PHOTO-SYNTHETICALLY ACTIVE RADIATION (PAR) EFFECTS ON BEAN PLANTS THAT DEVELOPED DUE TO RATION OF LIGHT : DARK (L:D) PERIODS (PHOTOPERIODS)
12th-December 2007

Abstract

Plants are bombarded by a myriad of signals, not just from their physical environment, but from friend and foe alike. As a consequence, they have evolved a remarkably sophisticated system of receptors and signal transduction pathways that generate appropriate responses, light plays a major signaling role in plant development. The plant’s ability to maximize its photosynthetic productivity depends on its capacity to sense, evaluate, and respond to light quality, quantity, and direction. Photo-synthetically active radiation (PAR) effects the general morphological development of bean plants with responses in leaf area, plant elongation (shoots and roots), and biomass accumulation measured as dry weight. The variables were assessed using Light : Dark (L : D) reception period ration, and the plants adjusted favorably to altered environmental illumination, in order to coincide or synchronize their growth to these conditions. The results show that the plant-growth tallied with the already existing hypothesis, ‘Plants synchronize to PAR with due changes in their morphological characters that is to say with changes in their photo-morphogenesis. The plants synchronized to altered PAR conditions and showed an reliably good pattern at growth conditions L : D = 60 out of 108 hours of PAR radiation per week. Although not statistically significant, the growth differences for continued growth variation of third and fourth week were not assessed due to time limitation.

1.0 Back ground
Research into photo-synthetic active radiation effect began more than 60 years ago when the existence of short-day and long-day plants was discovered (Borthwick., et al 1945). Its analytical stage started after botanical institutes had been equipped with climatic chambers where plants could be cultivated under controlled light and temperature programs. Plants have evolved to respond to light illumination differently using different receptors. A red, far-red reversible chromoprotein, phytochrome, were the first photoreceptor to having been identified. It is now known that multiple phytochromes (A, B, C, D, and E) exist and sometimes act independently of one another, sometimes redundantly, sometimes antagonistically, sometimes at the same time in development, and sometimes at different times. The first blue-light receptors to be identified were the two cryptochromes, and chromoproteins that mediate several responses, and more recently, another blue-light-absorbing chromoprotein accumulation (Briggs and Margaret. 2001) Photoperiodism was seen to involve synchronization of the photo-synthetic active radiation (PAR) and the rhythmical events in lifecycle of organisms. The major development activities of the plants were thus conclusively known as being affected by seasonal PAR changes, activities including among others diapauses, hibernation, cryptobiosis, sleep, migration, mating, dormancy, flowering, and germination. It was also discovered that it is not the day length (light period) that is decisive for stimulation of plant activities but the dark period, and hence a minimal light period was required for the production of enough assimilates. But the dark period alone can not exert both a stimulating and an inhibiting influence on general plant functions. In this study the effect of photoperiodism as the effect of more dark periods and light periods, was undertaken to define its effect on general bean plant growth relationships including leaf area, shoot and root elongation and biomass.

