Root restriction on the first stage of seedling growth decreases post-transplant biomass accumulation. Several studies in different vegetables and ornamental plants have suggested that this restriction would be overcome by means of a single 6-benzylaminopurine (BAP) spray. Based on this, the aim of this work was to evaluate the effects of both pre-transplant single BAP and dopamine (a cytokinin antagonist) sprays on the growth of Brussels sprout (Brassica oleracea var. gemmifera) plants grown in 200 plug cells tray-1, during different times of the annual cropping period. The responses of dopamine-sprayed plants were not completely similar to those previously reported. The different physiological processes studied in dopamine-sprayed plants through some growth parameters showed that dopamine affected mainly the rate of leaf appearance, which in turn positively affected both individual and total leaf area expansion. Dopamine-sprayed plants showed a higher leaf source, which allowed them to accumulate a higher biomass on both a fresh and dry weight basis during the initial vegetative growth stages. The effects of both BAP and dopamine were partially related to the environmental conditions during the experiments.

Vegetable yield is positively related to the environment and negatively affected by the pot root restriction during both the nursery and post-transplant stages. Root restriction is a physical stress imposed on the root system when plants are grown in small containers, which leads to a pronounced decrease in root and shoot growth at both the transplant and pot stages. Based on the assumption that the plant responses are mainly associated with a negative hormonal signaling from roots, some researchers have proposed that these abiotic stresses may be overridden by using a pre-transplant spray with benzyl amino purine (BAP), a synthetic cytokinin able to regulate plant metabolism. Although the physiological mechanisms induced by BAP have been described, the implementation of commercial applications of BAP for vegetables is still a pending issue. The aim of this work was to analyze growth changes in four lettuce genotypes in the presence of different root restrictions degree by the use of different plug cell volumes but sprayed with a single BAP spray under the hypothesis that it would play a role as abiotic stress alleviators. Our results showed that the higher biomass accumulation in lettuce plants non root-limited and BAP-sprayed ones are supported by higher photosynthetic rates, by higher leaf number initiation and expansion and by photo assimilate partition to shoots. Understanding the plant responses to this hormonal manipulation and the physiological mechanism involved will allow adjusting the agronomic advice for different vegetables and reaching commercial yields to each of them.

Although nursery lettuce information is plentiful, the use of exogenous cytokinin (6, benzyl amino purine, BAP) as a plant growth regulator, which would increase vegetable nursery yield has only recent suggested. On the other hand, biomass relationships between plug cell volumes, BAP uses and main temperature during nursery is quite scarce. Our results showed that the growth response of lettuce seedlings during nursery is firstly related to mean air temperature, independently the environmental characteristics of the cultivar tested. From the view of a nursery grower, yield can be optimize when higher plug cell number per tray was used or a BAP spray was applied to limited plug trays (128- or 288-cells). One of the novelty result of this work reside in the objective protocol proposed (the marginal root dry weight accumulation rate) to decide the time to transplant. Finally, our results showed that plug purchaser would be change their visual criteria when they buy BAP-treated lettuce for increased field yield.

Although vegetable nursery growers decide on plug sizes based on the types, production time and schedule of the crops to grow, they usually choose individual small cell sizes because these allow short plant-raising periods and reduced costs. However, larger plugs produce a finished plant after transplant in a shorter period of time than smaller plugs. Nursery growers end the propagation period when roots take up the plug cell and plantlets can be removed from the plug tray without damage. Thus, this study aimed to investigate the effect of three plug-cell sizes and a single BAP application (100 mg L-1) on the shoot and root growth and time to transplant of different lettuce genotypes. Nursery decreased as plug cell volume increased and with the single BAP spray. A novelty result was that the transplant dates were assigned when the marginal root dry weight accumulation decreased, i.e., based on objective rather than on subjective observations. The significant leaf area and dry weight accumulation found could be explained by growth parameters such as the rate of leaf appearance, the relative rate of leaf area expansion, the relative growth rate, the net assimilation rate and the partition coefficient from root: shoot allometries. Highlights Root restriction due to small plugs in transplant trays delay lettuce transplant. Exogenous cytokinin (BAP) overcame root restriction and decreased time to transplant. Time to transplant can be addressed on an objective basis: it were assigned when the marginal root dry weight accumulation decreased.