Atmospheric CO and VPD alter the diel oscillation of leaf elongation in perennial ryegrass: compensation of hydraulic limitation by stored-growth.

We explored the effects of atmospheric CO concentration (C ) and vapor pressure deficit (VPD) on putative mechanisms controlling leaf elongation in perennial ryegrass. Plants were grown in stands at a C of 200, 400 or 800 μmol mol combined with high (1.17 kPa) or low (0.59 kPa) VPD during the 16 h-day in well-watered conditions with reduced nitrogen supply. We measured day : night-variation of leaf elongation rate (LER  : LER ), final leaf length and width, epidermal cell number and length, stomatal conductance, transpiration, leaf water potential and water-soluble carbohydrates and osmotic potential in the leaf growth-and-differentiation zone (LGDZ). Daily mean LER or morphometric parameters did not differ between treatments, but LER strongly exceeded LER , particularly at low C and high VPD. Across treatments LER was negatively related to transpiration (R  = 0.75) and leaf water potential (R  = 0.81), while LER was independent of leaf water potential or turgor. Enhancement of LER over LER was proportional to the turgor-change between day and night (R  = 0.93). LGDZ sugar concentration was high throughout diel cycles, providing no evidence of source limitation in any treatment. Our data indicate a mechanism of diel cycling between daytime hydraulic and night-time stored-growth controls of LER, buffering C and daytime VPD effects on leaf elongation.