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CHAPTER 3 RESPONSE OF POTATO GROWN UNDER NON-INDUCTIVE GREENHOUSE CONDITIONS TO PACLOBUTRAZOL: SHOOT GROWTH, CHLOROPHYLL CONTENT, NET PHOTOSYNTHESIS, ASSIMILATE PARTITIONING, TUBER YIELD, QUALITY AND DORMANCY
ABSTRACT
The effect of foliar and soil applied PBZ on potato were examined under non-inductive conditions in a greenhouse. Single stemmed plants of the cultivar BP1 were grown at (+2)/20 (+2) ºC day/night temperatures, relative humidity of 60%, and a 16h photoperiod. Twenty-eight days after transplanting PBZ was applied as a foliar spray or soil drench at rates of 0, 45.0, 67.5, and 90.0 mg active ingredient PBZ per plant. Regardless of the method of application, PBZ increased chlorophyll a and b content of the leaf tissue, delayed physiological maturity, and increased tuber fresh mass, dry matter content, specific gravity, and dormancy period of the tubers. PBZ reduced the number of tubers per plant. A significant interaction between rates and methods of PBZ application were observed with respect to plant height and tuber crude protein content. Foliar application resulted in a higher rate of photosynthesis than the soil drench. PBZ significantly reduced total leaf area and increased assimilate partitioning to the tubers. The study clearly showed that PBZ is effective to suppress excessive vegetative growth, favour assimilation to the tubers, increase tuber yield, improve tuber quality and extend tuber dormancy of potato grown in high temperatures and long photoperiods.
Keywords: Crude protein; gibberellin; high temperature; long photoperiod; paclobutrazol
INTRODUCTION
High temperature is an important factor limiting potato production in some areas of the world (Morpurgo & Ortiz, 1988). The optimum temperatures for foliage growth and net photosynthesis are 20 – 25 ºC and 16 – 25 ºC, respectively. Low mean temperatures (15-19 °C) and short photoperiods (12 h) are favourable for tuberization and early tuber growth (Vandam et al., 1996). High temperatures inhibit tuberization in both short and long day conditions, but especially under long photoperiods (Jackson, 1999).
The carbon budget for potatoes developed by Leach et al. (1982) indicates that plant growth rate is strongly related to net photosynthesis and dark respiration. At elevated temperatures, foliage growth is promoted, rate of photosynthesis declines rapidly, assimilate partitioning to the tubers is reduced and dark respiration increases (Thornton et al., 1996). Tuber growth is completely inhibited at 29 ºC, above which point the carbohydrate consumed by respiration exceeds that produced by photosynthesis according to Levy (1992). Like high temperatures, long photoperiods also decrease partitioning of assimilates to the tubers and increase partitioning to other parts of the plant (Wolf et al., 1990).
Potatoes grown under high temperatures or long photoperiods are characterized by taller plants with longer internodes, increased leaf and stem growth, lower leaf: stem ratio, shorter and narrower leaves with smaller leaflets, and less assimilates partitioned to the tubers (Ben Khedher & Ewing, 1985; Manrique, 1989; Struik et al., 1989).
Induction to tuberization is promoted by short days, more specifically by long nights (Gregory, 1965) and cool temperatures (Ewing, 1981). Under such conditions a transmissible signal is activated that triggers cell division and elongation in the sub-apical region of the stolons to produce tuber initials (Xu et al., 1998; Amador et al., 2001). In this signal transduction pathway, the perception of appropriate environmental cues occurs in the leaves and is mediated by phytochrome and GA (Van den Berg et al., 1995; Jackson & Prat, 1996).
Amador et al. (2001) reported that endogenous GA is a component of the inhibitory signal in potato tuberization under long days. Previous studies on GA showed that the levels of GA-like activity decrease in leaves of potato upon transfer from long day to short day conditions (Railton & Wareing, 1973). Under short day conditions GA biosynthesis is reduced (Amador et al., 2001). Van den Berg et al. (1995) reported that a dwarf potato mutant tuberized under long days due to the incorporation of a gene that partially blocks the conversion of 13-hydroxylation of GA12-aldahyde to GA53, and treatment with GA biosynthesis inhibitors enhance tuberization in andigena spp. under long day conditions (Jackson & Prat, 1996).
Potato plants grown under non-inductive conditions are characterized by high levels of endogenous GA (Vreugdenhil & Sergeeva, 1999) that promotes shoot growth (Menzel, 1980) and delays or inhibits tuberization (Abdella et al., 1995; Vandam et al., 1996). In addition, accumulation of GA in tuber tissue can specifically impede starch accumulation (Booth & Lovell, 1972; Paiva et al., 1983; Vreugdenhil & Sergeeva, 1999), inhibits the accumulation of patatin and other tuber specific proteins (Vreugdenhil & Sergeeva, 1999), and in combination with other inhibitors it regulates potato tuber dormancy (Hemberg, 1970).
