An analysis of major U.S. crops shows that there is a large genetic potential for yield that is unrealized because of the need for better adaptation of the plants to the environments in which they are grown. Evidence from native populations suggests that high productivity can occur in these environments and that opportunities for improving production in unfavorable environments are substantial. Genotypic selection for adaptation to such environments has already played an important role in agriculture, but the fundamental mechanisms are poorly understood. Recent scientific advances make exploration of these mechanisms more feasible and could result in large gains in productivity.

Invertase (INV) hydrolyzes sucrose into glucose and fructose, thereby playing key roles in primary metabolism and plant development. Based on their pH optima and sub-cellular locations, INVs are categorized into cell wall, cytoplasmic, and vacuolar subgroups, abbreviated as CWIN, CIN, and VIN, respectively. The broad importance and implications of INVs in plant development and crop productivity have attracted enormous interest to examine INV function and regulation from multiple perspectives. Here, we review some exciting advances in this area over the last two decades, focusing on (1) new or emerging roles of INV in plant development and regulation at the post-translational level through interaction with inhibitors, (2) cross-talk between INV-mediated sugar signaling and hormonal control of development, and (3) sugar- and INV-mediated responses to drought and heat stresses and their impact on seed and fruit set. Finally, we discuss major questions arising from this new progress and outline future directions for unraveling mechanisms underlying INV-mediated plant development and their potential applications in plant biotechnology and agriculture.

Rates of photosynthesis, dark respiration, and leaf enlargement were studied in soil-grown corn (Zea mays), soybean (Glycine max), and sunflower (Helianthus annuus) plants at various leaf water potentials. As leaf water potentials decreased, leaf enlargement was inhibited earlier and more severely than photosynthesis or respiration. Except for low rates of enlargement, inhibition of leaf enlargement was similar in all three species, and was large when leaf water potentials dropped to about -4 bars.Intact sunflower leaves were held for 4 days at leaf water potentials which permitted maximal photosynthesis and respiration, but which inhibited leaf enlargement. Although leaf enlargement did not occur initially, enlargement resumed toward the end of the desiccation period. However, the rate of enlargement was not as rapid as in the well watered control, nor did it return to the control rate when the plant was rewatered.The present study is concerned with the ways in which leaf enlargement, photosynthesis, and respiration respond to reduced leaf water potentials in three plant species. It has been suggested that cell enlargement may be more sensitive than photosynthesis to reduced leaf water potential (2). In sunflower, at least, leaf enlargement is reduced at water potentials as high as -2.5 bars, and ceases at potentials of -4 bars (2). Photosynthesis in other species is usually unaffected at these levels (5). However, a recent study (18) of carbon fixation and leaf elongation in Lolium has shown that photosynthesis and leaf enlargement are affected similarly at moderate leaf desiccation and that elongation is inhibited more strongly than photosynthesis only at low leaf water contents. The work with Lolium suggested that conclusions regarding leaf enlargement in sunflower (2) Rates of net photosynthesis and respiration were measured daily in shoots of 4-to 5-week-old intact plants with the use of an infrared gas analyzer and assimilation chamber in a semiclosed system (3). Chamber temperature was 25 + 0.25 C, relative humidity was 77 i 2%, and wind speed was 1.7 m sec-'. Under these conditions, leaf temperatures were within 0.6 C of chamber temperature. For photosynthesis, seven 300-w incandescent spotlights provided a light intensity of 1.6 cal cm-2 min-' (measured with a Moil thermopile) at leaf height, which was saturating for soybean and sunflower. The rate of photosynthesis was determined at approximately 10-min intervals by measuring the time required for the shoot to decrease the CO2 concentration in the assimilation chamber from 270 to 230 ,Ml/liter. Respiration rate was determined similarly by measuring the time required for an increase in CO2 concentration from 230 to 270 ,l/liter in the dark. Between measurements of photosynthetic or respiratory rates, the CO2 concentration was held constant at 250 i 7 ,ul/liter.After steady rates of gas exchange were obtained, the assimilation chamber was opened and a leaf disc was rapidly removed from a lower leaf and placed in a thermocouple psychrometer cham...

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 ( 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 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 w , maintaining the sugar stream partially prevented the abortion, but invertase regulated the synthesis of ovary starch and partially prevented full recovery.