Original Research

Antioxidants, antistress compounds and ethylene inhibitors enhance plant defense against drought

Drought reduced photofunctions, destabilized membranes and increased ethylene synthesis, triggering senescence (Rajasekaran and Blake 1999 - J. Plant Growth Regul.18:175-181; Rajasekaran and Blake 2002 - Can. J. Plant Sci., 82: 195-202.). A search for various natural antistress, antioxidant compounds led to the discovery of “Bioprotect TM”, a phenolic derivative from conifers. Seed embryoconditioning using Bioprotect inhibited ethylene, protected membranes, maintained Pn and high WUE and promoted growth of pea seedlings under drought (Rajasekaran and Blake Patenting being actively considered). A synthetic antioxidant, AmbiolTM, a derivative of 5-hydroxybenzimidazole, spermidine and ABA all triggered plant defense against drought and promoted growth (Rajasekaran and Blake 1999 - J. Plant Growth Regul.18:175-181; Can. J. Plant Sci., 82: 195-202.). Inhibiting ethylene using various antiethylene compounds enhanced plant defense, protected membranes, maintained tissue water balance and elastic modulus (Islam et al. 2003 - Trees-Structure and Function 468: 238). Glycinebetaine (carrots and onion) and trigonelline (tomato) have been discovered to accumulate in significant quantities in response to drought (Caldwell, P. 2000-MSc Thesis, NSAC/Dalhousie; Pauley and Rajasekaran 2002). Extracting and reapplying these compounds to seeds, enhanced plant defense against drought, protecting membrane function. This is a significant contribution to science and has great impact in forestry, vegetable, cereal and floriculture sectors both at national and international levels providing protection against ravages of drought. Currently, nearly two million forestry seedlings are being treated and the seedlings that were out-planted and nearly 98 percent survived despite several episodes of natural drought. Ambiol Inc. Canada commercializes Bioprotect and Ambiol.

Thermogenic compounds, antistress, antioxidant compounds in promoting seed germination and seedling emergence at low temperature and low moisture status discovered

Stand establishment has been a greater challenge in carrot production. Low temperature and low moisture during seeding often results in uneven seed germination resulting in poor crop stand, challenging production, productivity, quality and profitability. A group of compounds, called salicylates such as DHBA, SA, ASA all enhanced germination at a limiting low temperature of 5oC (Rajasekaran et al. 2002- Can. J. Plant Sci., 82: 443-450). Embryoconditioning using various antistress, antioxidant compounds such as Ambiol (5-hydroxybenzimidazole derivative), Bioprotect (a natural antistress, antioxidant) and glycinebetaine (natural antistress compound) all promoted germination under low soil moisture of 25%FC. Field experiments using these compounds showed a significant advantage in promoting uniform emergence under natural conditions (Rajasekaran et al. 2003 - Acta Horticulturae 2003, 631: 105-116; Rajasekaran et al. 2005 – Scientia Horticulturae 106: 25-37). This technology offers greater promise for synchronizing and promoting seedling emergence under limited low temperature and low moisture conditions and is being commercially tested for adoption by the industry.

Endophytes in carrots discovered to enhance seedling growth

Endophytic bacteria belonging to 28 genera have been discovered from carrot crown tissues. Pseudomonas species were abundant compared to other species. Both Pseudomonas putida strain MLSN and Bacillus strain MLSN triggered shoot and root growth by nearly 72 per cent (Surette, M.A. et al. 2003. Plant and Soil 253: 1381-390). Their potential in abiotic and biotic stresses is being explored.This has great potential in naturally enhancing growth, triggering plant defense against fungal diseases and pests and promoting resistance to environmental stresses. This will have a great impact in food safety by reducing pesticide use and will have a positive impact on the environment and ecosystem stability.