The hormonal balance controlling potato tuberization can be altered using GA biosynthesis inhibitors such as 2-chloroethyl trimethyl ammonium chloride (CCC) (Menzel, 1980), B 995 (Bodlaender & Algra, 1966), and PBZ (Simko, 1994). PBZ is a triazole plant growth regulator known to interfere with ent-kaurene oxidase activity in the ent-kaurene oxidation pathway (Rademacher, 1997). Interference with the different isoforms of this enzyme could lead to inhibition of GA biosynthesis and abscisic acid (ABA) catabolism. In addition, it induces shoot growth reduction (Terri & Millie, 2000; Sebastian et al., 2002), enhances chlorophyll synthesis (Sebastian et al., 2002), delays leaf senescence (Davis & Curry, 1991) and increases assimilate partitioning to the underground parts (Balamani & Poovaiah, 1985; Davis & Curry, 1991; Bandara & Tanino, 1995; De Resende & De Souza, 2002).
It is postulated that PBZ blocks GA biosynthesis in potato plants grown under non-inductive growing conditions and modifies its growth to increase the productivity of the crop. Accordingly, the effects of foliar and soil applied PBZ on shoot growth, leaf chlorophyll content, assimilate production and allocation, tuber yield, and quality, and tuber dormancy period of potato grown under conditions of high temperatures and long photoperiod were investigated. The ultimate objective being to generate information to improve potato production in marginal areas where high temperatures and/or long photoperiods are limiting factors.
MATERIALS AND METHODS
Plant culture
Two experiments with similar procedures and treatments were conducted in 2002 on the experimental farm of the University of Pretoria, South Africa. Potato tubers of a medium maturing commercially cultivated variety BP1 were allowed to sprout, and seed cores of approximately 15 g containing the apical sprout were excised. Seed pieces were planted in crates with vermiculite and kept in a growth chamber at 35/20 oC day/night temperatures and a 16h photoperiod. A week after emergence, uniform plants were transplanted to 5-liter plastic pots filled with sand and coconut coir (50:50 by volume) and grown in a greenhouse at 35 (±2)/20 (±2) oC day/night temperatures, an average relative humidity of 60%, and a 16h photoperiod. The photoperiod was extended using a combination of Sylvania fluorescent tubes and incandescent lamps (PAR: 10 mmol m-2 s-1). In both experiments, the pots were arranged in a randomised complete block design with three replications and each replicate contained seven pots per treatment. Plants were fertilized with a standard Hoagland solution and watered regularly to avoid water stress.
Treatments
Twenty-eight days after planting (early stolon initiation) the plants were treated with PBZ at rates of 0, 45.0, 67.5 and 90.0 mg active ingredient (a.i.) per plant as a foliar spray or soil drench using the Cultar formulation (250 g a.i. PBZ per liter, Zeneca Agrochemicals SA (PTY.) LTD., South Africa). For the foliar treatment, the solution was applied as a fine spray using an atomizer. The drench solution was applied to the substrate around the base of the plants. The control plants were treated with distilled water.
Data recorded
Net photosynthesis and chlorophyll content
Two weeks after treatment the rate of photosynthesis was measured using a portable photosynthesis system (CIRAS-1, 1998, UK), and leaf chlorophyll content was determined. From each treatment, three plants were randomly selected and rate of photosynthesis was measured on the terminal leaflet of three fully expanded younger leaves. The photon flux density incident at the level of the leaf in the cuvette was 1050-1220 µmolm-2s-1 (PAR). Average saturated vapour pressure of water at cuvette temperature was 34.5 mbar and vapour pressure deficit of the air in the course of measurements was 6.05 mbar. To determine the concentrations of chlorophyll a and b spectrophotometer (Pharmacia LKB, Ultrospec III) readings of the density of 80% acetone chlorophyll extracts were taken at 663 and 645 nm and their respective values were assessed using the specific absorption coefficients given by MacKinney (1941).
Assimilate partitioning and total leaf area
Two, four, six, and eight weeks after treatment application one pot per treatment was harvested and separated into leaves, stems, tubers, and roots and stolons. Leaf area was measured with a LI-3000 leaf area meter (LI-Inc, Lincoln, Nebraska, USA) and the plant tissues oven dried at 72 °C to a constant mass. Dry matter partitioning was determined from the dry mass of individual plant parts as a percentage of total plant dry mass.
Plant height, senescence, tuber fresh mass and number
Plant height refers to the length from the base of the stem to shoot apex. Plants were regarded as physiologically mature when 50% of the leaves had senesced. Tuber fresh mass and numbers represent the average tuber mass and count of three plants at the time of final harvest.
Quality assessment
At harvest a representative tuber sample from each treatment group was taken and washed. Tuber specific gravity was determined by weighing in air and under water (Murphy & Goven, 1959). For dry matter content determination, the samples were chopped and dried at a temperature of 60 ºC for 15h, and followed by 105 ºC for 3h. Dry matter content of the tubers is the ratio between dry and fresh mass. Samples dried at 60 ºC were analysed for total nitrogen (Macro-Kjeldahl method, AOAC, 1984), and tuber crude protein content estimated by multiplying total nitrogen content by a conversion factor of 6.25 (Van Gelder, 1981).
