PMC, Pollen mother cells. Source: Ratnambal (1979). Pollen Morphology

PMC, Pollen mother cells. Source: Ratnambal (1979). Pollen Morphology

The earlier investigators (Stone et al., 1979; Zavada, 1983; Dahlgren et al., 1985) were of the opinion that the pollen grains of the family are exineless, possessing a structurally complex intine (Hesse and Waha, 1982). However later studies indicated that in the majority of the Zingiberaceae an exinous layer does exist, although it is poorly developed in many taxa (Kress and Stone, 1982; Skvaria and Rowely, 1988; Chen, 1989). Recent palynological studies have demonstrated differences in pollen structure between sections of Zingiber. The Sect. Zingiber has spherical pollen grains with cerebroid sculpturing, whereas Sect. Cryptanthium has ellipsoid pollen grains with spirostriate sculpturing (Liang, 1988; Chen, 1989).

The pollen of Zingiberaceae is usually classified as inaperturate, but Zingiber is an exception. Some workers described Zingiber pollen as monosulcate (Zavada, 1983; Dahl-gren et al., 1985; Mangaly and Nair, 1990), whereas others reported the pollen as being inaperturate (Liang, 1988; Chen, 1989). Theilade et al. (1993) made a detailed study of pollen morphology and structure in 18 species of Zingiber. The pollen is spherical or ellipsoidal. The spherical pollen grains have a cerebroid or reticulate sculpturing. The grains are 55 to 85 ^m in diameter. The elliptical pollen grains (in Sect. Cryptanthium) have a spirostriate sculpturing. The grains are 110 to 135 by 60 to 75 ^m. The pollen grains have 2 to 3 ^m thick coherent exine. The intine consists of two layers, a 5 ^m thick outer layer and 2 to 3 ^m thick inner layer adjacent to the protoplast. The outer layer is radially striated; the inner layer has a distinct, minute fine structure. No apertures are present. It has been indicated that the entire wall functions as a potential germination site (Hesse and Waha, 1982; Kress and Stone, 1982). Nayar (1995) studied germinating pollen grains of 22 taxa in Zingiberales including Z. roseum and Z. zerumbet and reported that the pollen grains possess an exine containing sporopollenin. Inside this layer there is a well-defined lamellatted cellulosic layer (described as the outer layer of intine by earlier workers), which is the medine. The intine is membraneous and consists of cellulose and protein and is in fact the protoplasmic membrane. At germination a solitary pollen tube develops that has the protoplasmic membrane (intine) as its wall and pierces the outer layers smoothly even in the absence of a germpore or aperture (Figure 2.13).

Figure 2.13 Germination of pollen grains.

The stainability percentage ranges from 14.7 (cv. Thingpuri) to 28.5 (in cv. Pottangi and China). Usha (1984) reported 12.5 and 16.4% stainability in cvs.Rio de Janeiro and Moran, respectively. Pollen germination ranged from 8 (cv. Sabarimala) to 24% (Moran) (Dhamayanthi et al., 2003). Pillai et al. (1978) reported 17% pollen germination in cv. Rio de Janeiro. The pollen tube growth under in vitro was maximum in cv. China (488 fxm) and minimum in cv. Nadia (328 fxm). The number of pollen tubes ranged from 6.5 (in cv. Nadia) to 16.7 (in cv. Varada) (Dhamayanthi et al., 2003).

Physiology of Ginger

Effect of Day Length on Flowering and Rhizome Swelling

Ginger is grown under varying climatic conditions and in many countries in both hemispheres. It is generally regarded as being insensitive to day length. Adaniya et al. (1989) carried out a study to determine the influence of day length on three Japanese cultivars (Kintoki, Sanshu, and Oshoga) by subjecting the plants to varying light periods in comparison with natural daylight. In the three cultivars, as the light periods decreased from 16 to 10 hours, there was inhibition of vegetative growth of shoots and the underground stem. The rhizome knobs became more rounded and smaller. As the day length increased to 16 hours, the plants grew more vigorously and the rhizome knobs were slender and larger and active as new sprouts continued to appear. When the light period was extended to 19 hours, there was reduction in all growth parameters, and it was on a par with the 13-hour light period. It seems that the vegetative growth was promoted by a longer light period up to a certain limit, whereas rhizome swelling was accelerated under a relatively short day length (Table 2.7). The results also suggested that a relatively short day length accelerated the progression of the reproductive growth, whereas relatively long day length decelerated it. Ginger is therefore described as a quantitative short-day plant for flowering and rhizome swelling (Adaniya et al., 1989). These workers have also observed intraspecific variations in photoperiodic response; cv. Sanshu responded most sensitively, and Kintoki was more sensitive than Oshoga. They concluded that such an intraspecific response to the photoperiod could be related to their traditional geographical distribution; Kintoki and Sanshu are early cultivars adapted to the northern part (Kanto district) and Oshoga is a late cultivar adapted to the south (Okinawa to Shikoku districts).

Sterling et al. (2002) studied the effect of photoperiod on flower bud initiation and development in Zingiber mioga (myoga, or Japanese ginger). Plants grown under long-day conditions (16 hours) and short-day conditions (8 hours) with a night break produced flower buds, whereas those under short-day conditions (8 hours) did not. This failure of flower bud production under short day was due to abortion of developing floral bud primordia rather than a failure to initiate inflorescences. It was concluded that although for flower development in myoga a quantitative long-day requirement must be satisfied, flower initiation was day neutral. Short-day conditions also resulted in premature senescence of foliage and reduced foliage dry weight.

Chlorophyll Content and Photosynthetic Rate in Relation to Leaf Maturity

Xizhen et al. (1998c) investigated the chlorophyll content, photosynthetic rate (Pn), MDA content, and the activities of the protective enzymes during leaf development. Both chlorophyll content and Pn increased with leaf expansion and reached a peak on

Table 2.7 Effect of photoperiod on the growth of underground parts

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