Rapid estimates of relative water content.

Relative water content may be accurately estimated using the ratio of tissue fresh weight to tissue turgid weight, termed here relative tissue weight. That relative water content and relative tissue weight are linearly related is demonstrated algebraically. The mean value of r(2) for grapevine (Vitis vinifera L. cv. Shiraz) leaf tissue over eight separate sampling occasions was 0.993. Similarly high values were obtained for maize (Zea mays cv. Cornell M-3) (0.998) and apple (Malus sylvestris cv. Northern Spy) (0.997) using a range of leaf ages. The proposal by Downey and Miller (1971. Rapid measurements of relative turgidity in maize (Zea mays L.). New Phytol. 70: 555-560) that relative water content in maize may be estimated from water uptake was also investigated for grapevine leaves; this was found to be a less reliable estimate than that obtained with relative tissue weight. With either method, there is a need for calibration, although this could be achieved for relative tissue weight at least with only a few subsamples.