Optimizing Environmental Conditions for Mass Application of Mechano-dwarfing Stimuli to Arabidopsis

Obtaining uniform mechano-dwarfing of Arabidopsis thaliana (L.) Heynh. seedlings within dense plantings is problematic. Alternative forms of mechano-stimulation were applied to seedlings in effort to obtain uniform growth reduction compared with undisturbed controls in both greenhouse and controlled growth environments. Arabidopsis grown under low photosynthetic photon flux (PPF) artificial light grew upright with limited leaf expansion, which enhanced mechano-responsiveness compared to that of rosette-growing plants under filtered sunlight or high PPF artificial light. Hypocotyls of seedlings grown at PPFs >60 μmol·m-2·s-1 elongated less and had 6% less sensitivity to mechanical stress than seedlings grown at PPFs <60 μmol·m-2·s-1. Fluorescent lamps alone (F) or fluorescent plus incandescent (F+I) lamps were compared for seedling responses to mechanical stress. Under F lighting, hypocotyl elongation was reduced 25% to 40% by twice-daily brush or plate treatments, and brushed seedlings exhibited more growth reduction than did plate treatments. Seedlings grown under F+I lamps exhibited similar stress-induced growth reduction compared to seedlings grown under F only, but stressed F+I seedlings lodged to a greater extent due to excessive hypocotyl elongation. Temperature-response studies using standardized F-only lighting indicated increased hypocotyl elongation but decreased leaf expansion, and decreased mechano-responsivity to brushing over the temperature range from 20 to 28 °C. Daylength studies indicated similar degrees of mechano-inhibition of hypocotyl elongation over the daylength range of 12, 16, 20, and 24 hours, whereas fresh weight of stressed seedling shoots declined compared to controls. A combination of environmental growth parameters that give repeatable, visual mechanical dwarfing of Arabidopsis include low-PPF fluorescent lighting from 55 to 60 μmol·m-2·s-1, ambient temperatures from 22 to 25 °C, and twice-daily brush treatments.

EFFECTS OF SALINITY ON INITIAL SEEDLING GROWTH OF CHICKPEA (CICER ARIETINUM L.)

In order to investigate the effects of salinity on the early seedling growth of chickpea, four chickpea cultivars, Jam, Hashem (kabuli type: large seeded genotypes with light salmon colour), Kaka and Pirooz (desi type: small seeded genotypes with different colours), were grown in pots containing soils with 0.9 (control), 2.6 and 4.9 dSm-1 salinity. The shoot/root ratio of Pirooz was consistently reduced by increasing salinity at all sampling stages. Under saline conditions, the reduction in seedling growth, shoot water content, root and shoot K+ concentration and the increase in root and shoot Na+ concentration were more severe in the kabuli type than in desi type cultivars. Considering path coefficients, increasing seedling K+ concentration and uptake of water from the soil favoured salt-stressed seedling growth. Increasing K+ content alleviated the deleterious effects of root Na+ to a greater extent than that of shoot Na+. On the other hand, a higher percentage of the decrease in seedling growth as the result of Na+ was due to shoot K+ deficiency than to root K+ deficiency.

Cortical Intracellular Electrical Potential in Roots of Unstressed and Stressed Sunflower Seedlings. II. Radial Profiles and Oscillations

Various models have been proposed to explain how plants can extract water from dry soil against a gradient in water potential. According to one model, it is hypothesised that water uptake from dry soil is associated with a radial gradient in the intracellular electrical potential (Ec) of root cortical cells such that the potential in the outer cells is relatively hyperpolarised. As a partial test of this model, a microelectrode was used to measure radial profiles of Ec in intact roots of sunflower (Helianthus annuus) seedlings. The seedlings were subjected to either a control (unstressed) or one of two stress treatments. Water stress was rapidly imposed and relieved by lowering and raising the level of nutrient solution in a tank which contained all the seedling roots except the lateral root in which Ec was measured. There was a gradient in Ec in roots of unstressed plants with the potential of the outer cortical cells being relatively depolarised. The imposition of stress shifted the gradient in accordance with the hypothesis. Increasing the magnitude of the osmotic potential of the solution perfusing the measured portion of a lateral root of a stressed seedling resulted in a discontinuity in the profile between the second and third cortical layers. The gradients in the profile of Ec indicate there is a limitation in intercellular electrical coupling and the appearance of a discontinuity may indicate a decrease in coupling. Three types of occasional, spontaneous oscillations in Ec are characterised. One type of oscillation may be additional evidence that the electrical coupling between the cortical cell layers is variable and depends on the transport and status of water in the roots.