Comparison of measurement methods of osmotic adjustment in rice cultivars

Osmotic adjustment (OA) is a major component of drought resistance. Four different methods for measuring OA in plants are in general use, but there is no information on the comparative performance of these methods. Two similar experiments were designed to evaluate the four methods for measuring OA in diverse indica and japonica cultivars of rice (Oryza sativa L.) subjected to a drying cycle in large pots in the greenhouse. The four methods were: (i) derivation of OA from regressions of leaf relative water content (RWC) on leaf osmotic potential (OP); (ii) estimation of OA from OP of stressed plants calculated to rehydrated state; (iii) estimation of OA from OP of stressed plants that have been rehydrated; and (iv) estimation (from data used in Method 1) of OA capacity by the sustained RWC at a given OP of -3.5 MPa. Method 1 was a priori considered as the best estimate. Under relatively mild atmospheric conditions and a slow development of water deficit (first experiment), mean OA over 12 cultivars was 0.89, 0.51, and 0.72 MPa by Methods 1, 2, and 3, respectively. Mean RWC at -3.5 MPa was 69.3%. Significant (P less than or equal to 0.05) variation in OA among cultivars was observed by all methods, up to a four-fold difference in OA among cultivars (0.35-1.51 MPa) by Method 1. Simple correlation for OA across 12 cultivars with Method 1 was significantly higher for Method 3 (r = 0.76; P = 0.04) and Method 4 (r = 0.87: P < 0.01) than for Method 2 (r = 054; P = 0.07). OA by Method 4 was better correlated with Method 3 (r = 0.80; P < 0.01) than with Method 2 (r = 0.67; P = 0.02). The coefficient of variation (CV) as a measure of error was greater for Method 1 (47%) and Method 2 (31%) than for Method 3 (21%) or 4 (24%), Both Methods 2 and 3 were less demanding on labor and plant materials than Methods 1 and 4. The results support the use of Method 3 (the "rehydration method") as a faster and an economical replacement for Method 1.