Cytokinin action revisited: leaf anatomical changes play a key role in 6-benzylaminopurine-driven growth promotion in pot-grown lettuce

Abstract

Plants raised in small cell trays are often subjected to root growth restrictions. Root tips are a source of cytokinin, and insufficient biosynthesis and transport of this hormone to the aerial part severely impairs shoot development. Exogenous supply of cytokinin to the foliage has been shown to effectively counteract these effects in several horticultural crops, but the physiological processes involved are still unclear. The aim of this work was to study the effect of spraying the cytokinin 6-benzylaminopurine (BAP) on growth and development of plug lettuce seedlings focusing on the morpho-physiological mechanisms involved in plant response. Two experiments were conducted in a greenhouse. Lettuce (Lactuca sativa L. Crimor INTA) seedlings were grown for 35 days in 128-, 200- and 288-cells trays (17.37, 13.90 and 6.18 cm3·cell-1 respectively), after which they were sprayed with BAP (6-benzylaminopurine) solutions (0, 5, 50 or 100 mg·L-1) either immediately before transplant, or 7 days after transplant. Seedlings were transplanted into larger (3,000 cm3) pots in which they grew for further 60 days. Decreasing plug cell volume resulted in a steep decrease in plant net assimilation rate and leaf net photosynthetic rate, but the impact on the relative growth rate was somewhat lower due to an increased leaf area ratio. BAP sprays increased plant biomass accumulation and enhanced the development of photosynthetic area, in parallel with a strong promotion of carbon assimilation, and these effects were more remarkable in plants raised in smaller plugs, and when hormone was supplied at the pre-transplant stage.