Starch and the control of kernel number in maize at low water potentials

Abstract

After reproduction is initiated in plants, subsequent reproductive development is sometimes interrupted, which decreases the final number of seeds and fruits. We subjected maize (Zea mays L.) to low water potentials (psi(w)) that frequently cause this kind of failure. We observed metabolite pools and enzyme activities in the developing ovaries while we manipulated the sugar stream by feeding sucrose (Suc) to the stems. Low psi(w) imposed for 5 d around pollination allowed embryos to form, but abortion occurred and kernel number decreased markedly. The ovary contained starch that nearly disappeared during this abortion. Analyses showed that all of the intermediates in starch synthesis were depleted. However, when labeled Suc was fed to the stems, label arrived at the ovaries. Solute accumulated and caused osmotic adjustment. Suc accumulated, but other intermediates did not, showing that a partial block in starch synthesis occurred at the first step in Suc utilization. This step was mediated by invertase, which had low activity. Because of the block, Suc feeding only partially prevented starch disappearance and abortion. These results indicate that young embryos abort when the sugar stream is interrupted sufficiently to deplete starch during early ovary development, and this abortion results in a loss of mature seeds and fruits. At low psi(w), maintaining the sugar stream partially prevented the abortion, but invertase regulated the synthesis of ovary starch and partially prevented full recovery.

Figure 1

Schedule of watering and infusions for the low ψ_w treatment. Water was withheld at the times shown by the black bar. Infusions were made at the end of the dark periods, as shown by the white bar. Arrowheads 1 through 5 on the plant show infusion sites at each internode and correspond with arrowheads 1 through 5 on the white bar. A single infusion was made at sites 1 and 2. Two infusions were made at sites 3 to 5 to ensure uptake of 6.8 g of Suc.

Figure 2

Starch location and amount in maize ovaries and developing kernels. A, Ovary on d −5 before pollination. Starch (region stained black) is located at the ovary base around the phloem and in the ovary tissues around the nucellus (ovary wall). The nucellus occupies the center of the ovary and is starchless. The embryo sac is not present in this section. Silk normally attached to the ovary apex has been removed. W, Ovary wall; N, nucellus, V, vein. B, Ovary at pollination on d 0. The location of starch is similar to that in A. Magnification is the same as in A. C, Ovary on d 10 after pollination. Endosperm occupies most of the ovary interior, and the nucellus is at the periphery, inside the starch-containing ovary wall. D, Ovary on d 12. Starch deposition is beginning in the endosperm. The nucellus is at the periphery, inside the starch-containing ovary wall. Magnification is the same as in C. E, Ovary on d 23. Starch is being rapidly deposited in the endosperm. The nucellus and ovary wall are forming the outer covering of the caryopsis. F, Starch content of the ovaries as they develop into mature caryopses. Bars in A, C, and E indicate 1 mm for all micrographs.

Figure 3

Starch location and amount in maize ovaries around the time of pollination, showing the effects of Suc fed to the stems at low ψ_w. A, Ovary on d −5 before pollination. Bar indicates 1 mm for all micrographs. B, Ovary on d −2 before pollination. C, Ovary on d 0 at pollination. D, Ovary on d 2 after pollination. Note that starch (region stained black) is present at the ovary base around the phloem and in the ovary tissues around the nucellus. E through G, Same as B through D except water was withheld on d −5 before pollination and resupplied on d 1 after pollination, and stems were fed with Suc solution starting on d −4. Starch location is unchanged from B through D. H through J, Same as E through G except no Suc was fed to the stems. In I and J, starch has nearly disappeared from the ovary base around the phloem and the ovary tissues around the nucellus. K, Starch contents of ovaries in micrographs. The black bar on the x axis shows time when water was withheld from the soil and the white bar shows time when Suc was infused into the stems, as in Figure 1. Data are means ± sd of six plants. ○, Control; ▪, low ψ_w + Suc infusion; •, low ψ_w.

