Variation of Leaf Characteristics With Level of Insertion on a Grass Tiller. III. Tissue Water Relations

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Abstract
Plants of a grass, green panic (Panicum maximum var. trichoglume) were grown under controlled environmental and nutritional regimes. Throughout the growth of the plants, leaves from varying levels of insertion on the main stem were sampled at comparable physiological age (viz, when just fully expanded) and their water relations characteristics measured over a range from full turgidity to a moderately stressed condition to determine whether there were inherent differences between leaves on the shoot. Leaf water potential, osmotic potential, and pressure potential fell in a similar manner for the leaves from each insertion level as leaf relative water content declined. A small increase in the average water and osmotic potential, and also in the average matric potential component, estimated for a common reIative water content, was evident with increasingly higher level of leaf insertion. Despite marked increases in the proportion of cell wall material and a trend towards increasing lignification in leaves of higher insertion, the bulk coefficient of elasticity of the tissues showed no gradient of change, and the overall change in the matric potential component was small. Various other chemical and morphological characteristics showed definite gradients with insertion level: for example, stomatal frequency increased from 91 to 287 mm-2 from leaf 5 to the flag leaf. There was no evidence of a progressive variation with insertion level in the water status of leaves sampled fresh from the plants, and thus no support for the theory that upper leaves on a tiller develop under inherently higher water stress, which could account for their xeromorphic chemical, anatomical and morphological features. It appears that in green panic at least, the water relations characteristics measured, with the exception of stomatal frequency, exhibit no strong inherent gradients with insertion level of leaves which could seriously confound comparisons of water status between plants in different treatments.