Physiological Significance of Tuber and Bulb Formation in Plants

In this article we will discuss about the physiological significances of tuber and bulb formation in plants.

The formation of tubers as the stem and bulb swellings of leaf base is governed mostly by the mobilization activities resulting in the storage of carbohydrates and fats thereby degrading the polarity in such organs.

Most of the induction for mobilization starts in the leaves which later on is transmitted to the swelling organs and is analogous to the flowering and fruiting activities of plants. Morphological differentiation begins to form tuber and bulb like fruits which will pass on to the ripening process. The storage organs may be tubers, bulbs, corms and enlarged basal regions of the stem or an enlargement of the root itself.

Potato tuber development is initiated by the lateral enlargement of the cortical cells lying just behind the stolon followed by the mitotic activity in the cortical cells which continue till the last aspect of tuber growth is completed.

The tuber development of potato stolon retains the nodes and internodes. The initiation of tuber growth takes place normally at the tip of stolons (which are the ageotropic stems) by the lack of polar elongation associated with recumbent stem growth.

The so called “eyes” on tubers are the nodes with a leaf bract or the scar and the subtended bud. The onion bulbs are initiated in the younger leaves as a consequence of the mobilization of carbohydrates at their base.

Hearth and Holdsworth (1948), suggested that the bulb formation was associated with the cessation of apical meristem and root growth with the cessation of cell division initiating lateral swelling of younger leaves.

The localized lateral enlargement of cells associated with the tuber formation relates to the storage of polysaccharides, mostly the starch and only sometimes inulin. Therefore, the tuber formation and the starch deposition are the two dependent processes. At the time of the sprouting of bulbs the young leaves enclosed inside the bud scales start elongating with the restoration of the mitotic activity in them.

The bud scales after enlargement deteriorate by yielding their stored carbohydrates. Starch deposition is the primary controlling factor in tuber formation; once it is initiated, starch biosynthesis and the mode of translocation of principal carbohydrates to the site of synthesis are some of the important points which need thorough discussion.

It is known that the sucrose translocated to the tuber is finally converted into starch. The most logical turnover of sucrose to starch is through the formation of glucose nucleotides. The sucrose is then converted to uridine diphosphate (UDP) glucose which is thought to be the immediate precursor of starch and the reaction is catalyzed by sucrose synthetase.

UDP glucose then gets incorporated to the end of polysaccharide chain as catalyzed by the enzyme amylose synthetase:

I. Sucrose + Uridine diphosphate + ATPD Uridine dipsosphate glucose + fructose + ADP

II. UDP glucose + (glucose)_n” UDP + (glucose)_n+1

Fructose so formed in I-reaction also gets converted to UDP glucose by following its phosphorylation with ATP, first getting converted into glucose 1-phosphate as catalyzed by hexokinase and which finally gets converted to UDP glucose through the utilization of one more ATP.

This provides another glucose unit for the synthesis of starch. It has been proposed that for the conversion of one molecule of sucrose into two amylose links in starch grains two high-energy phosphate bonds are required as given in the reaction.

Sucrose + UDP + 2ATP” 2 amylose links + UDP + 2ADP + 2Pi

Besides amylose synthetase enzyme there are two more enzymes which have an important role to play in starch synthesis e.g., in potato there is a starter enzyme called D enzyme. This enzyme helps in making some primer from the simple polysaccharides e.g. maltotriose. Q enzyme has been reported to catalyze the branching of amylose chain.

It is reported that when tuber formation starts in potato there is high accumulation of starch systematically in leaves, stems and stolons. It is pointed out that the tuber formation is in response to sugar accumulation in plants which occurs under CO₂ pressure and light from outsides.

Besides the starch accumulation in relation to tuber formation, it is known that with the initiation of storage organs like, tubers, bulbs, corms and enlarged stems, the elongation growth is suppressed likewise with the bulb formation in onion and there occurs the suppression of growth of the above ground parts.

The tuber growth is known to be promoted by the application of growth hormones e.g., cytokinin, ethylene, abscisic acid and certain growth retardants. Besides this, environmental factors like phatoperiods, light and temperature also regulate the growth of tubers and other storage organs. The tuberization stimulus is probably sugar itself.

Tuberization does not occur under high sucrose concentration in the absence of a proper photoperiodic exposure, instead it helped in the stem growth. Besides sugar other also control the tuberization. All the plant hormones are thought to play a role in regulation of tuber and bulb formation though some of them may be promotoiy and still others have inhibitory effects on the formation of storage organs.

Tuber growth progress is shown to follow an exponential curve with the great bulk of tuber-filling activity occurring near the end of growth season. The changes associated with the ripening of tubers are very common in nature to those of the ripening of the fleshy fruits.

There is a marked decrease in the sugar content by an increase in starch content during ripening. Climcteric rise has not been reported in ripening tubers and respiration remains alomost steady. The tuber ripening is normally associated with the active senescence of plants.

