L. Huché-Thélier

Genotypic differences in architectural and physiological responses to water restriction in rose bush

The shape and, therefore, the architecture of the plant are dependent on genetic and environmental factors such as water supply. The architecture determines the visual quality, a key criterion underlying the decision to purchase an ornamental potted plant. The aim of this study was to analyze genotypic responses of eight rose bush cultivars to alternation of water restriction and re-watering periods, with soil water potential of -20 and -10 kPa respectively. Responses were evaluated at the architectural level through 3D digitalization using six architectural variables and at the physiological level by measuring stomatal conductance, water content, hormones [abscisic acid (ABA), auxin, cytokinins, jasmonic acid, and salicylic acid (SA)], sugars (sucrose, fructose, and glucose), and proline. Highly significant genotype and watering effects were revealed for all the architectural variables measured, as well as genotype × watering interaction, with three distinct genotypic architectural responses to water restriction – weak, moderate and strong – represented by Hw336, ‘Baipome’ and ‘The Fairy,’ respectively. The physiological analysis explained, at least in part, the more moderate architectural response of ‘Baipome’ compared to ‘The Fairy,’ but not that of Hw336 which is an interspecific hybrid. Such physiological responses in ‘Baipome’ could be related to: (i) the maintenance of the stimulation of budbreak and photosynthetic activity during water restriction periods due to a higher concentration in conjugated cytokinins (cCK) and to a lower concentration in SA; (ii) a better resumption of budbreak during the re-watering periods due to a lower concentration in ABA during this period. When associated with the six architectural descriptors, cCK, SA and ABA, which explained the genotypic differences in this study, could be used as selection criteria for breeding programs aimed at improving plant shape and tolerance to water restriction.

Impacts of contrasting light on bud burst and on RwMAX1 and RwMAX2 expression in rose

Bud burst is a crucial factor in plant architecture and is strongly induced by light. In Rosa sp., this light effect was correlated with the growth of axillary buds and RwMAX1 and RwMAX2 expression within buds. In this paper, we investigated whether strigolactone pathway is involved in the regulation of axillary bud in response to light intensity. Hence, young roses were subjected to two contrasting light intensity regimes: high/high and high/low. The phenotype was characterized in both conditions and the expression of RwMAX1 and RwMAX2 genes was measured in the basal, middle and apical parts of rose primary branch. Light treatments showed a strong impact on axillary bud. The percentage of bud burst was severely reduced in the treatment high/low compared to the treatment high/high in all branch parts. In addition, the expression of RwMAX1 and RwMAX2 was strongly inhibited by high/high light regime and was conversely correlated with the rate of bud burst. In in vitro-grown axillary buds supplied with sucrose, glucose and fructose, RwMAX1 expression was significantly stimulated whereas that of RwMAX2 was significantly inhibited. Our results suggest that although RwMAX1 and RwMAX2 expression can be regulated by light, this expression does not explain the ability of bud burst.

Effect of fertigation timetable on growth and nitrogen content of an ornamental shrub (Ligustrum ovalifolium L.)

In order to limit nitrate leaching without affecting growth of soilless culture, a better adjustment between fertilisation timetable and nitrogen (N) seasonal demand of plants is required. One-year-old Ligustrum ovalifolium L. were grown outdoors in containers for two years. The first year plants received three different fertigation -water and nutrients- timings (continuous, during spring and autumn or only during spring). The second year, only water was provided, compelling plants to use their reserves for regrowth. Growth and total N were determined for nineteen destructive harvests along the two years. Stopping fertigation in summer had no immediate effect on growth. Restoring fertigation in autumn (did not) increase current growth but allowed N pool reconstitution. Plants receiving continuous fertigation and spring and autumn fertigation had an identical regrowth. Regrowth was linked to an identical N status of both plants in winter. Treatment combining spring and autumn fertigation allowed to limit nitrate supply and to ensure a sufficient N storage to support regrowth.

Absorption, storage and mobilisation of nitrogen and phosphorus in an ornamental shrub (Ligustrum ovalifolium) during a two year fertilisation experiment.

For soilless ligneous plant culture, improving the fertilisation schedule requires better knowledge of storage dynamics and nutrient remobilisation. Ligustrum ovalifolium was grown for fifteen months with two different fertilisation treatments during the first year [spring, summer and autumn (SsA) or spring and autumn (SA) and no fertilisation the second year. Growth, nitrogen (N) and phosphorus (P) contents were measured throughout the experiment (18 sampling dates) in roots, old and young leaves, old and young stems. Stopping fertilisation during the summer of the first year affected neither the next spring plant regrowth nor the N and P remobilisation towards young ramifications. The evolution of N and P contents in the whole plant was strongly linked. Nitrogen was preferentially stored in and mobilised from the perennial aerial parts of the plant whereas phosphorus was stored in and mobilised from all the perennial parts of the shrub.