Concentration and Delivery of Abscisic Acid in Xylem Sap are Greater at the Shoot Base than at a Target Leaf Nearer to the Shoot Apex

Abstract: Samples of xylem sap from 5-week-old Ricinus corn-munis L. were obtained after severing a lamina, or shoot, from plants pressurized at the roots with air to raise hydrostatic xylem water potentials to atmospheric. In situ sap flow gauges, and mass flow measurements, showed that removing the lamina approximately doubted sap flow rate through the petiole stub that remained attached to the plant. This was a consequence of flow out of the roots being diverted along this low-resistance pathway and away from leaves higher in the canopy. Leaf and whole shoot excision temporarily released extra solutes in to sap as it discharged from the cut petiole or from the hypo-cotyl stump. This contamination prevented the use of sap extracted from detached lamina by overpressurizing in a Scholan-der bomb. To minimise distortions to sap flow and wound-induced contamination, estimates of in planta concentration and delivery (concentration × sap flow rate) of ABA and osmolality in xylem sap were made using sap flow rates measured before excision and concentrations in flowing sap collected approximately 30 mm after excision. At this time, effects of excision on solute contamination had subsided. The approach revealed that withholding water from upper roots increased ABA delivery from roots into the shoot base 3-fold. However, approximately half this ABA was lost en route to the youngest fully open leaf. This loss of ABA may explain the slow stomatal response to drying of upper roots shown by R. communis .     

Control of Leaf Growth and its Role in Determining Variation in Plant Growth Rate from an Ecological Perspective

Abstract: Plants vary widely in their relative growth rate (RGR), be it dependent on environmental conditions or due to their genetic background. In a comparison of the RGR of grasses growing under different environmental conditions, variation in RGR tends to correlate with that in the leaf elongation rate (LER). When different species or genotypes thereof are compared under identical growing conditions, variation in LER may or may not correlate with that in RGR, depending on the comparison. However, since RGR is described by an exponential equation, whereas LER is mainly a linear process, we conclude that any correlation between RGR and LER must be fortuitous. That is, exponential growth must be due to increases with time in plant traits such as 1) leaf dry mass per unit leaf length invested per unit time, and/or 2), i.e., the total LER of all the growing leaves at one point in time. The latter can be achieved as follows: 1) each subsequent leaf has a higher LER than the preceding one; 2) leaves appear at an increasing rate; 3) the duration of the process of leaf elongation increases for subsequent leaves. In this review, we only explore possible factors that account for changes in with time, in different genotypes and under different environmental conditions. Inherent variation in LER of individual leaves and variation due to environmental factors may reflect variation in the rate of cell division and/or in cell elongation.