Commercial use of PGRs
Phytohormones are organic compounds, other than nutrients, that produced naturally in higher plants, controlling growth or other physiological functions at a site remote from its place of production and active in minute amounts, modify plant physiological process (Bisht, Rawat, Chakraborty, & Yadav, 2018). Plant growth regulator (PGR) includes synthetic compounds as well as naturally occurring hormones (Bisht et al., 2018; Kaur et al., 2018). Plant growth regulators generally include Auxins, Gibberellins, Cytokinin, Ethylene, Growth retardants and Growth inhibitors (Kaur et al., 2018).
PGRs also acts as a growth promoter or growth inhibitors, act inside plant cells to stimulate or inhibit specific enzymes or enzymes systems and help regulate plant metabolism (Kaur et al., 2018). PGRs which have a catalytical effect, i.e. take a vital role in plant growth are called growth promoter e.g. Auxins, Gibberellins and Cytokinin whereas PGRs which selectively interfere with the normal hormonal promotion of growth are called Growth Inhibitor e.g. Abscisic acid and Ethylene (Bisht et al., 2018).
Some of the important usages of PGRs includes:
Ikuma & Thimann (1960), found that GA3 at 60 ppm induces maximal germination of Lettuce (requiring Red light for natural germination) even in the absence of Red light. Balaguera-López, Cárdenas-Hernández, & Álvarez-Herrera, (2009) found that soaking seeds in 900 ppm GA3 allowed obtaining more vigorous tomato seedlings in less time and with a better development in the field.
Breaking of Potato tuber buds dormancy
Growing potato tubers or freshly harvested mature tubers have a dormant apical bud, this dormancy is spontaneously broken after a period of maturation of the tuber (Claassens, Verhees, Van Der Plas, Van Der Krol, & Vreugdenhil, 2005). Tuber maturation may take 1−15 weeks; small tubers have longer periods of dormancy (Lommen, 1993 in Hassani, Zareian, & Rezvani, 2014). Hassani et al., (2014) found that Potato tubers can be induced to sprouting by immersing in 50 ppm GA3 for 3 hours. Alexopoulos, Aivalakis, Akoumianakis, & Passam, (2008) concluded that Potato tuber dormancy breaking is dependent on the duration of treatment rather than GA concentration (no significant difference over the range 1–50 ppm) and the optimum time of immersion in GA to achieve dormancy breakage is 2 hour.
Reduction of bolting and storage sprouting of bulb crops
Flowering is favoured in production of fruit vegetable but not in the production of the bulb or leafy vegetable; higher vegetative growth is preferred. Flowering in such crop is termed as bolting. Kwon, Kim, Kim, Moon, & Yoo, (2016) found that bolting causes a decrease in bulb weight of Onion. Untreated control plants exhibited about 40 percent bolting and there was a progressive suppression of bolting following top spraying as the concentration of Maleic Hydrazide (MH) increased from 0.025 to 0.25 percent; when sprayed with 0.25 % MH results into no bolting (Choudhri & Bhatnagar, 1953). Fadl, Abu-Goukh, & El-Balla, (2005) reported that Maleic hydrazide also reduces the storage spouting of Onion bulbs.
Parthenocarpic fruit development
Unfavourable conditions, such as extreme temperatures, may prevent pollination and hence also fruit set. Parthenocarpy has been recognized for a long time as an interesting trait to avoid low fruit set in harsh conditions (Gorguet, Van Heusden, & Lindhout, 2005). The shelf life of seedless fruits is longer than seeded fruits because seeds produce hormones that trigger senescence. Lower endogenous Cytokinin (CK) and GA levels have been found in the male-sterile mutant of tomato SL-2 (stamen- less-2; Sawhney, 1974; Sawhney and Shukla, 1994 in Gorguet et al., (2005). Moreover, tomato GA-deficient mutants (gib-1 and gib-2) are male sterile and fertility can be restored by exogenous application of GAs (Nester and Zeevaart,1988; Jacobsen and Olszewski,1991 in Gorguet et al., (2005).
Ding et al., (2013) found that applications of CPPU (a Cytokinin) at 100 ppm and GA3 at 200 ppm (one at a time or together) induced the parthenocarpic fruit development in Tomato and concluded that CK may induce parthenocarpy in tomato partially through modulation of GA and IAA metabolisms.
