The neutral form of CBD and THC were extracted fast at the beginning of the extraction,while the CBDA and THCA were found in higher concentration in the late subsequent samples suggesting their lower solubilities.The fractionation of the extracts into two separators was investigated by changing the pressure of 1st separator on the example of Sample 3.The 2nd separator was kept at 4 MPa,while the pressure and temperature of extractor set the same in all experiments.The effect of separator pressure on the fractionation of each compound-family in shown in Figs.5 and 6.Unfortunately,the separator pressure had no effect on the separation of PCs from NPCs.The cannabinoids were collected mainly in the 1st separator at lower pressures or mainly in the second separator when the pressure of the 1st separator was higher.Phytocannabinoids were however efficiently separated from the highly scented fraction of hemp operating the 1st separator at lower pressure and 40 ◦C temperature.However,not only the pressure of the separator,but the combination of the extraction pressure,the pressure of first separator and the temperature of the first separator might have synergetic effects on the separation.Their combined effect on the total content of unwanted components was evaluated by multiple linear regression using best subset method.By residual analysis it was verified that the error terms were independent,identically and normally distributed thus the OLS method was applicable.UW was defined as the sum of the yield of CBD and CBDA in the first separator the yield of the non-CBDs in the second separator.As the experiments were conducted on different types of samples,its potential effect was also investigated.The effect of PS1 and the “sample” were found to be significant at 5% significance level,while a significant effect of TS1 and Pext cannot be detected.In fibre cannabis grow tray two types of bast fibres occur,primary and secondary fibres.The classification is made according to their origin.
The cell walls of both types are enforced with layers of cellulose,and both types are organised in bundles.In a cross‒section of a hemp stem,the outer fibre bundle layer consists of primary fibres,the inner layer,if present,of secondary fibres.For spinning high–quality textile hemp yarns only the primary or ‘long’ fibres are valuable.Secondary or ‘short’ fibres are unwanted because these fibres are too short for spinning and their presence hampers the production of fine and homogeneous yarns from the primary or long fibres.Kundu stated that the presence of a few layers of parenchyma cells between the primary and secondary fibre bundles enables the isolation of the primary fibre bundles by a retting process,but although secondary fibres can easily be distinguished under a microscope,and methods are available to isolate these short fibres in the laboratory,it is technically difficult to separate them from the primary fibres during commercial fibre processing.For this reason it should be known how the development of secondary fibres above stubble height can be avoided in the raw materials aimed at textile yarn production.Cells are initiated before or during stem elongation and fibre cells mainly grow with the surrounding tissue; they elongate when stems elongate.While the neighbouring cells continue to divide,the multi-nuclear primary fibre cells can reach a length of 5–55 mm and a width of 10–60 μm before cell division occurs.The individual or elementary primary fibre cells are held together in bundles by binding substances that mainly consist of hemicelluloses,lignin,and pectins.The long fibres of hemp,desired for textile processing,are collectives of such primary bast fibre bundles.The high length–to–- diameter ratio of the cells,between 250–1000,and the cell and bundle architecture make these fibres fit to be spun into yarns,and then woven or knitted into fabrics.
To introduce hemp long fibres into the fashion textile sector,fibres should be produced allowing the spinning of yarns between Nm 20 and Nm 40,in which Nm is the Number metric,the yarn length in meters per 1 g of mass.The finer the yarns that can be spun,the higher the value of the raw material is.Primary fibre bundles are already present in very young hemp stems.They run longitudinally along the stem from bottom to top and can reach almost the full length of the plants.These fibres have to be strong enough before the bundles can be extracted.This so–called ‘ripeness’ or maturity of the primary fibres is closely connected with the cellulose filling degree of the cells,which progresses from bottom to top,and from the outer to the inner part of the stem.At maturity the lumen of the tapering fibre cells is very small and protoplasm is absent.Hemp for textile use is usually harvested around the time of flowering,when the primary fibres are filled and stem dry matter yield and fibre yield are highest.Secondary fibres might derive from tangential division of vascular cambium cells when a stem part has reached its maximum length.These uninucleate fibre cells are no longer than 2 mm,which is too short for spinning.Besides,secondary fibres contain too much lignin,which is detrimental for the production of fine,flexible,and homogeneous yarns.Secondary fibres are absent in young hemp plants or only present in a thin layer at the stem base.However,around the usual harvest time,hence around flowering,secondary fibres are found higher up along the stems,with a thicker layer towards the bottom part of the stem.Mediavilla et al. stated that with the induction of the generative phase secondary fibre formation increases quickly,at first in female plants.Schäfer and Honermeier also related the observed increased secondary fibre formation and thicker secondary fibre layers to a phenological stage of the plant: the beginning of flowering.However,the observed coincidence of enhanced secondary fibre formation with the transition from the vegetative to the generative growing stage of the plants does not necessarily point at a causal relationship between the two.Although it cannot be excluded yet that flowering accelerates the process of secondary fibre formation,it seems more likely that the size of the plant determines the amount of secondary fibres.Botanically the bast fibres in hemp belong to the sclerenchyma tissue which gives mechanical support to the plants and the need for such support increases when plants grow taller and when tops become heavier,due to the development of inflorescences and the filling of the seeds.
