Effects of water deficit on radicle apex elongation and solute accumulation in Zea mays L

In this study, we examined the effects of water deficit on the elongation of radicles of maize seedlings and on the accumulation of solutes in the radicle apices of two maize varieties: VS-22 (tolerant) and AMCCG-2 (susceptible). Sections of radicle corresponding to the first 2 mm of the primary roots were marked with black ink, and the seedlings were allowed to grow for 24, 48, and 72 h in polyvinyl chloride (PVC) tubes filled with vermiculite at three different water potentials (Ψw, −0.03, −1.0, and −1.5 MPa). The radicle elongation, sugar accumulation, and proline accumulation were determined after each of the growth periods specified above. The Ψw of the substrate affected the dynamics of primary root elongation in both varieties. In particular, the lowest Ψw (−1.5 MPa) inhibited root development by 72% and 90% for the VS-22 and AMCCG-2 varieties, respectively. The osmotic potential (Ψo) was reduced substantially in both varieties to maintain root turgor; however, VS-22 had a higher root turgor (0.67 MPa) than AMCCG-2 (0.2 MPa). These results suggest that both varieties possess a capacity for osmotic adjustment. Sugar began to accumulate within the first 24 h of radicle apex growth. The sugar concentration was higher in VS-22 root apices compared to AMCCG-2, and the amount of sugar accumulation increased with a decrease in Ψw. Significant amounts of trehalose accumulated in VS-22 and AMCCG-2 (29.8 μmol/g fresh weight [FW] and 5.24 μmol/g FW, respectively). Starch accumulation in the root apices of these two maize varieties also differed significantly, with a lower level in VS-22. In both varieties, the proline concentration also increased as a consequence of the water deficit. At 72 h, the proline concentration in VS-22 (16.2 μmol/g FW) was almost 3 times greater than that in AMCCG-2 (5.19 μmol/g FW). Trehalose also showed a 3-fold increase in the tolerant variety. Accumulation of these solutes in the root growth zone may indicate an osmotic adjustment (OA) to maintain turgor pressure.