Shade promotes thorn development in a tropical liana, Artabotrys hexapatalus (Annonaceae)

The liana Artabotrys hexapetalus (L.f.) Bhand., which is widely planted in the Tropics and native to African rain forests, produced new reiterations (new leader shoots) normally and after damage induced by Hurricane Andrew (August 24, 1992). In each new orthotropic shoot, there is a gradient in lateral branch structures from basal thorns, to vegetative leafy branches, to distal leafy flowering branches. We noted that reiterations developing in shade had more thorns than similar reiterations developing in full sun. Tents with clear (66% photosynthetically active radiation [PAR]) and shaded plastic film (12%–14% PAR) were placed over nodes when the axillary buds began to expand to produce reiteration shoots. After 2 mo of growth inside the tents and in the open, the types of lateral outgrowths (thorn vs. branch) were recorded. Shoots in spectrally neutral shade (red to far red of full sun) and spectrally altered shade (red to far red of canopy shade) produced significantly more thorns at the lower nodes of the shoots as compared to those in full sun. Shoots in control clear plastic tents were the same as those in full sun. We conclude that the fate of lateral bud development is controlled by irradiance (light level) but not by light quality. Increased thorn production in shade could be advantageous to plants growing in the deep shade of rain forests. Thorns in the self-shaded regions of the plant, and well below the forest canopy, could aid in protection from herbivory and in climbing by acting as hooks.

The significance of chloroplast movement in leaves of tropical plants

The purpose of this research project was to contribute to the understanding of chloroplast movement in plants. Chloroplast movement in leaves from twenty tropical plant species ranging from cycads to monocots and varying in shade tolerance was examined by measuring changes in transmittance following 30 min. of exposure to white light at 1000 μmol m−2 s −1 in the wavelength range of 400–700 nm (photosynthetically active radiation, PAR). Leaf anatomical characteristics were also measured. Eighteen species increased significantly in transmittance (Δ T) at this level of illumination. ^ Chloroplast movement was significantly correlated with palisade cell width suggesting that cell dimensions are a significant constraint on chloroplast movement in the species examined. In addition, Δ T values were strongly correlated with values of an index of shade tolerance. ^ To further examine the relationship between palisade width and chloroplast movement, additional studies were conducted with a tropical aroid vine, Scindapsus aureus Schott. Scindapsus plants were grown under three different light treatments: 63% (control), 9.0% and 2.7% of full sunlight. Under these growing conditions plants produced markedly different palisade cell widths. Palisade cell width was again found to be correlated with transmittance changes. In addition, the observed increases in transmittance following exposure to the above illumination condition were correlated with absorbance of PAR. Fluorescence studies demonstrated that chloroplast movement helps protect Scindapsus aureus from the effects of photoinhibition when it is exposed to light at a higher intensity relative to the intensity of its normal environment. Ratios of variable fluorescence (Fv) to maximal fluorescence (Fm ) were higher in plants exposed to high light when chloroplasts moved than in plants where chloroplasts did not. ^ To further explore the role of chloroplast movement, studies were conducted to determine if transmittance changes could be induced in ten xerophytes at (1000 μmol m−2 s−1), as well as two stronger light intensities (1800 μmol m−2 s−1 and 2200 μmol m−2 s −1). Transmittance changes in the ten xerophytes were dependent upon the illumination intensity; nine out of the ten xerophytes changed in transmittance at 1800 μmol m−2 s−1. For the other two intensity levels, only three out of the ten xerophytes tested exhibited transmittance changes, and for two species, a negative Δ T value was obtained at 1000 μmol m−2 s−1 . No relationship was found between cell dimensions and chloroplast movement, although all species had large enough chlorenchyma cells to allow such movements. ^ The results of the study clearly show that in non-xerophytes, palisade cell anatomy is a strong constraint on chloroplast movement. This relationship may be the basis for the relationship between chloroplast movement and shade tolerance. Although absorbance changes are relatively small, chloroplast movement was clearly shown to reduce photoinhibition. ^

Anatomical and morphological responses of papaya, Carica papaya L., to various light conditions

Plants that develop under foliar shade encounter both low photosynthetically active radiation (PAR) and low red to far red ratios (R:FR). Both of these factors are important in determining developmental responses to shade. Papaya (Carica papaya L.) seedlings grown under filtered shade (low PAR and low R:FR) were compared with seedlings grown under neutral shade (low PAR with R:FR similar to that of full sunlight), and high light (moderate PAR with R:FR similar to that of full sunlight). The results indicated that papaya exhibits a light seeking strategy as evidenced by morphological and anatomical differences between treatments. Based on past research the results also indicate shade developmental responses in papaya to be phytochrome mediated.