Impact of Light Intensity on Leaf Initiation in Young Cucumber and Tomato Plants: A Matter of Photosynthates Availability?

Presence of light per se is essential for triggering the process of leaf initiation in the shoot apical meristem (SAM). Light is also essential for photosynthates production and thus its intensity largely determines the energy available for plant growth and development. The aim of this study is to quantify the effect of light intensity on leaf initiation in tomato and cucumber. In greenhouses these two species are grown under almost similar conditions. However, previous research indicated that, in contrast to cucumber, leaf initiation in tomato is hardly affected by photosynthates availability. In this study vegetative plants of both species were subjected to a range of low to intermediate light intensities (40 - 240 µmol PAR.m-2.s-1) and leaf initiation rates were quantified. Higher rates were observed in cucumber. Both species showed no change in leaf initiation rate above 100 µmol.m-2.s-1 and a similar relative decline in leaf initiation rate below ~100 µmol.m-2.s-1. At 80 and 40 µmol.m-2.s-1 leaf initiation rate was reduced by ~10% and ~20% respectively compared to 240 µmol.m-2.s-1. Additional measurements were conducted at the highest (240 µmol.m-2.s-1; HL) and lowest applied light intensity (40 µmol.m-2.s-1; LL). Both species showed a decline of about 80% in shoot biomass between HL and LL. Diel respiration, indicator of the photosynthates utilization, was measured on terminal buds (SAM + surrounding folded leaves). Respiration was substantially lower at LL than at HL in both species. For both species total soluble sugars and starch concentrations in the 6th leaf (sunlit leaf) and in the terminal bud were significantly lower at LL than at HL, indicating a lower carbohydrates availability. Our results suggest that leaf initiation rates in vegetative cucumber and tomato are only affected when light intensity is very low, implying priority of these species in producing new leaves.