Dormancy
To determine the effect of PBZ on dormancy, six healthy tubers per treatment were selected at the final harvest and labelled. Each treatment was replicated three times and samples were randomly distributed on shelves in a dark room. The dormancy of a particular tuber was deemed to have ended when at least one 2mm long sprout was present (Bandara & Tanino, 1995).
Data analysis
The analyses of variance were carried out using MSTAT-C statistical software (MSTAT-C, 1991). Combined analysis of variance did not shown significant treatments by experiment interactions. Hence, for all of the parameters considered, the data of the two experiments were combined. Means were compared using the least significant difference (LSD) test at 1% probability level. Correlations between parameters were computed when applicable.
LIST OF TABLES
LIST OF FIGURES
ABSTRACT
ACKNOWLEDGMENTS
CHAPTER 1 GENERAL INTRODUCTION
CHAPTER 2 LITERATURE REVIEW
2.1 SEXUAL REPRODUCTIVE GROWTH IN POTATO
2.2 TUBERIZATION
2.3 PACLOBUTRAZOL
CHAPTER 3 RESPONSE OF POTATO GROWN UNDER NON-INDUCTIVE GREENHOUSE CONDITIONS TO PACLOBUTRAZOL: SHOOT GROWTH, CHLOROPHYLL CONTENT, NET PHOTOSYNTHESIS, ASSIMILATE PARTITIONING, TUBER YIELD, QUALITY AND DORMANCY
3.1 ABSTRACT
3.2 INTRODUCTION
3.3 MATERIALS AND METHODS
3.4 RESULTS
3.5 DISCUSSION
3.6 CONCLUSION
CHAPTER 4 PACLOBUTRAZOL INDUCED LEAF, STEM, AND ROOT ANATOMICAL MODIFICATIONS IN POTATO
4.1 ABSTRACT
4.2 INTRODUCTION
4.3 MATERIALS AND METHODS
4.4 RESULTS
4.5 DISCUSSION
4.6 CONCLUSION
CHAPTER 5 RESPONSE OF POTATO GROWN IN A HOT TROPICAL LOWLAND TO PACLOBUTRAZOL. I: SHOOT ATTRIBUTES, PRODUCTION AND ALLOCATION OF ASSIMILATES
5.1 ABSTRACT
5.2 INTRODUCTION
5.3 MATERIALS AND METHODS
5.4 RESULTS
5.5 DISCUSSION
5.6 CONCLUSIONS
CHAPTER 6 RESPONSE OF POTATO GROWN IN A HOT TROPICAL LOWLAND TO PACLOBUTRAZOL. II: GROWTH ANALYSES
6.1 ABSTRACT
6.2 INTRODUCTION
6.3 MATERIALS AND METHODS
6.4 RESULTS
6.5 DISCUSSION
6.6 CONCLUSION
CHAPTER 7 RESPONSE OF POTATO GROWN IN A HOT TROPICAL LOWLAND TO PACLOBUTRAZOL. III: TUBER ATTRIBUTES
7.1 ABSTRACT
7.2 INTRODUCTION
7.3 MATERIALS AND METHODS
7.4 RESULTS
7.5 DISCUSSION
7.6 CONCLUSION
CHAPTER 8, GROWTH AND PRODUCTIVITY OF POTATO AS INFLUENCED BY CULTIVAR AND REPRODUCTIVE GROWTH: I. STOMATAL CONDUCTANCE, RATE OF TRANSPIRATION, NET PHOTOSYNTHESIS, AND DRY MATTER PRODUCTION AND ALLOCATION
8.1 ABSTRACT
8.2 INTRODUCTION
8.3 MATERIALS AND METHODS
8.4 RESULTS
8.5 DISCUSSION
8.6 CONCLUSION
CHAPTER 9 GROWTH AND PRODUCTIVITY OF POTATO AS INFLUENCED BY CULTIVAR AND REPRODUCTIVE GROWTH: II. GROWTH ANALYSIS, TUBER YIELD AND QUALITY
9.1 ABSTRACT
9.2 INTRODUCTION
9.3 MATERIALS AND METHODS
9.4 RESULTS
9.5 DISCUSSION
9.6 CONCLUSION
CHAPTER 10 THE EFFECT OF MCPA AND PACLOBUTRAZOL ON FLOWERING, BERRY SET, BIOMASS PRODUCTION, TUBER YIELD AND QUALITY OF POTATO
10.1 ABSTRACT
10.2 INTRODUCTION
10.3 MATERIAL AND METHODS
10.4 RESULTS
10.5 DISCUSSION
10.6 CONCLUSION
CHAPTER 11 GENERAL DISCUSSION
REFERENCES
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