Modification of yield and chlorophyll content in leaf lettuce by HPS radiation and nitrogen treatments
1991
Mitchell, C.A. | Leakakos, T. | Ford, T.L.
This study evaluated the potential of high photosynthetic photon flux (PPF) from high-pressure sodium (HPS) lamps, alone or in combination with metal halide (MH) plus quartz iodide (QI) incandescent lamps, to support lettuce growth, with or without N supplementation. Varying exposures to radiation from combined HPS, MH, and QI lamps influenced dry weight gain and photosynthetic pigment content of hydroponically grown 'Black-seeded Simpson' lettuce (Lactuca sativa L.) seedlings. Cumulative leaf dry weight declined with increasing exposure, up to 20 hours per day, to 660 micromole.m-2.s-1 of photosynthetically active radiation (PAR) from HPS lamps concomitant with constant 20 hours per day of 400 micromole.-m-2.s-1 from MH + QI lamps. Leaves progressively yellowed with increasing exposure to radiation from the three-lamp combination, corresponding to lower specific chlorophyll content but not to specific carotenoid content. Lettuce grown under 20-hour photoperiods of 400, 473, or 668 micromole.m-2.s-1 from HPS radiation alone had the highest leaf dry weight at a PPF of 473 micromole.m-2.s-1. Chlorophyll, but not carotenoid specific content, decreased with each incremental increase in PPF from HPS lamps. Doubling the level of N in nutrient solution and supplying it as a combination of NH4(+) and NO3(-) partially ameliorated adverse effects of high PPF on growth and pigment content relative to treatments using single-strength N as NO3(-).
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