2.0 Introduction

Plants’ photo-responses to light include; photo-taxis, photo-morphogenesis, and photo-periodism. Photo-taxis (or phototropism) is a light-induced movement of organisms consisting of a single or just a few cells towards the light source. Plants show a positive photo-tactic growth towards a light, and this is very typical for multicellular plants though many chlorophyll-free, non-plant egg cell, plus the sporangiophore of the fungi Phycomyces also perform a similar response (Halliwell, 1981). Photo-morphogenesis is PAR light-induced activity of plant growth and differentiation. Certain wave lengths of visible light function as a signal causing the generation of information within the cell that is used for the selective activation of certain genes. Photoperiodism on the other hand is the ability of plants to measure the length of periods of light. Certain species (short-day plants) stop certain growth activities such as flowering as soon as the day length has attained a critical value, whereas other species (long-day plants) begin to flower only after such a value has been passed. With the onset of a light-period, plants begin a physiological activity called the photophilic (Light-loving) stage. After 9 -to-12 hours of light regime, development of plant being exposed is thought to be inhibited by all further exposure to light. After this stimulation the plant enters its skotophile, that’s to say its darkness-loving phase. The next photophilic phase begins just a few hours later, long before dawn and thus quite independent of the actual conditions of alternation of day and night. A reduced light program where the plant receives just a few hours of light every 168 hours of the total week duration, were hypothetically thought to be varying before the start of this experiment and the variations in bean study groups were to be synchronized to the alternation of longer skotophile and shorter photophilic phase all week round. But in natural sense growth and differentiation are dependent on light so do physiological activities of plant need a dark period as speculated in the hypothesis? Yes, during this period its thought that a pool of Phytochrome red (PR) is replenished. This pool does not grow unlimited but is broken down again into far red (PFR), during long periods of darkness so that the skotophile phase returns periodically. Phytochrome is a homodimer, (or two identical protein molecules) each conjugated to a light-absorbing molecule. Plants make 5 phytochromes: Phy-A, Phy-B, as well as C, D, and E, and there is some redundancy in function of the different phytochromes but there also seems to be functions that are unique to one or another. The phytochromes also differ in their absorption spectrum; that is to say, which wavelengths (e.g., red vs. far-red) they absorb best. phytochromes exist in two interconvertible forms; PR because it absorbs red (R; 660 nm) light, and PFR because it absorbs far red (FR; 730 nm) light. These have relationships in that, absorption of red light by PR converts it into PFR, and absorption of far red light by PFR converts it into PR, and in the dark, PFR spontaneously converts back to PR. Thus in description long-day plants (such as Nicotiana sylvestris), are long day + adequate PFR hence growth is accelerated in short skotophile phase, while short-day plants (such as Kalanchoe blossfeldiana)are, short day + enough PFR so growth is limited due to short the skotophile phase.
Beans just like any other flowering plant use the pigment phytochrome to sense seasonal changes in day length, but beans behave are known to be neutral plants because they were found to flower irrespective of photoperiodicity, they rather use PAR-induced temperature conditions (vernalization) to control the different growth activities, such as flowering, transpiration, and the generally plant growth. The behavior of phytochrome described above provides the study model — called the hourglass model — of the mechanism of photoperiodism in plants, but this model fails to account for the fact that night-time exposure even two hours extra of darkness when all the PFR has already been converted to PR affects plant function. Light : Dark ratio responses were studied by varying bean plants’ groups to 1, 2, 3, 4, 5, 6, 7, and 8, (or A-to-H), for 0 –to- 7 days of the week illumination ration respectively, after which treatment four (4) samples in each group were harvested and various growth variable changes assessed and these included among others; the number of leaves, the leaf area of lower leaves, the wet weight, and the dry weight and elongation of the shoot and roots in bean plants for two (2) consecutive weeks.

3.0 Objectives

3.1 Major objective
Study photo-synthetically active radiation (PAR) effects in bean plants developed on basis of rationed of light : dark periods (photoperiods) of the two weeks’ growth period.

3.2 Specific objectives
 Grow model bean crops in eight different buckets that were subjected to different Light : Dark (L:D) ratio through the week.
 Weekly harvest of four plants from each bucket and then measuring the growth variables that were initially set to be number of leaves, shoot and root length, leaf area of the lowest leaves, wet weight of the plant, and the dry weight of the plant.
 Collect results in two (2) consecutive weeks, take average of each group treatment and then analyze them.