Figure 4

Ovary dry mass showing effects of Suc fed to the stems at low ψ_w. The black bar on the x axis shows time when water was withheld from the soil and the white bar shows time when Suc was infused into the stems, as in Figures 1 and 3. Data are means ± sd for six plants. ○, Control; •, low ψ_w; ▪, low ψ_w + Suc infusion.

Figure 5

Ovary δ¹³C ratios showing effects of sugar beet Suc fed to the stems at low ψ_w around the time of pollination. The black bar on the x axis shows time when water was withheld from the soil and the white bar shows time when Suc was infused into the stems, as in Figures 1 and 3. Data are means ± sd for three plants. ○, High ψ_w; ▵, high ψ_w + sugar beet Suc; •, low ψ_w; ▪, low ψ_w + sugar beet Suc.

Figure 6

Effects of withholding water and feeding Suc to the stems around the time of pollination on ψ_w (A), ψ_s (B), and ψ_p (C). On the x axis, the black bar shows time when water was withheld from the soil and the white bar shows time when Suc was infused into the stems, as in Figures 1 and 3. Data are means ± sd for six plants. ○, Control; •, low ψ_w; ▪, low ψ_w + Suc infusion.

Figure 7

Ovary Suc content, acid invertase activity, and reducing sugar content showing effects of withholding water and feeding Suc to the stems around the time of pollination. A, Suc (invertase substrate). B, Soluble acid invertase activity. C, Insoluble acid invertase activity. D, Reducing sugars (invertase product). The black bar on the x axis shows time when water was withheld from the soil and the white bar shows time when Suc was infused into the stems, as in Figures 1 and 3. Data are means ± sd for six plants. ○, Control; •, low ψ_w; ▪, low ψ_w + Suc infusion.

Figure 8

Ovary contents of Glc-6-P (A), Fru-6-P (B), and Glc-1-P (C) showing effects of withholding water and feeding Suc to the stems around the time of pollination. The black bar on the x axis shows time when water was withheld from the soil and the white bar shows time when Suc was infused into the stems, as in Figures 1 and 3. Data are means ± sd for six plants. ○, Control; •, low ψ_w; ▪, low ψ_w + Suc infusion.

Figure 9

Ovary contents of UDP-Glc (A), and Pi (B) showing effects of withholding water and feeding Suc to the stems around the time of pollination. The black bar on the x axis shows time when water was withheld from the soil and the white bar shows time when Suc was infused into the stems, as in Figures 1 and 3. Data are means ± sd for six plants. ○, Control; •, low ψ_w; ▪, low ψ_w + Suc infusion.

Figure 10

Ovary concentrations of 3-phosphoglyceric acid (A) and Pi (B) showing effects of withholding water and feeding Suc to the stems around the time of pollination. Concentrations are based on the total water content of the ovaries. The black bar on the x axis shows time when water was withheld from the soil and the white bar shows time when Suc was infused into the stems, as in Figures 1 and 3. Data are means ± sd for six plants. ○, Control; •, low ψ_w; ▪, low ψ_w + Suc infusion.

Figure 11

Amylase activity in ovaries showing effects of withholding water and feeding Suc to the stems around the time of pollination. The black bar on the x axis shows time when water was withheld from the soil and the white bar shows time when Suc was infused into the stems, as in Figures 1 and 3. Data are means ± sd for six plants. ○, Control; •, low ψ_w; ▪, low ψ_w + Suc infusion.

Figure 12

Dynamics of ovary starch leading to embryo abortion at low ψ_w. A, Suc is delivered to the tissues below the ovary (large arrow) and either used in developing structures or stored as starch around the veins and in the ovary tissues around the nucellus (small arrows). B, With inhibited photosynthesis, Suc delivery is curtailed (▪ ▪ ▪), starch is mobilized (small arrows), and ovary development continues. C, When the starch is depleted, ovary development ceases irreversibly. The final kernel number is diminished accordingly.

Figure 13

Likely pathway of starch biosynthesis in maize ovaries around the time of pollination. Bold print shows that Suc accumulated with Suc feeding at low ψ_w. Light print shows that all other metabolites were depleted despite Suc feeding at low ψ_w. The depletion indicates that a block occurs at the first step in the pathway (acid invertase).