In Hyacinthus orientalis the propagation is affected by scooping of the basal plate of the bulb or cross-cutting when cuts are made across the base of the bulb. Further each cut is deep enough to pass through the basal plate and the growing point. The mother bulbs are kept at high temperature to produce adventitious bulblets.

This system is used to study the role of different growth regulators on bulblet differentiation. It is well known that one of the endogenous hormones of intact hyacinthus bulbs is ethylene. Some authors have studied the effect of ethral and morphactin on bulblet differentiation in scooped out hyacinthus bulbs.

It is also reported that wounding of bulbs strongly increased the peroxidase activity possibly due to increased wounding. The ethral treatment decreased bulblet growth and this could be due to optimal level of ethylene. Morphactin prevented root formation and caused formation of abnormal bulb shoots in regenerated plantlets. Perhaps morphactins interacted with auxin or cytokinins.

In potato, the apical bud of stolons at the time of tuber initiation loses its polarity and thus starts producing cells in the radial direction instead of elongating lengthwise. This change in polarity is also accompanied by the enlargement of the pith and cortical cells in radial direction. It is known why the apical bud behaves in this way.

By exposing the plants to variable temperatures, this behaviour of the bud can be induced. It has been suggested that the existence of a tuber inducing substance and the changed behaviour of the bud may be attributed to this factor.

When cycocel (CCC) is applied it induces early initiation and growth of tubers. Further, the application of GA resulted in the formation of elongated tubers in a variety bearing round tubers. GA apparently induced the cells of buds to elongate longitudinally to some extent.

The accumulation of fresh and dry matter in all varieties is markedly influenced by the agroclimatic conditions. Further the accumulation of fresh and dry matter was more in leaves and stem till the tuber enlargement phase started and thereafter it decreased. When the varieties growing in hills and plains were compared, highest weight in leaves and stem was recorded in hills and it was low in Eastern plains.

The tuber weight showed opposite trends. Further it is reported that tuber bulking was faster in plains than in hills in all varieties. It is concluded that all these varieties are more suitable for plains than for hills and are adapted to relatively shorter days as autumn crop.

In Colocasia the apical bud produces cells in the radial direction and thus produces tubers as in potato. Application of GA induces the apical bud to grow lengthwise and consequently elongated tubers are produced.

The studies given above pose some interesting questions: firstly, any factor(s) having an effect opposite to that of GA is involved in tuber development. Secondly, could it be anti-gibberellin which is produced endogenously in the developing tubers.

Sinha and his collaborators at IARI demonstrated that when cycocel (antigibberellin) was applied, it produced tubers in one Colocasia species which did not produce tubers otherwise.

The saffron plant (Crocus sativus L.) is an important cash crop of Jammu and Kashmir state and is grown over 3500 acres. The plant is sexually sterile and is propagated vegetatively by corms.

Development of the corms in the plant is completed by early May, followed by a summer dormancy which lasts till early September, when these corms sprout again. Crude extracts prepared from dormant corms possessed strong inhibitor activity as assessed by the cucumber seed germination test.

From aqueous extracts, the inhibitors could be effectively positioned into ethyl ether or ethyl acetate. The crude extracts were subjected to paper chromatography with isopropanol-ammonia-water (3:1:1) and it was possible to locate strong inhibitor activity on chromatograms between Rf 0.3 to 0.5. This inhibition of cucumber seed germination could be reversed by the addition of GA.

These characteristics indicated the occurrence of inhibitor-B complex in the dormant corms. The preliminary studies by the author have indicated that these inhibitors could be phenols or abscisic acid. The significance of these endogenous growth inhibitors in controlling the resting period of plant species has been studied in Gladiolus, Dioscorea and some other species.

The sprouting of partially stratified Dioscorea batatas bulbils was promoted by exogenous application of benzyl-adenine, cycocel, IAA and GA₃ suppressed it. GA₃ raised the content of batatasins but not that of ABA.

It was deduced that the sprouting-inhibiting actions of GA₃ and light were exerted by raising the batatasin level in bulbils. According to these workers batatasin may be the native dormancy-inducing principle in this plant.

Further, it is also assumed and similar explanation may be afforded for the aerial tubers of Begonia evansiana and winter buds of some woody plants including grape vine. As many as three (I, II, III) batatasins have been isolated from Dioscorea bulbils.

The instant sprouting of immature and half-dormant mature bulbils and rhizomes in some species of Dioscorea was promoted by treatment with inhibitors of protein synthesis, although the sprouting in the completely awake mature bulbils was inhibited.

GA₃ inhibition of the sprouting was releived by cycloheximide and some base and amino acid analogues. Further, when the bulbils were treated with cycloheximide there was a decrease in the polyphenol oxidase activity. Apparently dormancy was induced by some proteins.