EFFECT OF IDGH TEMPERATURES ON MOISTURE DEPLETION, IMBIBITION AND GERMINATION OF SEEDS OF MIMOSA HAMATA WILLD.

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1 EFFECT OF IDGH TEMPERATURES ON MOISTURE DEPLETION, IMBIBITION AND GERMINATION OF SEEDS OF MIMOSA HAMATA WILLD. U. N. CHATTERJI AND ACHALA MUKHERJEE (nee' CHATTERJEE) Botany Department, Jodhpnr University, Jodhpur INTRODUCTION Crosier '(1956), Koller (1957), Koller and Roth (1963), Lahiri and Kharabanda (1964), Chatterji and Baxi (1966) tested the tolerance of seeds to high temperature and some of them indicated the beneficial effect of such treatment. Chatterji and Mohnot (1967) initiated the study on the influence of temperatures on germination of seeds of Mimosa hamata, a common weed of the gravely soil in the semi-arid zone. The work was carried out further to ascertain the tolerance of seeds exposed to very high temperatures prior to germination and the results are incorporated in the paper. MATERIALS AND METHODS The seeds of Mimosa hamata Willd. were collected from natural pasture land of the Research Farm maintained by the Central Arid Zone Research Institute at Pali during the month of February, The seeds were separated mechanically by hand and were divided into two groups. (i) Large (L), 5 mm. in diameter and weighing 4.05 gm. to 4.20 gm. per 100 seeds. seeds. (ii) Small (S), 3 mm. in diameter and weighing 2.05gm. to 2.35 gmper 100 The seeds of M. hamata besides having two protective layers, viz., testa and tegmen, possess one more coating in between the seedcoat and cotyledon. The layer is mucilaginous in nature, and the amount of ml.1cilageis about 13.5.percent by dry weight. Twenty seeds of each kind were weighed and kept in a ventilated oven previously adjusted to 70 C, 90 C, and 100 C for 24, 48, 96 and 144 hours for each case for dry heat treatment. The seeds were reweighed again after the treatments to determine the loss of moisture due to the heat treatments given. After dry heat treatment the seeds were kept for germination in ordinary laboratory condi-

2 U. N. CHATTERJI AND ACHALA MUKHERJEE tions at 30 C± 2 C. In each case three replications with 20 seeds each were taken. The process was repeated thrice and the data presented indicate the mean value of the three replications. The rate of imbibition and germination was noted, and the total imbibition and germination percentage were calculated after eight days after the start of the experiments. RESULTS Moisture depletion from the seeds due to dry heat treatments Table 1. Percentage of moisture depletion from large 'nd small seeds of Mimosa hamata due to dry heat treatments for various intervals of time Duration in hours L Percentage of moisture depletion 90 C L S L The maximum amount of moisture loss, i.e was ooserved when small seeds' were subjected to 100 C for 24 hours, and the minimum amount of moisture (2.83) was lost when the large seed were subject to 70 C for 24 hours The data thus indicated that the loss of moisture from the seeds treated for different intervals of time, viz., 24, 48, 96 and 144 hours with temperatures, varying from 70 C -100 C, was comparatively more in case of small seeds than in the case of large seeds. As regards variation in temperature there has been greater water loss at 100 C and comparatively less at 70 C; intermediate values were obtained at 90 C. The loss of moisture is less at 70 C in both large and small seeds after 24 hours as compared to that after 144 hours at the same temperature. But during the same interval of time when the temperature was raised, the loss of water increased; for example, the moisture depletion in large seeds increased from 2.83 percent to 3.54 percent and to 4.01 percent as the temperature was raised from 70 C to 90 n C and then to 100 C; Deviations from results reported above was also noted; for example, the loss of water (3.89 percent) was less when the large seeds were treated for 96 hours in comparison to that (3;94 percent) when the same kind of seeds were treated for 48 hours, the temperature in both the cases being 90 C. The seeds which indicated more moisture depletion germinated poorly in comparison to those which lost less amout olmoisture (Table 3). This might be due to some metabolic disturbances within the seed: Similar phenomenon was also noted in the case of small seeds:

