Elsje Londers

Glucuronoarabinoxylan structure in the walls of Aechmea leaf chlorenchyma cells is related to wall strength

In CAM-plants rising levels of malic acid in the early morning cause elevated turgor pressures in leaf chlorenchyma cells. Under specific conditions this process is lethal for sensitive plants resulting in chlorenchyma cell burst while other species can cope with these high pressures and do not show cell burst under comparable conditions. The non-cellulosic polysaccharide composition of chlorenchyma cell walls was investigated and compared in three cultivars of Aechmea with high sensitivity for chlorenchyma cell burst and three cultivars with low sensitivity. Chlorenchyma layers were cut from the leaf and the non-cellulosic carbohydrate fraction of the cell wall fraction was analyzed by gas-liquid chromatography. Glucuronoarabinoxylans (GAXs) were the major non-cellulosic polysaccharides in Aechmea. The fine structure of these GAXs was strongly related to chlorenchyma wall strength. Chlorenchyma cell walls from cultivars with low sensitivity to cell burst were characterized by an A/X ratio of ca. 0.13 while those from cultivars with high sensitivity showed an A/X ratio of ca. 0.23. Xylose chains from cultivars with high cell burst sensitivity were ca. 40% more substituted with arabinose compared to cultivars with low sensitivity for cell burst. The results indicate a relationship in vivo between glucuronoarabinoxylan fine structure and chlorenchyma cell wall strength in Aechmea. The evidence obtained supports the hypothesis that GAXs with low degrees of substitution cross-link cellulose microfibrils, while GAXs with high degrees of substitution do not. A lower degree of arabinose substitution on the xylose backbone implies stronger cell walls and the possibility of withstanding higher internal turgor pressures without cell bursting.

Impact of developmental stage on CAM expression and growth in an Aechmea hybrid under greenhouse conditions

Summary Besides environmental factors, expression of crassulacean acid metabolism (CAM) and the related production of malic acid may depend on plant developmental stage. This topic has not yet been investigated under commercial greenhouse conditions. In CAM plant cultivation, malic acid is a central determinant of plant growth, but can also cause physiological leaf damage problems. Here, we present data on diurnal leaf malic acid contents, relative water content dynamics, and measurements of growth at four stages of the plant growth cycle (i.e., ex vitro-acclimated plantlets, 6-month, 12-month, and 18-month-old plants) of Aechmea ‘Maya’. The results obtained showed that Aechmea ‘Maya’ was an obligate CAM plant at all developmental stages during cultivation. Nevertheless, there was some developmental control on the expression of CAM. Under the same environmental conditions, 6-month-old plants accumulated significantly higher amounts of malic acid in their leaves, thus supporting higher growth rates, than in the other growth stages. This larger malic acid pool implied a higher risk of physiological leaf damage, but since remobilisation of malic acid in the early morning remained unaffected during growth, physiological leaf damage might occur throughout vegetative growth.

Impact of fertiliser level on plant growth, plant shape, and physiological leaf damage in two cultivars of Aechmea characterised by crassulacean acid metabolism.

Summary The efficacy of different fertiliser levels was evaluated in terms of plant growth, plant shape, and leaf quality. Experiments were conducted on two cultivars of Aechmea of known sensitivity (high and low, respectively) to physiological leaf damage. Plants were grown under two fertiliser levels (a fertigation solution of 1.2 mS cm−1, or a fertigation solution of 2.4 mS cm−1) over a 9-month treatment period. In general, the lower level of fertiliser resulted in more desirable, compact plants, with leaf length as the major parameter determining plant shape. The higher level of fertiliser improved the growth rate significantly, but also increased the risk of leaf quality problems. The extent of this effect was strongly cultivar-dependent. In practice, growers will have to compromise between plant shape and leaf quality on the one hand, and growth rate on the other. Based on this work, either of two plant-related parameters can be used to evaluate the risk of leaf quality problems when changing fertiliser levels: (i) chlorenchymal cell dimensions at the leaf anatomical level, or (ii) turgor pressure at the leaf physiological level.