It could be considered an example of ‘mechanoperception’,the perception of mechanical stimuli that keep plants in balance with their physical environment.The findings of Amaducci et al.,Bredemann et al.,Höppner et al.,and Van der Werf et al. that increased amounts of secondary fibres were present in plants with higher stem dry weight and in lower internodes could support our view.However,secondary fibre formation was not related to phenological stage in these experiments,hence flowering as an accelerator of secondary fibre formation cannot be excluded.Flowering as the exclusive trigger to secondary fibre formation must be excluded as secondary fibres are also observed in non–flowering hemp.It can be hypothesised that either the change from the vegetative to the generative phase accelerates the formation of secondary fibres higher up along the stems or that the increasing height and weight of the plant,which obviously are strongly correlated,cause the formation of secondary fibres.To avoid the presence of the unwanted secondary fibres above stubble height and to optimise the production of uncontaminated valuable primary fibres,we need to know whether hemp should be harvested before flowering or before the plants become too tall.Therefore,the background of the development of secondary fibre formation during the growing cycle should be elucidated.To discriminate between flowering and plant size as the cause of enhanced secondary fibre formation,a broad size range of flowering and non–flowering plants is required.Such a test set cannot easily be achieved in a field experiment,as flowering plants grown under natural climate and day‒length conditions at higher latitudes in general are relatively tall at the end of the growing season.However,in a greenhouse with day‒length control it is rather simple to disconnect phenological stage and plant size.The reason is that hemp is a short‒day plant and can be kept vegetative and growing for a prolonged period of time under long day conditions,while a transition to short‒days triggers the plant to flower within days.Around this transition,the longitudinal growth of the plant slows down or stops,hence by transferring plants from a long–day compartment to a short–day compartment at different moments in time,the desired test set with flowering and non‒flowering plants of different sizes can be achieved.
Harvesting plants from different treatments and microscopic analyses from stem cross–sections regarding the presence or absence of secondary fibres then provides the opportunity to find out whether or not flowering and secondary fibre formation are related.The results of an earlier conducted field experiment were analysed for comparison.The aim of the day–length treatments thus is simply and solely to obtain a size‒range of flowering and non‒flowering plants.Microscopic measurements on plants of this test set were not related to the day–- length treatments the plants underwent,but only to the result of the treatments: the size characteristics of the plants and their phenological stage.As expected,a size range of flowering and non‒flowering plants could be created with different day–length treatments and harvest times.Short days triggered flowering,vertical grow systems for sale while plants remained shorter.Long days kept the plants vegetative and extending in height.Successive transfer treatments produced flowering plants of increasing height and weight,providing a more–or–less continuous data set for both variables.Almost the full plant height and weight ranges were covered by flowering as well as non‒flowering plants.With increasing thermal time,the height of the secondary fibre front became increasingly variable for both flowering and non–flowering plants hence thermal time as such did not explain the height up to which secondary fibres were present in fibre hemp stems.In the field experiment,flowering plants showed much higher secondary fibre fronts than non–flowering plants.Flowering plants,however,were consistently taller as well.The disentanglement of plant size and phenological stage in the greenhouse experiment revealed that the height up to which secondary fibres are present in fibre hemp stems increases with increasing plant weight and height.During early sampling very often no secondary fibres were observed at 10 cm height or higher,comparable to what is shown in Fig.5 for the field experiment.These samples were not included in the regression analyses as they would lead to non–linearity in the relations.Although plant weight explains the height of the secondary fibrefront in the simplest way with a single regression line that is valid for both flowering and non–flowering plants,providing a biologically nice and simple model,for practical reasons the relation with plant height is more important because primary producers need non–destructive means to know at which height the crop should be harvested to avoid that the unwanted secondary fibres contaminate the valuable primary fibres.This is more complicated as the regression lines for flowering and non–flowering plants are different.
The secondary fibre front is found a little higher in flowering plants when compared to non–flowering plants of the same height.This must be due to the higher weight or momentum of flowering plants as compared to non–flowering plants of the same height.Consequently,to maximise the length of the stem parts fit for the production of high–quality hemp yarns the crop should be harvested before flowering and before it becomes too tall.Increasing plant weight as a trigger to secondary fibre growth,a form of mechanoperception,was also found in Arabidopsis where auxins were identified as the downstream signal transducer.Although the change from the vegetative to the generative growing phase as such does not enhance secondary fibre formation higher up along the stems,it can be understood why these phenomena under field conditions often occur roughly at the same time and therefore seemed to be related.In Fig.6 our conceptual model is drawn.During the growing season plant height and stem weight increase and consequently the secondary fibre front moves upwards.Possibly the force that is exerted by the stem part above the secondary fibre front is rather constant,as it seems likely that passing a certain threshold value triggers the development of secondary fibres to resist an increased force.At the transition from the vegetative to the generative phase length growth slows down and the weight of the top of the plant increases due to the development of the inflorescence and the subsequent filling of the seeds.An accelerated development of secondary fibres as compared to the longitudinal growth,but keeping pace with the increasing weight is hypothesised.Consequently,the length of the stem part that is valuable for high–quality yarn spinning becomes shorter from around flowering onwards.