4.0 Justification

The research project findings were aimed in disclosing the influence of Photo-synthetically active radiation (PAR) responses of a well renowned photo-period neutral plant (Phaseolus vulgaris, Beans). These results will help to describe the cause and find a solution to early maturity, declining in productivity and food-quality yields, increased susceptibility to pests and infections, and poor adaptability of some breeds of beans in some areas, witnessed in tropical zone of the world today. These changes can be thought at as being determined by the entrained photo-period rhythms, which is due to a reputation global climatic change in temperature (global-warming). Global temperature increases are estimated at 0.6-0.7oC rise, this has caused a lockstep within the PAR cycle of day and night. The tropics have balanced exactly 12 hours for light and dark regimes of illumination, but with altered cycles of PAR-induced temperature, variations in plant modules of growth and general environmental impact on life is thought by many scientists to result into ‘environmental disease’, there fore much efforts like one sited in this work still need a hand resolve before the environmental problem worsen.

5.0 Methods and materials

The reliability of this study was mostly dependent on the basis of good, health, and homogeneous-ness of the bean planting material. The conditions of growth were standardized, such growth factors included; fertile loam soils, similar watering regimes, well screened sowing seeds, and equally aerated light and dark gardens. Neither fertilizer application, nor pesticides were required in any trial experiment. In the optimizations of the study, a few bean seeds were planted out in an open garden and the period they took to sprout was recorded. In the main study this period of sprouting was deducted from the experimental due life of the first week’s trial, meaning it was not accounted for as part of the light response in terms of duration of bean seedling growth. The main study plants were grown in well labeled buckets signifying the ration study conditions in weekly hours for Light : Dark = 0:168, 12:156, 24:144, 36:132, 48:120, 60:108, 72:96, and 84:84, named as conditions A, B, C, D, E, F, G, and H (or 1-to-8) respectively. In each study condition, the bean garden was arranged as shown in the diagram below.

A diagram showing the lay-out of the sample gardens in each of the eight labeled buckets (A-to-H)
Garden (bucket)
Plant

As shown in the diagram bean plants were grown out in two (2) rows thus each garden held to a minimum of eight plants, therefore a total population of 56 bean plants were assessed. Equal spacing was utilized to avoid any competitions for light and nutrients. For two weeks, the plants were treated to varying ratios of both light and dark, this was done by standing the gardens outside of the laboratory window for light regimes (during day), and for night regimes plants were transferred to a dark room for artificially manipulated darkness which were used to supplement normal nights for the dark regimes. A weekly harvest of the four plants in one row was made, washed of any soil on the roots, and the measured for the variables which were; number of leaves, shoot and root length, leaf area of the lowest leaves, wet weight of the plant, and the dry weight of each sample plant.

6.0 Results
Garden

Condition A
(1) B
(2) C
(3) D
(4) E
(5) F
(6) G
(7) H
(8)
Exposures
d=day, n= night 0d
14n 1d
13n 2d
12n 3d
11n 4d
10n 5d
9n 6d
8n 7d
7n
Weekly exposure rational (Hours) L=light, D=dark 0L
168D 12L
156D 24L
144D 36L
132D 48L
120D 60L
108D 72L
96D 84L
84D
Week 1
Number of leaves 2

(small open) 2

(small open) 2

(small open) 2

(big open) 2

(big open) 2

(large open) 2

(broad
open) 2

(broad
open)
Average length Roots 16.8 11.3 13.8 13.3 12.5 13.5 12.5 9.3
Shoot 35.5 26.5 22.0 24.5 24.3 20.3 22.8 19.5
Total 52.3 37.8 35.8 37.8 36.8 33.8 35.3 28.8
Lower leaf area (cm2) 7.7 14.1 24.1 31.6 30.3 38.1 47.5 50.2
Wet weight (g) 2.7 2.9 2.7 2.5 3.4 4.0 3.8 3.4
Dry weight (g) 0.15 0.20 0.22 0.28 0.30 0.45 0.40 0.35
Week 2
Number of leaves Wilted 2
(chlorosis)