3 EFFECT OF HIGH TEMPERATURES ON MOISTURE DEPLETION 95 Imbibition of water by seeds after dry heat treatments Table 2. Percentage of Imbibition of water in large and small seeds of Mimosa hamata after dry heat treatments Percentage of imbibition of water Duration 70 C. 90 C 100 C in hours. L S L S L S ~ OJ The percentage of imbibition was higher in the case of large seeds; it was always more than 63 percent in every case; whereas the value in the case of small seeds was percent. The imbibition percentage increased with the increase in degree and duration of temperature. The percentage of seeds which imbibed water was more after 144 hours as compared to that after 24 hours. A probable explanation might possibly be the duration of dry heat treatment might have some part in making impermeable seed coats permeable. Percentage of germination of seeds after dry heat treatments The seeds both large and small were examined for germination after they were given dry heat treatments. Table 3. Percentage of germination of large and small seeds of Mimosa hamata after dry heat treatments Percentage of germination Duration 70 C 90 C 100 C in hours. L S L S L S Nil Nil The maximum number of large and small seeds which germinated were 90 percent and percent respectively when they were~treated with dry heat for 144 hours at a temperature of 70 C. This temperature both in degree and duration may

4 96 U. N. CHATTERJI AND ACHALA MUKHERJEE be noted as the optimum tolerable temperature by the seeds without causing any injury to the embryos. The germination percentage decreased with the increase in temperature and duration of treatment. When the large and small seeds were treated for 24 hours at 90 C, the number of seeds which germinated were 50 percent and 61.6 percent respectively. The percentage of germination was further lowered to 3.30 and 30 percent respectively in the case of large and small seeds at the same temperature when the treatment continued for 144 hours. In the present study it was observed that both large and small seeds failed to genriinate when Hey were subjected to loo Cfor 144 hours. The results further indicated that the degree of tolerance to high temperature was more in small seeds than in large seeds. It was apparent, therefore, that the critical temperature to kill the embryos of Mimosa hamata varied from 90 C toloo C when the seeds were exposed from 96 to 144 hours. High temperature treatment and radicle growth During the course of this study it was also found that dry heat for longer periods at higher temperature affected the radicle growth. In the germinated seedlings of small seeds treated with dryheat at looocfor 96 hours, the radicle growth was very meagre even after eight days of g~rmination; the hypocotyl region elongated some what and cotyledons became wrjnkled. It was also noted that as a result of dry heat tretment at loo C,the overall protecting function of the seed coat was lost and as a result the whole embryo came out of the seed coat after imbibition of water by bursting it. It was also observed that the colour of the seed coat turned from brown to dark brown. DISCUSSION In laboratory conditions(temperature 30 C±2 C) the germination percen tage of freshly harvested seeds of Mimosa hamata was This was due to the fact that the seeds have hard and impermeable seed coats, a general characteristic of desert plants. The seed coat dormancy developed as the seeds matured, but after-ripening and storage gradully made the seeds permeable. Koller (1957), Koller and Roth (1963) observed the beneficial effect of desiccation prior to germination. In desert area, the seeds in their natural habitats, are often exposed to hot and dry environments before they can possibly germinate. Predrying thus constitute a sort of special adaptation prior to germination. Chatterji and Baxi (1966) bave also reported the beneficial effect of dry heat on the germination percentage of some leguminous seeds. Desiccation to tbe extent of 3 percent in tbe case of Mimosa hamata increased the germination percentage from 10 to 90; but further deisccaion indicated poor germination result. Tbis reduction in germination due