Ethylene and polyamines trigger bulking in carrots

The phenomenon of bulking in carrots is less well understood and the signals that trigger bulking are not known. Our research revealed that ethylene, a senescence hormone and polyamine may be involved in triggering and enhancing bulking. Exogenous application of ethrel (a ethylene releasing compound) and polyamine, spermine, enhanced bulking and increased root girth. Yield enhancement was observed to be around 8 t/ha (Neuteboom, C.E. 2002; Neuteboom et al. 2002 Plant Growth Regulator Society of America, 30). This will have a great impact in carrot production under northern climate where longer, cloudy days and shorter growing season are the major production constraints. Further, this will provide a window for growing cover crops to reduce soil erosion. These phytohormone signals are being commercially tested for industry

Micro-environment alters carrot photosyntheis

Carrot photosynthesis is significantly influenced by irradiance, carbon dioxide, soil moisture and to a lesser extent by nutrients. Genotypes are less resilient to changes in environment and little changes in Pn other photofunctions found. Increasing irradiance increases Pn non-linearly and none of the genotypes reached saturation at 1000 µmole m-2 s-1. Increasing Pn however, reduces Gs and Tl which contributing to an increase in WUE (Kyei-Bohen et al. 2003. Photosynthetica 41; 301-305). Increasing CO2concentration increases Pn non-linearly. Genotypes do not differ significantly in their response to increasing CO2 concentration. Increasing CO2 reduces Gs and Tl and increases WUE. Pn max has been found to be 650 µmol mol-1 (Kyei-Bohen et al. 2003. Photosynthetica 41: 597-603). Photosynthesis parameters aren’t significantly altered by changes in NPK concentrations in any of the genotype (Pattipas et al. 2005. Proc. ASHS 2005; Rajasekaran et al. 2005. Proc. ASHS 2005). Declining soil moisture below -33 cbars reduce photosynthetic parameters significantly (Caldwell and Rajasekaran 2002. Proc. Intl. Carrot Conf. Bakersfield, CA). These discoveries are of great importance for optimizing carrot micro-environment for maximizing yield and quality.

Carrots are sensitive to drought and natural and synthetic Antistress, antioxidants can protect carrot seedlings from drought

Soil moisture decline from 70-30%FC significantly reduces xylem pressure potential, membrane capacitance, Pn, WUE leading to a significant reduction in leaf area expansion, elongation and shoot and root dw. Ambiol, a synthetic antioxidant protects seedlings and promotes growth under drought. Ambiol completely alleviates drought induced reduction in shoot and root dry matter growth by 214% compared to untreated controls (Rajasekaran and Blake 2002. Can J Plant Sci. 82: 195-202). A natural antistress compound, glycine betaine has been discovered in carrots. Preconditioning carrot seedlings with glycinebetaine enhanced membrane stability and protected the seedlings (Caldwell 2001, M.Sc. Thesis, NSAC-Dalhousie; Caldwell and Rajasekaran 2002. Proc. Int. Carrot Conf. Bakersfield, CA). Seed preconditioning using BP2 and Ambiol increased seedling emergence and yield and recovery of fancy grades in oranza by 71 and 146% compared to the untreated control (Rajasekaran et al. 2005. Scientia Horticulturae106: 25-37). These compounds have great potential in protecting crops during unanticipated drought and for commercialization.

Carrot can adjust well for nutrient deficient environment

Carrots are less responsive to both macro and micro nutrients both under sand culture and under field conditions. Increasing N, P K or S has no significant yield advantage. However, significant relationship between tissue and soil P, Mg, S, Fe, Zn and B was found at active bulking. Petiole Mg, Fe concentrations and leaf Zn concentration at bulking initiation stage showed a significant correlation with yield. At bulking stage, leaf N, P concentrations have a significant negative correlation. Tissue Ca, Mg, S, Mn concentrations significantly correlated with root weight (Pattipas et al. 2004. Hort Science 39: 870; Pattipas et al. 2006. Comm. Plant and Soil Analy.- In Press). Tissue nutrient diagnostics has direct impact on crop yield and quality and on the environment by optimization of required nutrients.