Delay of senescence
Richmand and Lang (1957) in Bisht et al., (2018) found that kinetin can postpone the senescence. Gibberellins and Cytokinins have a positive effect on delaying chlorophyll degradation, while Abscisic acid and Ethylene induce senescence (Nooden, 1988). 1-Methylcyclopropene (1-MCP) is an Ethylene action inhibitor that can minimise the negative effect of Ethylene on leafy vegetables to extend their shelf-life (Blankenship and Dole, 2003). Jiang, Sheng, Zhou, Zhang and Liu (2002) found that 1-MCP at a concentration of 100 nL/L delayed postharvest senescence of coriander during storage in the presence of ethylene (10μL/L). Pak Choy sprayed with 50 ppm kinetin 1 hour before harvest had a shelf life of 23 days (Jiang and Pearce 2005).
Modification of Cucurbit sex expression
Sex expression in the cucumber is influenced by genotype and plant hormones (Takahashi et al. 1983; Atsmon and Galun 1960 in Yamasaki, Fujii, & Takahashi, 2003). Hormone influencing sex expression in the cucumber is Ethylene (Yin and Quinn, 1995 in Yamasaki et al., 2003). Application of 2-chloroethylphosphonic acid (ethephon), an ethylene-releasing agent, induces the formation of female flowers (Iwahori et al. 1970; MacMurray and Miller 1968 in Yamasaki et al., 2003). In contrast, aminoethoxyvinyl glycine (AVG), an inhibitor of ethylene biosynthesis, or AgNO3, an inhibitor of ethylene action, suppresses the development of female flowers and induces male ones (Takahashi and Jaffe 1984; Takahashi and Suge 1980; Atsmon and Tabbak 1979; Beyer 1976 in Yamasaki et al., 2003).
Baset Mia, Islam, & Shamsuddin, (2014) found that application of CEPA (Ethhylene source) at 150 ppm and NAA at 50 ppm is the best treatments for reducing sex ratio by increasing the female flowers in monoecious Bitter gourd. Golabadi, Golkar, & Eghtedari, (2018) found that 300 ppm of Silver nitrate and 500 ppm of Silver thiosulphate were effective in significantly increasing the number of male flowers in gynoecious Cucumber.
Flower induction in Cabbage
Seed vernalisation (2.0° – 4.5°C for 8 weeks), followed by GA3 treatment (250 ppm for 7 times at weekly interval after 45 days of transplanting), induced flowering in both the seed grown and ratoon cabbage variety ‘HRT 009617’ where neither treatment (GA3 or seed vernalisation), applied without the other, was able to induce flowering (George, Alderson, Craigon, & Sparkes, 2007). Furthermore, Foliar spray of 200 ppm GA3 induced flowering in 70 % of Carrot plants grown from 8 weeks old roots where no flowering was observed in control (Bandara & Tanino, 1995).
Advantages and limitations in the use of PGRs
PGRs facilitates a fast and efficient way of plant growth and development: From vigorous germination of seeds (Balaguera-López et al., 2009; Ikuma & Thimann, 1960) to higher yield (Baset Mia et al., 2014; George et al., 2007; Bandara & Tanino, 1995; Choudhri & Bhatnagar, 1953). They are also reported to increase the shelf life of the harvested vegetables (Bisht et al., 2018) and also induces fruit development even in the absence of natural pollination (Ding et al., 2013). So, PGRs are very useful in the field of commercial Olericulture.
PGRs are crop as well as dose-specific in nature (Golabadi et al., 2018; Baset Mia et al., 2014; Choudhri & Bhatnagar, 1953). The same product at the same dose may affect positively in a certain crop may harm or do not affect another crop. So, the PGRs need proper field trial before adopting on the commercial scale.
Recent advances in the PGR related studies in Nepal
- Bhattarai, (2017) found that spraying Paclobutrazol 6 weeks after transplanting at 50 ppm gave the significantly higher number and larger size of potato mini-tuber during pre-basic seed potato production under glasshouse.
- 2,4-D 2 ppm or Triacontanol 5 ppm are most effective to reduce flower and fruit drops in winter and rainy season Chilli (Chaudhary, Sharma, Shakya, & Gautam, 2006).
- Above are the recently published PGR related studies from Nepal.
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