5 EFFECT OF HIGH TEMPERATURES ON MOISTURE DEPLETION 97. to greater desiccation seemed to be brought about by the direct effect produced on the embryo by exposure to higher temperatures for longer periods. In this study exposure of seeds to 70 C for 144 hours was found to be the optimum tolerable treatment prior to germination. Benzeevand Zemenhof (1962) reported survivalof some annual crop seeds when they were subjected to dryheat of 122 C to 138 C in a vacuum. Such treatment brought about delay and inhibition of germination. Some of the embryos in this case also tolerated dry heat treatment of 100 C for 24 hours, but tbe embryos lost their vaibility as the duration of treatment was increased. Crosier (1956) observed that some seeds could tolerate a temperature of 80 0e for 11 days, while Asparagus and cabbage seeds were severely injured by exposure to 70 0e for three days. Hence it may be concluded that desiccation which inactivates the self-regulatory mecbanismwithin the seed decreases the percentage of germination. In tbe present study when 3.94 precent of moisturewas lost due to dry heat treatment at 90 C for 48 hours. the germination of seeds - was found to be reduced to percent. This may be due to some metabolic breakdown within the seed. Henckel (1964) expressed opinion that severe overheating- caused hydrolysis of proteins to amino acids and produced ammonia in toxic amounts. At the same time polysaccharides were also hydrolysed. For this reason trealment with 100 C for longer durations was 'fatal' to the embroy of the seeds as has been found in the present study. Prolonged dry heat treatment also brought about some morphological change. The radicle growth became stunted. The possible reasons might be that the meristematic cells of the roots had been destroyed on account of exposure to high temperature. It was found that the small seeds were more resistant to dry heat than large ones. the The third mucilaginous protective layer in between seed coat and the cotyledons helped in moisture retention during imbibition. This might be possible due to one of the adaptations of the seeds of desert plants which have to withstand a high degree of desiccation in natural environments. The seed coat controls the water relations between the embryo and the environment. Water can enter the seed only if the seed coatis permeable. Dry heat treatment helped in breaking the seedcoat impermeability and made it permeable to water. Physiologically there did not appear to exist much difference between large and small seeds.

6 98 U. N. CHATTERJI AND ACHALA MUKHERJEE SUMMAR Y To study the effect of exposing seeds to high temperature on germination, the seeds of Mimosa hamata were subjected to temperatures of 70 C, 90 C and 100 C. for 24, 48, 96 and 144 hours. Treatment with 70 C for all these intervals of time promoted germination in comparison with unt reated controls. But the treatement of seeds with 100 C did not bring ahout any acceleration of germination; in fact exposure of seeds to this temperature for longer periods was seen to be fatal to the embryos. The treatment of the seeds with 90 C for all the intervals of time denoted intermediate between treatment with 70 C and 100 C results. There was a direct correlation between the rate of imbibition and the degree and duration of the temperature treatment. This indicated that imbibition was independent of tbe viability of the embryo; it might be probably a physical process related to colloidal particles found in the cells composing the seed. Some morphological changes also accompanied with the physiological changes; radicle growth was found to be stunted on account of exposure to high temperature. The third protective mucilaginous layer in between seedcoat and cotyledons might be helpful to the seed to avoid desiccation and render it capable of retaining moisture. There was no significant differnce in the behaviour of large and small seeds with regard to their capacities for imbibition and germination as a result of their exposure to dry heat treatment. REFERENCES Benzeev, N. and Zamenhof, S Effects of high temperature on dry seeds. Plant Physiology, 37 : Chatterji, U. N. and Baxi, D Ecophysiological studies on arid-zone plants. III. Effect of dry heat on germination of certain leguminous seeds. Proc. Ind. Sci. Congo III: 285. Chatterji, U. N. and Mohnot, K Ecophysiological studies on some aridzone plants I. Germination of Parkinsonia aculeata seeds. Proc. Arid Zone Symposium (In press). Chatterji, U.N. and Mohnot, K Thermo-Physiological investig,'lltions on the imbibition and germination of seeds of certain arid zone plants. I. The seeds of Mimosa hamata. Acta. Agron. Acad. Sci Hung Tomus. 16: Crosier, W Longevity of seeds exposed to dry heat. Froc. Assoc. Seed. Analysts, 46 :

7 " EFFECT OF HIGH TEMPERATURES ON MOISTURE DEPLETION 99 Henckel, P. A Physiology of plants under drought. In Ann. Rev. Plant Physiol. 15 : Koller, D Germination regulating mechanism in some, desert seeds; IV. Atriplex dirnorphoslegia Kar at Kit:. Ecology, 38 : Koller, D. and Roth, N Germination regulating mechanisms in some de~ert seeds. VII. Panicum iurgidu1n (Gnlminae). Isreal J. Bot. 12 : Lahiri, A. N. and Kharabanda, B. C Germination studies on arid zone plants III. Some factors influencing the germination of grass seeds. Proc. Rat. Sci. Ind. 30R ( 3, 4) _;".. '/ {: " ",';' ~;.-...,-,.~... ' ;.