 171 ± 79 g after 334 days of development. For Ipomoea batatas these patterns were the opposite, because the CNC of the reserve storage tissue, the storage roots, was significantly lower than that of the stems . The CNC of both tissues was lower than that of the Ipomoea leaves. The N:C ratio of the leaves was 0.045 ± 0.0014, of the stem 0.017 ± 0.000006, and that of the storage roots was 0.013 ± 0.0011. For this species, as for Arundo, the biomass of the tissue with the lowest CNC, the storage roots, was significantly higher at 181.8 ± 23.8 g DW, than that of the tissue with a higher CNC, the stems, which reached 28.9 ± 7.5 g DW, when the plants had matured.The results of this experiment indicate that the leaf N:C ratio and their CNC can be used as an indicator for good timing of systemic herbicide application, such as that of the glyphosate based Rodeo©. When the N:C ratio of the leaves has been reduced to their CNC, the only major tissues that initially continue growth are the roots, the rhizomes and the stem. The applied glyphosate will be transported to these tissues, with the flow of photosynthates in the phloem supplied by the photosynthetically active leaves. If the glyphosate accumulates in and kills the roots, this will kill the Arundo stand, because the uptake of soil water and nutrients will not be possible any more.
This is not the most likely scenario, because root growth being reduced as they reach their own CNC. After root growth has tapered off, the roots can still function in the uptake of water and nutrients. The glyphosate will also be transported to those tissues that support spring regrowth, thestem and the rhizomes, because their growth continues after the leaves have reached their CNC. Based on physiological studies into the amount of reserves stored in the rhizomes of Arundo throughout the growing season, transport of photosynthates to and storage of reserves in the rhizomes, the major underground plant structure that will support regrowth if the above ground parts of the plant are killed or removed, starts near the end of July and continues through October/November . If this tissue accumulates enough glyphosate due to the use the correct herbicide concentrations and good timing of application, recovery should be reduced to a minimum.Soybean is an important biotech food, vegetable, and field crop that provides oil , protein , and carbohydrate to millions of people worldwide. Furthermore, soybean is a promising sustainable source of biofuels in North America, South America, and Europe . Zinc deficiency has been recognized globally as a major micro-nutrient stress that lowers crop yield and productivity around the world . Zn deficient soils occur in nearly 30% of the world’s arable lands. Selection and breeding of plant genotypes for Zn efficiency , defined as the ability of plants to maintain reasonable yield under Zn deficiency, is considered a sustainable approach to increase plant production on low Zn soils . Considerable differences in response to low Zn stress are known to exist among genotypes of bread wheat , rye, triticale , rice,dutch bucket for tomatoes , and common bean . Variations in shoot or leaf based parameters together with higher internal Zn utilization can be the principal factors in differential ZE in crop plants . Preliminary studies in common bean indicated that leaf physiological parameters such as leaf area are a useful criteria for ZE screening . Currently, there is little information regarding response of stomatal conductance to low Zn stress. Many earlier studies of low Zn stress focused on economically important cereal species. Few studies have been conducted in soybean, and fewer have tested hydroponics as a growing media. It has been shown that critical Zn deficiency level for soybean leaves was 15 µg g-1 . In a field study in Central Turkey, Zn deficient calcareous soils were shown to reduce yield and cause the development of visual symptoms on young leaves of soybean plants . Many soybean genotypes are being developed in the U.S. but little is known about their reaction to low Zn stress. Therefore, the objectives of this study were to: develop a suitable hydroponics-based method for ZE screening of soybean plants to identify more Zn efficient and less Zn efficient genotypes; and detect genotypic ZE variation in soybean using physiological parameters such as leaf area, chlorophyll contents, stomatal conductance, nutrient concentration, and plant biomass. Available Zn concentrations around 1 to 2 pM has already been shown to induce Zn deficiency in bread wheat and common beans .
Accordingly, our experiments successfully induced Zn deficiency at this concentration level in hydroponics. Based on our results, it appears that hydroponics with chelate buffers is feasible for screening soybean ZE trait. The soybean genotypes tested in this study had considerable variability and physiological responses to low Zn stress in hydroponics. Total leaf area, chlorophyll content, and leaf Zn concentration levels were all high in MZE genotypes. At the same time LZE soybean genotypes had various visible symptoms which indicated unfavorable Zn levels. This is consistent with previous findings that soybean plants showed chlorosis and brown leaf patches in calcareous soils . In terms of overall assessment genotypes “Williams” and “Hampton” were the most Zn efficient and inefficient, respectively . Although chlorosis is the most prominent symptom of low Zn stress, there is limited info on the effect of Zn deficiency on chlorophyll content levels. Leaf chlorophyll content was greater for MZE genotypes such as “Williams” and “Pella86” compared with LZE genotypes. Our results suggest that increased chlorosis was the cause of reduced SPAD levels. This is in agreement with the previous findings on wheat and common beans . The variability in both stomatal conductance and shoot Fe concentration was considerably large . It is interesting to note that Fe concentrations were considerably high for some genotypes such as “Thomas” . The lack of correlation with ZE trait across the genotypes tested may indicate that stomatal conductance could not be used for early detection of Zn stress in soybean. Significant differences between soybean genotypes in shoot Zn and N concentration were observed in low-Zn grown plants in hydroponics. Although there was no significant correlation between shoot nutrient concentration and ZE trait, LZE genotypes were characterized by slightly lower concentration of Zn, Fe, and N . This data are in agreement with previous findings showing that Zn efficient wheat varieties transported more Zn from roots to shoots than Zn inefficient varieties under Zn deficiency in the early field growth stages in bread wheat .As sessile organisms, plants are presented with numerous biotic challenges such as herbivory and pathogen attack. Plants initiate responses to these challenges by harnessing tightly regulated phytohormone networks. Salicylic acid levels increase in plants following pathogen infection and SA is critical for the development of systemic acquired resistance . There are two enzymatic pathways for the generation of SA: one via phenylalanine ammonia lyase and the other via isochorismate synthase . In tomato , Arabidopsis and Nicotiana benthamiana, most pathogen-induced SA appears to be synthesized via the ICS pathway .