2 + bud 5 5 + bud 5 + 3 young ones 8 + 2 buds 8 + 2 buds
Average length Roots 5.0 5.5 12.0 15.0 11.0 17.0 18.0 9.0
Shoot 39.0 24.5 21.0 26.5 28.5 33.0 25.0 23.0
Total 44.0 30.0 33.0 41.5 39.5 50.0 43.0 32.0
Lower leaf area (cm2) - 21.5 22.5 31.2 36.0 52.5 50.0 50.2
Wet weight (g) 2.3 2.6 3.3 4.2 3.6 4.3 4.0 3.8
Dry weight (g) 0.4 0.7 0.8 1.0 1.1 1.5 1.1 1.2

7.0 Discussion
7.1 Plant-elongation variable and PAR growth response in the examined bean plants.

Plants in groups 1 showed abnormal maximum elongation in the length because of elongation in the body cells, and also because these plants are positively photo-tactic they try to reach out in areas of maximal illumination. Such a characteristic is also shown by the forest twinning climbers and there implies climber plants actually act abnormally due to increased dark stimulation, but they have elongated roots which enable them collect water from below sub-surface level, as shown in figure 1.

Fig 1: Total plant (shoot + root) elongation within the two weeks

Other plant groups such as group 3, 4, and 5 show a positive correlation with the increase in illumination regimes signifying that plant elongation factor is directly proportional to duration of light reception and but plant length is affected negatively by increased period of dark stimulation there fore breeds of short stem beans should be planted in open gardens which must also be well watered due to short root length which can not collect water from very deep the soil. However there is a remarkable fall in plant length in groups 7 and 8, this can be explained as a direct effect of increased temperature on plant growth there fore this extreme of PAR stimulation lessens growth and thus will affect normal activities of the plants. In addition to increase in temperature, decay of roots, root excretion and consumption of roots by parasites and symbionts can have led to shortening of roots, and this directly translated in low net primary productivity (NPP).
7.2 Plant biomass-accumulation variable and PAR growth response by the bean plants.

Plant bio-mass accumulation (or NPP) was measured as the plant dry weight. And it was found to be positively proportional to the light illumination in the treatment condition. Therefore plants in groups 1-to-6 show a positive coloration with increase in light than with dark stimulation as shown in the figure 2.

Fig 2: Plant dry weight growth in the two weeks

But plant groups 7 and 8 show an abnormal negative response to increase in light treatment; this can due to increase in temperature conditions caused by the increase in PAR stimulation.

7.3 Plant lower-leaf area variable and PAR growth response by the bean plants.

Lower-leaf area growth is proportional to light stimulation, and there fore its affected by dark stimulation as shown in figure 3. Plants in lower illumination conditions 1, 2, and 3, the plants lost chlorophyll (or they developed chlorosis) and many leaves dropped off the plants. Low leaf area growth in 1-to-5 is an affecting factor in net primary productivity (NPP). Other factors that affect NPP are shading of leaves, and decay of some parts of the leaf.

Fig 3: Lower leaf area growth in the two weeks

8.0 Conclusion

Bean plants show maximum elongation and accumulation of biomass at L : D = 60:108 hours in a week, this condition was optimum for growth, but since they showed shorter root system, they there fore require a sub-optimal water supply. These conditions can help revive normal growth in plants especially those in a tropical countries such as Uganda. But these plants at the end of week 2 they had less development of root nodules than in their counterparts of groups 7 and 8. There fore such conditions if they are to be integrated national agriculture, efforts have to be taken to improve on their nitrogen fixing ability. Groups 6 and 7 showed maximum leaf area growth, there fore such plants can be grown for forage (or leaf harvest) so normal conditions of balanced L : D = 84:84 hours can be utilized. But since such conditions occur with minimized root length, it implies that adequate water supply should be availed since such plant will not fetch water from deep crust of earth.

9.0 Appendix

Fig 4: Root elongation growth within the two weeks

Fig 5: Shoot elongation growth within the two weeks

Shoot length growth influences total plant length than does the root length growth but still bean plants in conditions 6 and 7 show the normal patterns of elongation.