Info

Source: Xizhen et al. (1998b).

Source: Xizhen et al. (1998b).

out the relationship between photosynthesis and changes in microclimate. Stomatal conductance (gsc) increased and was saturated at relatively low values of high intensity (400 fxmol-1). At different leaf temperatures, gsc peaked at 29°C, but transpiration (tr) increased with increasing irradiance and temperature. Increasing external CO2 concentrations caused gsc to increase but were relatively insensitive to increasing soil moisture availability until a threshold was reached (0.5 to 2 g/g). At a soil moisture content of 2 to 3.5 g/g, gsc increased approximately linearly with increasing tr. Fluorescence (Fv/Fm, electron transfer in PS II) decreased with increasing photon flux density (PFD). In leaves exposed to high PFD and different temperatures, Fv/Fm was the lowest at 15°C, and the highest at more than 25°C. In leaves exposed to low PFD, Fv/Fm remained at a similar value over all temperatures tested.

Photosynthesis and Photorespiration

Zhenxian et al. (2000) measured, using a portable photosynthetic system and a plant efficiency analyzer, the photosystem inhibition of photosynthesis and the diurnal variation of photosynthetic efficiency under shade and field conditions. There were marked photoinhibition phenomena under high light stress at mid-day. The apparent quantum yield (AQY) and photochemical efficiency of PS II (Fv/Fm) decreased at midday, and there was a marked diurnal variation. The extent of photoinhibition due to higher light intensity was severe in the seedling stage. After shading, AQY and Fv/Fn increased and the degree of photoinhibition declined markedly. However, under heavier shade, the photosynthetic rate declined because the carboxylation efficiency declined after shading.

Shi-jie et al. (1999) investigated the seasonal and diurnal changes in photorespiration (Pr) and the xanthophyll cycle (L) in ginger leaves under field conditions in order to understand the role of L and Pr in protecting leaves against photoinhibitory damage. The seasonal and diurnal changes of Pr and L of ginger leaves were marked, and Pr showed diurnal changes in response to PFD, and its peak was around 10:00 a.m. to

12:00 noon. Pr declined with increasing shade intensity. The L cycle showed a diurnal variation in response to PFD and xanthophyll cycle pool. Both increased during the midday period, and peaked around 12:00 noon. The results, in general, indicated that Pr and the xanthophyll cycle had positive roles in dissipating excessive light energy and in protecting the photosynthetic apparatus of ginger leaves from midday high-light stress.

Xizhen et al. (2000) have also investigated the role of SOD in protecting ginger leaves from photoinhibition damage under high-light intensity. They observed that on a sunny day the photochemical efficiency of PS ll (Fv/Fm) and AQY of ginger leaves declined gradually in the morning, but rose progressively after 12:00 noon. The MDA content in ginger leaves increased but the Pn declined under midday high-light stress. SOD activity in ginger leaves increased gradually before 1400 hours, and then decreased. At 60% shading in the seedling stage, Fv/Fm and AQY of ginger leaves increased but the MDA content, SOD activity, and Pn decreased. Pn, AQY, and Fv/Fm of ginger leaves treated with diethyldithio carbamic acid (DDTC) decreased whether shaded or not, but the effect of DDTC on shaded plants was less than that on unshaded plants. These workers concluded that midday high-light intensity imposed a stress on ginger plants and caused photoinhibition and lipid peroxidation. SOD and shading played important roles in protecting the photosynthetic apparatus of ginger leaves against high light stress.

Xizhen et al. (1998a) have investigated the effect of temperature on photosynthesis of ginger leaf. They showed that the highest photosynthetic rate and apparent quantum efficiency was under 25°C. The light compensation point of photosynthesis was in the range of 25 to 69 ^mol.m2s-1; it increased with increasing temperature. The light saturation point was also temperature dependent. The low-light saturation point was noted at temperatures below 25°C. The CO2 compensation point and the saturation point were 25 to 72 and 1343 to 1566 (xl/l, respectively, and both increased with the increase in leaf temperature.

Xianchang et al. (1996) studied the relationship between canopy, canopy photosynthesis, and yield formation in ginger. They found that canopy photosynthesis was closely related to yield. In a field experiment using a plant population of 5,000 to 10,000 per 666.7 m2 area, they had a yield increase from 1,733 to 2,626 kg. The Pn increased from 8.16 ((mol CO2 m-2 1 (ground) s-1) to 14.66; the leaf area index from 3.21 (m2/m2) to 7.02 m2/m2 (Table 2.9). The unit area of branches (tillers) and leaf area index were over 150/m2 and 6 m2/m2, respectively, in the canopy of the higher yield class. The canopies over 7,000 plants per 666.67 m2 satisfied these two criteria and among them there were no significant differences in height, tillers, leaf area index, canopy photosynthesis, and yield. Diurnal changes in the canopy Pn showed a typical single-peak curve, which was different from the double-peak curve obtained from the single-leaf Pn.

Effect of Growth Regulators

Studies have been carried out to find out the effect of various growth regulators on ginger growth, flowering and rhizome development. The main aims of such studies are to break the rhizome dormancy, to induce flowering and seed set, and to enlarge the rhizome followed by increased yield. Islam et al. (1978) studied the influence of 2-chloroethyl phosphonic acid (ethrel or ethephon) and elevated temperature treatments. Exposure of ginger rhizome pieces to 35°C for 24 hours or to 250 ppm ethrel for 15 min caused a

Table 2.9 Effect of plant density on growth, photosynthesis, and yield of ginger

Plant density

Plant

Tillers

Leaf area

Photosynthetic rate

Yield

(per 66.67 m2)

ht. (cm)

(no/m2)

index (m2lm2)

( ^mol CO2 m2 (ground) • s

(kg/666.7 m2)

5,000

62a*

103a

3.21a

8.16a

1733.42a

6,000

64a

122b

4.25b

10.17b

1948.25b

7,000

67a

138c

5.31c

12.23c

2211.22c

8,000

73b

152d

6.15d

14.56d

2637.17d

9,000

74b

159d

6.63d

14.78d

2614.95d

10,000

74b

163d

7.02d

14.66d

2626.13d

aSame letters are not statistically significant Source: Xianchang et al. (1996).

aSame letters are not statistically significant Source: Xianchang et al. (1996).

Table 2.10 Effect of temperature and ethrel on germination and early growth of ginger

Growth parameters Day 16 Pretreated Day 23 Pretreated

Table 2.10 Effect of temperature and ethrel on germination and early growth of ginger

Growth parameters Day 16 Pretreated Day 23 Pretreated

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