Leaf mass per area predicts palisade structural properties linked to mesophyll conductance in balsam poplar (Populus balsamifera L.)

Modifications to leaf structural components that drive variation in leaf mass per area (LMA) may substantially impact leaf physiology by changing how easily CO2 diffuses through intercellular air space to carboxylation sites in mesophyll tissues. Mesophyll conductance (gm) is inversely proportional to the total pathway length for CO2, including the structural resistances encountered. In balsam poplar (Populus balsamifera L.), gm increases with latitude, paralleled by an increase in LMA. We investigated a family of P. balsamifera (K4×C) with high variation in LMA for different characteristics (tissues, nitrogen content, ultrastructural attributes). We interpreted trait variability using a developmental scale quantified by the leaf plastochron index (LPI). Developmental age significantly affected LMA, but those effects were lost at LPI ≥ 6. We outlined contributions of anatomical components to LMA and found palisade mesophyll properties were the primary drivers of variation in LMA within mature leaves (LPI ≥ 6). Using anatomical data, we derived components corresponding to structural resistances for gm. Perimeters of palisade cells and surface area of palisade exposed to intercellular air space, which may strongly influence CO2 diffusion, were correlated to LMA. Variation in LMA is positively related to differences in structural features expected to increase the conductance to CO2 diffusion within palisade mesophyll.

Deactivation of aquaporins decreases internal conductance to CO2 diffusion in tobacco leaves grown under long-term drought

We compared the diffusion conductance to CO2 from the intercellular air space to the chloroplasts (internal conductance (g i)) between tobacco leaves acclimated to long-term drought (drought-acclimated (DA)) and those grown under sufficient irrigation (well-watered (WW)), and analysed the changes in g i in relation to the leaf anatomical characteristics and a possible CO2 transporter, aquaporin. The g i, which was estimated by combined analyses of CO2 gas exchange with chlorophyll fluorescence, in the DA plants was approximately half of that in the WW plants. The mesophyll and chloroplast surface areas exposing the intercellular air space, which potentially affect g i, were not significantly different between the WW and DA plants. The amounts of plasma membrane aquaporins (PIP), immunochemically determined using radish PIP antibodies, were unrelated to g i. After treatment with HgCl2, an aquaporin inhibitor, the water permeability of the leaf tissues (measured as the weight loss of fully-turgid leaf disks without the abaxial epidermis in 1 m sorbitol) in WW plants decreased with an increase in HgCl2 concentration. The g i in the WW plants decreased to similar levels to the DA plants when the detached leaflets were fed with 0.5 mm HgCl2. In contrast, both water permeability and g i were insensitive to HgCl2 treatments in DA plants. These results suggest that deactivation of aquaporins is responsible for the significant reduction in g i observed in plants growing under long-term drought.

Firmness Decline in `Gala' Apple during Fruit Development

Puncture force was measured in `Gala'apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] fruit from 16 to 175 days after full bloom over 2 years using a range of circular flat-tipped probes (1 to 11 mm diameter) to test the firmness of each fruit. The area-dependent (Ka) and perimeter-dependent (Kp) coefficients of puncture force were determined and were used to calculate the indicative puncture force approximating a standard 11.1-mm-diameter Effegi/Magness-Taylor probe for even the smallest fruit. Ka declined exponentially throughout fruit development with much greater changes occurring closer to bloom. In contrast, maximum Kp occurred at 107 to 119 days after full bloom before declining progressively. Estimated firmness (using a 11.1-mm-diameter probe) declined constantly from 16 days after full bloom. Ka was associated with developmental changes in cortical tissue intercellular air space, cell volume and cell packing density although relationships changed throughout fruit growth. However seasonal change in Kp was not associated with any obvious anatomical change in the cortex.

LEAF ANATOMY OF MICROPROPAGATED GRAPE AFFECTED BY REDUCED WATER POTENTIAL

Grape cv. Valiant was micropropagated in an MS medium with and without 2% (W/V) of polyethylene glycol (PEG, MW 8000). Leaf anatomy of control (in vitro, no PEG), treated (in vitro, PEG), field grown and greenhouse grown plants were compared under light microscopy. Cell size, palisade layer formation, relative intercellular air space and apparent chloroplast number varied between the leaves of control and PEG treated (high osmoticum) plantlets. These leaf characteristics in the high osmoticum medium appeared more similar to the leaves of the greenhouse and field grown plants. Leaves from control plantlets contained cells of larger size, lacked normal palisade layer formation, greater intercellular pore spaces and fewer chloroplasts. Leaves of PEG treated plantlets had smaller cells, a more defined palisade layer, reduced intercellular pore spaces and greater number of chloroplasts. Leaves of greenhouse and field grown plants had small cells, a well-defined palisade layer, least intercellular pore space and greatest number of chloroplasts. These results demonstrate that a high osmoticum medium may be used to induce more normal leaf development.