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International audience; Leaf development is affected by both internal (genetic) and external (environmental) regulatory factors. The aim of this work was to investigate how leaf growth variables are related to one another in a range of environments. The leaf growth variables of wild-type Arabidopsis thaliana and leaf development mutants (ang4, ron2-1, elo1, elo2 and elo4) were studied under different incident light treatments (light and shade). The leaves studied were altered in various leaf development variables, such as the duration of expansion, relative and absolute expansion rates, epidermal cell size, epidermal cell number and initiation rate. Final leaf area was correlated to maximal absolute leaf expansion rate and cell number, but not to duration of leaf expansion or cell size. These relationships were common to all studied genotypes and light conditions, suggesting that leaf size is determined early in development. In addition, the early variables involved in leaf development were correlated to one another, and initial relative expansion rate was negatively correlated to the duration of expansion. These relationships between the leaf development variables were used to construct a conceptual model of leaf size control.

We have investigated the way in which the radiation absorbed by leaves affects the rate of elongation of maize (Zea mays L.) roots. In five repeated growth chamber experiments, plants previously grown at a photon irradiance of 23 mol m–2 d–1 received either 7 or 34 mol m–2 d–1 from day 10 to day 20 after germination. The elongation rate of primary roots steadily decreased for 4 d after reduction in irradiance and then stabilized at 60% of that in plants at high irradiance. The elongating zone was slightly shorter after 2 d at low irradiance, and was further reduced after 8 d. The concentrations of sucrose and glucose in the elongating zone were greatly decreased after 2 d at low irradiance and the gradient of both sugars was suppressed. The longer period at low irradiance affected neither sugar content nor gradient. In the same way, cell production rate was reduced after 2 d at low irradiance and was not appreciably decreased afterwards. The root zone with cell division was shorter in plants at low irradiance, but cell division rate remained nearly constant temporally and spatially, and was unaffected by the irradiance treatment. Our results suggest that primary events after a reduction in irradiance were a change in cell flux and sugar content in the elongating zone. Change in elongation rate was slower and probably the result of a time-related developmental effect, which may be related to the change in cell production.

The stomatal conductance of several anisohydric plant species, including field-grown sunflower, frequently correlates with leaf water potential ([psi]1), suggesting that chemical messages travelling from roots to shoots may not play an important role in stomatal control. We have performed a series of experiments in which evaporative demand, soil water status and ABA origin (endogenous or artificial) were varied in order to analyse stomatal control. Sunflower plants were subjected to a range of soil water potentials under contrasting air vapour pressure deficits (VPD, from 0.5 to 2.5 kPa) in the field, in the glasshouse or in a humid chamber. Sunflower plants were also fed through the xylem with varying concentrations of artificial ABA, in the glasshouse and in the field. Finally, detached leaves were fed directly with varying concentrations of ABA under three contrasting VPDs. A unique relationship between stomatal conductance (gs) and the concentration of ABA in the xylem sap (xylem [ABA]) was observed in all cases. In contrast, the relationship between [psi]1 and gs varied substantially among experiments. Its slope was positive for droughted plants and negative for ABA-fed whole plants or detached leaves, and also varied appreciably with air VPD. All observed relationships could be modelled on the basis of the assumption that had no controlling effect on gs. We conclude that stomatal control depended only on the concentration of ABA in the xylem sap, and that [psi]1 was controlled by water flux through the plant (itself con-trolled by stomatal conductance). The possibility is also raised that differences in stomatal 'strategy' between isohydric plants (such as maize, where daytime [psi]1 does not vary appreciably with soil water status) and anisohydric plants (such as sunflower) may be accounted for by the degree of influence of [psi]1 on stomatal control, for a given level of xylem [ABA]. We propose that statistical relationships between [psi]1 and gs are only observed when [psi]1 has no controlling action on stomatal behaviour. (Resume d'auteur)

To test whether the inhibition of leaf expansion by high evaporative demand is a result of hydraulic processes, we have followed both leaf elongation rate (LER) and cell turgor in leaves of maize plants either normally watered or in water-saturated soil in which hydraulic resistance at the soil-root interface was abolished. Cell turgor was measured in situ with a pressure probe in the elongating zone of the first and sixth leaves, and LERs of the same leaves were measured continuously with transducers or by following displacements of marks along the growing leaves. Both variables displayed spatial variations along the leaf and positively correlated within the elongating zone. Values peaked at mid-distance of this zone, where the response of turgor to evaporative demand was further dissected. High evaporative demand decreased both LER and turgor for at least 5 h, with dose-effect linear relations. This was observed in five genotypes with appreciable differences in turgor maintenance among genotypes. In contrast, the depressing effects of evaporative demand on both turgor and LER disappeared when the soil was saturated, thereby opposing a negligible resistance to water flow at the soil-root interface. These results suggest that the response of LER to evaporative demand has a hydraulic origin, enhanced by the resistance to water flux at the soil-root interface. They also suggest that turgor is not completely maintained under high evaporative demand, and may therefore contribute to the reductions in LER observed in non-saturated soils.